The build2 Toolchain Introduction

Revision 0.8, October 2018
This revision of the document describes the build2 toolchain 0.8.X series and is available in the following formats: XHTML, PDF/A4, PDF/Letter, PostScript/A4, and PostScript/Letter.

Copyright © 2014-2018 Code Synthesis Ltd
Permission is granted to copy, distribute and/or modify this document under the terms of the MIT License.

Table of Contents

Preface
1TL;DR
2Getting Started Guide
2.1Hello, World
2.2Package Repositories
2.3Adding and Removing Dependencies
2.4Upgrading and Downgrading Dependencies
2.5Versioning and Release Management
2.6Developing Multiple Packages and Projects
2.7Package Consumption
2.8Using System-Installed Dependencies
2.9Using Unpackaged Dependencies
3Canonical Project Structure
3.1Source Directory
3.2Source Naming
3.3Source Contents
3.4Tests
3.5Build Output

Preface

This document is an overall introduction to the build2 toolchain that shows how the main components, namely the build system, the package dependency manager, and the project dependency manager are used together to handle the entire C++ project development lifecycle: creation, development, testing, and delivery. For additional information, including documentation for individual toolchain components, man pages, etc., refer to the build2 project Documentation page.

1 TL;DR

$ git clone ssh://example.org/hello.git
$ tree hello
hello/
├── hello/
│   ├── hello.cxx
│   └── buildfile
├── manifest
└── repositories.manifest

$ cd hello
$ bdep init --config-create ../hello-gcc cc config.cxx=g++
initializing in project /tmp/hello/
created configuration /tmp/hello-gcc/ default,auto-synchronized
synchronizing:
  new hello/0.1.0

$ b
c++ hello/cxx{hello}@../hello-gcc/hello/hello/
ld ../hello-gcc/hello/hello/exe{hello}
ln ../hello-gcc/hello/hello/exe{hello} -> hello/

$ hello/hello World
Hello, World!

$ edit repositories.manifest   # add https://example.org/libhello.git
$ edit manifest                # add 'depends: libhello ^1.0.0'
$ edit hello/buildfile         # import libhello
$ edit hello/hello.cxx         # use libhello

$ b
fetching from https://example.org/libhello.git
synchronizing /tmp/hello-gcc/:
  new libhello/1.0.0 (required by hello)
  reconfigure hello/0.1.0
c++ ../hello-gcc/libhello-1.0.0/libhello/cxx{hello}
ld ../hello-gcc/libhello-1.0.0/libhello/libs{hello}
c++ hello/cxx{hello}@../hello-gcc/hello/hello/
ld ../hello-gcc/hello/hello/exe{hello}
ln ../hello-gcc/hello/hello/exe{hello} -> hello/

$ bdep fetch                   # refresh available versions
$ bdep status -i               # review available versions
hello configured 0.1.0
  libhello ^1.0.0 configured 1.0.0 available [1.1.0]

$ bdep sync libhello           # upgrade to latest
synchronizing:
  new libformat/1.0.0 (required by libhello)
  new libprint/1.0.0 (required by libhello)
  upgrade libhello/1.1.0
  reconfigure hello/0.1.0

$ bdep sync libhello/1.0.0     # downgrade
synchronizing:
  drop libprint/1.0.0 (unused)
  drop libformat/1.0.0 (unused)
  downgrade libhello/1.0.0
  reconfigure hello/0.1.0

2 Getting Started Guide

The aim of this guide is to get you started developing C/C++ projects with the build2 toolchain. All the examples in this section include the relevant command output so if you just want to get a sense of what build2 is about, then you don't have to install the toolchain and run the commands in order to follow along. Or, alternatively, you can take a short detour to the Installation Instructions and then try the examples for yourself.

One of the primary goals of the build2 toolchain is to provide a uniform interface across all the platforms and compilers. While most of the examples in this document assume a UNIX-like operation system, they will look pretty similar if you are on Windows. You just have to use appropriate paths, compilers, and options.

The question we will try to answer in this section can be summarized as:

$ git clone .../hello.git && now-what?

That is, we clone an existing C/C++ project or would like to create a new one and then start hacking on it. We want to spend as little time and energy as possible on the initial and ongoing infrastructure maintenance: setting up build configurations, managing dependencies, continuous integration and testing, release management, etc. Or, as one C++ user aptly put it, "All I want to do is program."

2.1 Hello, World

Let's see what programming with build2 feels like by starting with a customary "Hello, World!" program (here we assume our current working directory is /tmp):

$ bdep new -t exe -l c++ hello
created new executable project hello in /tmp/hello/

The bdep-new(1) command creates a canonical build2 project. In our case it is an executable implemented in C++.

To create a library, pass -t lib. By default new also initializes a git repository and generates suitable .gitignore files (pass -s none if you don't want that). And for details on naming your projects, see Package Name.

Note to Windows users: the build2-baseutils package includes core git utilities that are sufficient for the bdep functionality.

Let's take a look inside our new project:

$ tree hello
hello/
├── .git/
├── .bdep/
├── build/
├── hello/
│   ├── hello.cxx
│   ├── buildfile
│   └── testscript
├── buildfile
├── manifest
└── repositories.manifest

While the canonical project structure is strongly recommended, especially for new projects, build2 is flexible enough to allow most commonly used arrangements. See Canonical Project Structure for the more detailed discussion and rationale behing this layout.

Similar to version control tools, we normally run all build2 tools from the project's source directory or one of its subdirectories, so:

$ cd hello

While the project layout is discussed in more detail in later sections, let's examine a couple of interesting files to get a sense of what's going on. We start with the source file which should look familiar:

$ cat hello/hello.cxx

#include <iostream>

int main (int argc, char* argv[])
{
  using namespace std;

  if (argc < 2)
  {
    cerr << "error: missing name" << endl;
    return 1;
  }

  cout << "Hello, " << argv[1] << '!' << endl;
}

If you prefer the .?pp extensions over .?xx for your C++ source files, pass -l c++,cpp to the new command. See bdep-new(1) for details on this and other customization options.

Let's take a look at the accompanying buildfile:

$ cat hello/buildfile

libs =
#import libs += libhello%lib{hello}

exe{hello}: {hxx ixx txx cxx}{**} $libs testscript

As the name suggests, this file describes how to build things. While its content might look a bit cryptic, let's try to infer a couple of points without going into too much detail (for details see the build system Introduction).

That exe{hello} on the left of : is a target (executable named hello) and what we have on the right are prerequisites (C++ source files, libraries, etc). This buildfile uses wildcard patterns (that **) to automatically locate all the C++ source files. This means we don't have to edit our buildfile every time we add, remove, or rename a source file in our project. There also appears to be some (commented out) infrastructure for importing and linking libraries (that libs variable). We will see how to use it in a moment.

In simple projects that follow the canonical structure we can often completely ignore the presence of the build definition files thus approaching the build system-less workflow found in languages like Rust and Go.

Finally, the buildfile also lists testscript as a prerequisite of hello. This file tests our target. Let's take a look inside:

$ cat hello/testscript

: basics
:
$* 'World' >'Hello, World!'

: missing-name
:
$* 2>>EOE != 0
error: missing name
EOE

Again, we are not going into detail here (see Testscript Introduction for a proper introduction), but to give you an idea, here we have two tests: the first (with id basics) verifies that our program prints the expected greeting while the second makes sure it handles the missing name error condition. Tests written in Testscript are concise, portable, and executed in parallel.

Next up is manifest:

$ cat manifest
: 1
name: hello
version: 0.1.0-a.0.z
summary: hello executable
license: TODO
url: https://example.org/hello
email: you@example.org
#depends: libhello ^1.0.0

The manifest file is what makes a build system project a package. It contains all the metadata that a user of a package might need to know: its name, version, license, dependencies, etc., all in one place.

Refer to Manifest Format for the general format of build2 manifest files and to Package Manifest for details on the package manifest values.

As you can see, manifest created by bdep-new(1) contains some dummy values which you would want to adjust before publishing your package. But let's resist the urge to adjust that strange looking 0.1.0-a.0.z until we discuss package versioning.

Next to manifest you might have noticed the repositories.manifest file – we will discuss its function later, when we talk about dependencies and where they come from.

Project in hand, let's build it. Unlike other programming languages, C++ development usually involves juggling a handful of build configurations: several compilers and/or targets (build2 is big on cross-compiling), debug/release, different sanitizers and/or static analysis tools, and so on. As a result, build2 is optimized for multi-configuration usage. However, as we will see shortly, one build configuration can be designated as the default with additional conveniences.

The bdep-init(1) command is used to initialize a project in a build configuration. As a shortcut, it can also create a new build configuration in the process, which is just what we need here. Let's start with GCC (remember we are in the project's root directory):

$ bdep init -C ../hello-gcc @gcc cc config.cxx=g++
initializing in project /tmp/hello/
created configuration @gcc /tmp/hello-gcc/ default,auto-synchronized
synchronizing:
  new hello/0.1.0-a.0.19700101000000

The --create|-C option instructs init to create a new configuration in the specified directory (../hello-gcc in our case). To make referring to configurations easier, we can give it a name, which is what we do with @gcc. The next argument (cc, stands for C-common) is the build system module we would like to configure. It implements compilation and linking rules for the C and C++ languages. Finally, config.cxx=g++ is (one of) this module's configuration variables that specifies the C++ compiler we would like to use (the corresponding C compiler will be determined automatically). Let's for now also ignore that synchronizing:... bit along with strange-looking 19700101000000 in the version – it will become clear what's going on here in a moment.

Note to Windows users: a command line argument with leading @ has a special meaning in PowerShell. To work around this, you can use the alternative -@gcc syntax or the -n gcc option.

Now the same for Clang:

$ bdep init -C ../hello-clang @clang cc config.cxx=clang++
initializing in project /tmp/hello/
created configuration @clang /tmp/hello-clang/ auto-synchronized
synchronizing:
  new hello/0.1.0-a.0.19700101000000

If we check the parent directory, we should now see two build configurations next to our project:

$ ls ..
hello/
hello-gcc/
hello-clang/

If, as in the above examples, our configuration directories are next to the project and their names are in the prj-name-cfg-name form, then we can use the shortcut version of the init command:

$ bdep init -C @clang cc config.cxx=clang++

Things will also look pretty similar if you are on Windows instead of a UNIX-like operating system. For example, to initialize our project on Windows with Visual Studio, start the Visual Studio development command prompt and then run:

Currently we have to run build2 tools from a suitable Visual Studio development command prompt. This requirement will likely be removed in the future.

> bdep init -C ..\hello-debug @debug cc     ^
  config.cxx=cl                             ^
  "config.cc.coptions=/MDd /Od /Zi"         ^
  config.cc.loptions=/DEBUG

> bdep init -C ..\hello-release @release cc ^
  config.cxx=cl                             ^
  config.cc.coptions=/O2

Besides the coptions (compile options) and loptions (link options), other commonly used cc module configuration variables are poptions (preprocess options) and libs (extra libraries to link). We can also use their config.c.* (C compilation) and config.cxx.* (C++ compilation) variants if we only want them applied during the respective language compilation. For example:

$ bdep init ... cc       \
  config.cxx=clang++     \
  config.cc.coptions=-g  \
  config.cxx.coptions=-stdlib=libc++

One difference you might have noticed when creating the gcc and clang configurations above is that the first one was designated as the default. The default configuration is used by bdep commands if no configuration is specified explicitly (see bdep-projects-configs(1) for details). It is also the configuration that is used if we run the build system in the project's source directory. So, normally, you would make your every day development configuration the default. Let's try that:

$ bdep status
hello configured 0.1.0-a.0.19700101000000

$ b
c++ hello/cxx{hello}@../hello-gcc/hello/hello/
ld ../hello-gcc/hello/hello/exe{hello}
ln ../hello-gcc/hello/hello/exe{hello} -> hello/

$ b test
test hello/testscript{testscript} ../hello-gcc/hello/hello/exe{hello}

$ hello/hello World
Hello, World!

To see the actual compilation command lines, run b -v and for even more details, run b -V. See b(1) for more information on these and other build system options.

In contrast, the Clang configuration has to be requested explicitly:

$ bdep status @clang
hello configured 0.1.0-a.0.19700101000000

$ b ../hello-clang/hello/
c++ hello/cxx{hello}@../hello-clang/hello/hello/
ld ../hello-clang/hello/hello/exe{hello}

$ b test: ../hello-clang/hello/
test hello/testscript{testscript} ../hello-clang/hello/hello/exe{hello}

$ ../hello-clang/hello/hello/hello World
Hello, World!

As you can see, using the build system directly on configurations other than the default requires explicitly specifying their paths. It would have been more convenient if we could refer to them by names. The bdep-update(1) and bdep-test(1) commands allow us to do exactly that:

$ bdep test @clang
c++ hello/cxx{hello}@../hello-clang/hello/hello/
ld ../hello-clang/hello/hello/exe{hello}
test hello/testscript{testscript} ../hello-clang/hello/hello/exe{hello}

And we can also perform the desired build system operation on several (or --all|-a) configurations at once:

$ bdep test @gcc @clang
in configuration @gcc:
test hello/testscript{testscript} ../hello-gcc/hello/hello/exe{hello}

in configuration @clang:
test hello/testscript{testscript} ../hello-clang/hello/hello/exe{hello}

As we will see later, the bdep-test(1) command also allows us to test immediate (--immediate|-i) or all (--recursive|-r) dependencies of our project. We call it deep testing.

While we are here, let's also check how hard it would be to cross-compile:

$ bdep init -C ../hello-mingw @mingw cc config.cxx=x86_64-w64-mingw32-g++
initializing in project /tmp/hello/
created configuration @mingw /tmp/hello-mingw/ auto-synchronized
synchronizing:
  new hello/0.1.0-a.0.19700101000000

$ bdep update @mingw
c++ hello/cxx{hello}@../hello-mingw/hello/hello/
ld ../hello-mingw/hello/hello/exe{hello}

As you can see, cross-compiling in build2 is nothing special. In our case, on a properly setup GNU/Linux machine (that automatically uses wine as an .exe interpreter) we can even run tests (in build2 this is called cross-testing):

$ bdep test @mingw
test hello/testscript{testscript} ../hello-mingw/hello/hello/exe{hello}

$ ../hello-mingw/hello/hello/hello.exe Windows
Hello, Windows!

Let's review what it takes to initialize a project's infrastructure and perform the first build. For an existing project:

$ git clone .../hello.git
$ cd hello
$ bdep init -C ../hello-gcc @gcc cc config.cxx=g++
$ b

For a new project:

$ bdep new -t exe -l c++ hello
$ cd hello
$ bdep init -C ../hello-gcc @gcc cc config.cxx=g++
$ b

If you prefer, the new and init steps can be combined into a single command:

$ bdep new -t exe -l c++ hello -C hello-gcc @gcc cc config.cxx=g++

And if you need to deinitialize a project in one or more build configurations, there is the bdep-deinit(1) command for that:

$ bdep deinit @gcc @clang
deinitializing in project /tmp/hello/
in configuration @gcc:
synchronizing:
  drop hello

in configuration @clang:
synchronizing:
  drop hello

As mentioned earlier, by default bdep-new(1) initializes a git repository for us. Now that we have successfully built and tested our project, it might be a good idea to make a first commit and publish it to a remote repository where others can find it. Using GitHub as an example:

$ git add .
$ git commit -m "Initial implementation"
$ git remote add origin git@github.com:john-doe/hello.git
$ git push -u

While we have managed to test a couple of platforms (Linux and Windows) and compiler versions (Clang and GCC) locally, there are quite a few combinations that we haven't tried (Mac OS with Apple Clang and Windows with MSVC, to name the major ones). We could test them manually, some with the help of virtualization while for others (such as Mac OS) we may need physical hardware. Add a few versions for each compiler and we are looking at a dozen build configurations. Needless to say, testing on all of them manually is a lot of work. Now that we have our project available from a public remote repository, we can instead use the remote testing functionality offered by the bdep-ci(1) command. For example:

$ bdep ci
submitting:
  to:      https://ci.cppget.org
  in:      https://github.com/john-doe/hello.git#master@93e1dbc94baa
  package: hello
  version: 0.1.0-a.0.20180907091517.93e1dbc94baa
continue? [y/n] y
############################################################# 100.0%
CI request is queued:
  https://ci.cppget.org/@d6ee90f4-21a9-47a0-ab5a-7cd2f521d3d8

Let's see what's going on here. By default ci submits a test request to ci.cppget.org, a public CI service run by the build2 project (see available Build Configurations and Use Policies). It is testing the current working tree state (branch and commit) of our package which should be available from our remote repository (on GitHub in this example) since that's where the CI service expects to find it. In response we get a URL where we can see the build and test results, logs, etc.

This push CI model works particularly well with the "feature branch" development workflow. Specifically, you would develop a new feature in a separate branch, publishing and remote-testing it as necessary. When the feature is ready, you would merge any changes from master, test the result one more time, and then merge the feature into master.

Now is a good time to get an overview of the build2 toolchain. After all, we have already used two of its tools (bdep and b) without a clear understanding of what they actually are.

Unlike most other programming languages that encapsulate the build system, package dependency manager, and project dependency manager into a single tool (such as Rust's cargo or Go's go), build2 is a hierarchy of several tools that you will be using directly and which together with your version control system (VCS) will constitute the core of your project management toolset.

While build2 can work without a VCS, this will result in reduced functionality.

At the bottom of the hierarchy is the build system, b(1). Next comes the package dependency manager, bpkg(1). It is primarily used for package consumption and depends on the build system. The top of the hierarchy is the project dependency manager, bdep(1). It is used for project development and relies on bpkg for building project packages and their dependencies.

The main reason for this separation is modularity and the resulting flexibility: there are situations where we only need the build system (for example, when building a package for a system package manager where all the dependencies should be satisfied from the system repository), or only the build system and package manager (for example, when a build bot is building a package for testing).

Note also that strictly speaking build2 is not C/C++-specific; its build model is general enough to handle any DAG-based operations and its package/project dependency management can be used for any compiled language.

As we will see in a moment, build2 also integrates with your VCS in order to automate project versioning. Note that currently only git(1) is supported.

Now that we understand the tooling, let's also revisit the notion of build configuration (those hello-gcc and hello-clang directories). A bdep build configuration is actually a bpkg build configuration which, in the build system terms, is an amalgamation – a project that contains subprojects. In our case, the subprojects in these amalgamations will be the projects we have initialized with init and, as we will see later, packages that they depend on. For example, here is what our hello-gcc contains:

$ tree hello-gcc
hello-gcc/
├── .bpkg/
├── build/
│   └── config.build
└── hello/
    ├── build/
    │   └── config.build
    └── hello/
        ├── hello
        └── hello.o

Underneath bdep-init(1) with the --config-create|-C option calls bpkg-cfg-create(1) which, in turn, performs the build system create meta-operation (see b(1) for details).

The important point here is that the bdep build configuration is not a black box that you should never look inside of. On the contrary, it is a normal and predictable concept of the package manager and the build system and as long as you understand what you are doing, you should feel free to interact with it directly.

Let's now move on to the reason why there is dep in the bdep name: dependency management.

2.2 Package Repositories

Say we have realized that writing "Hello, World!" programs is a fairly common task and that someone must have written a library to help with that. So let's see if we can find something suitable to use in our project.

Where should we look? That's a good question. But before we can try to answer it, we need to understand where build2 can source dependencies. In build2 packages come from package repositories. Two commonly used repository types are version control and archive-based (see bpkg-repository-types(1) for details).

As the name suggests, a version control-based repository uses a VCS as its distribution mechanism. Currently, only git is supported. Such a repository normally contains multiple versions of a single package or, perhaps, of a few related packages.

An archive-based repository contains multiple, potentially unrelated packages/versions as archives along with some meta information (package list, prerequisite/complement repositories, signatures, etc) that are all accessible via HTTP(S).

Version control and archive-based repositories have different trade-offs. Version control-based repositories are great for package developers: With services like GitHub they are trivial to setup. In fact, your project's (already existing) VCS repository will normally be the build2 package repository – you might need to add a few files, but that's about it.

However, version control-based repositories are not without drawbacks: It will be hard for your users to discover your packages (try searching for "hello library" on GitHub – most of the results are not even in C++ let alone packaged for build2). There is also the issue of continuous availability: users can delete their repositories, services may change their policies or go out of business, and so on. Version control-based repositories also lack repository authentication and package signing. Finally, obtaining the available package list for such repositories can be slow.

A central, archive-based repository would address all these drawbacks: It would be a single place to search for packages. Published packages will never disappear and can be easily mirrored. Packages are signed and the repository is authenticated (see bpkg-repository-signing(1) for details). And, last, but not least, archive-based repositories are fast.

cppget.org is the build2 community's central package repository. While centralized, it is also easy to mirror since its contents are accessible via plain HTTPS (you can browse pkg.cppget.org to get an idea). As an added benefit, packages on cppget.org are continuously built and tested on all the major platform/compiler combinations with the results available as part of the package description.

The main drawback of archive-based repositories is the setup cost. Getting a basic repository going is relatively easy – all you need is an HTTP(S) server. Adding a repository web interface like that on cppget.org will require running brep. And adding CI will require running a bunch of build bots (bbot). Note also that in build2 archive-based repositories can be federated with different sections of the repository being hosted/managed potentially independently.

To summarize, version control-based repositories are great for package developers while a central, archive-based repository is convenient for package consumers. A reasonable strategy is then for package developers to publish their releases to a central repository. Package consumers can then decide which repository to use based on their needs. For example, one could use cppget.org as a (fast, reliable, and secure) source of stable versions but also add, say, git repositories for select packages (perhaps with the #HEAD fragment filter to improve download speed) for testing development snapshots. In this model the two repository types complement each other.

Publishing of packages to archive-based repositories is discussed in Versioning and Release Management.

Let's see how all this works in practice. Go over to cppget.org and type "hello library" in the search box. At the top of the search result you should see the libhello package and if you follow the link you will see the package description page along with a list of available versions. Pick a version that you like and you will see the package version description page with quite a bit of information, including the list of platform/compiler combinations that this version has been successfully (or unsuccessfully) tested with. If you like what you see, copy the location value – this is the repository location where this package version can be sourced from.

The cppget.org repository is split into several sections: stable, testing, beta, alpha and legacy, with each section having its own repository location (see the repository's about page for details on each section's policies). Note also that testing is complemented by stable, beta by testing, and so on, so you only need to choose the lowest stability level and you will automatically "see" packages from the more stable sections.

The cppget.org stable sections will always contain the libhello library version 1.0.X that was generated using the following bdep-new(1) command line:

$ bdep new -t lib -l c++ libhello

It can be used as a predictable test dependency when setting up new projects.

Let's say we've visited the libhello project's home page (for example by following a link from the package details page) and noticed that it is being developed in a git repository. How can we see what's available there? If the releases are tagged, then we can infer the available released versions from the tags. But that doesn't tell us anything about what's happening on the HEAD or in the branches. For that we can use the package manager's bpkg-rep-info(1) command:

$ bpkg rep-info https://git.build2.org/hello/libhello.git
libhello/1.0.0
libhello/1.1.0

As you can see, besides 1.0.0 that we have seen in cppget.org/stable, there is also 1.1.0 (which is perhaps being tested in cppget.org/testing). We can also check what might be available on the HEAD (see bpkg-repository-types(1) for details on the git repository URL format):

$ bpkg rep-info https://git.build2.org/hello/libhello.git#HEAD
libhello/1.1.1-a.0.20180504111511.2e82f7378519

We can also use the rep-info command on archive-based repositories, however, if available, the web interface is usually more convenient and provides more information.

To summarize, we found two repositories for the libhello package: the archive-based cppget.org that contains the released versions as well as its development git repository where we can get the bleeding edge stuff. Let's now see how we can add libhello to our project.

2.3 Adding and Removing Dependencies

So we found libhello that we would like to use in our hello project. First, we edit the repositories.manifest file found in the root directory of our project and add one of the libhello repositories as a prerequisite. Let's start with cppget.org:

role: prerequisite
location: https://pkg.cppget.org/1/stable

Refer to Repository Manifest for details on the repository manifest values.

Next, we edit the manifest file (again, found in the root of our project) and specify the dependency on libhello with optional version constraint. For example:

depends: libhello ^1.0.0

Let's briefly discuss version constraints (for details see the depends value documentation). A version constraint can be expressed with a comparison operator (==, >, <, >=, <=), a range shortcut operator (~ and ^), or a range. Here are a few examples:

depends: libhello == 1.2.3
depends: libhello >= 1.2.3

depends: libhello ~1.2.3
depends: libhello ^1.2.3

depends: libhello [1.2.3 1.2.9)

You may already be familiar with the tilde (~) and caret (^) constraints from dependency managers for other languages. To recap, tilde allows upgrades to any further patch versions while caret also allows upgrades to further minor versions. They are equivalent to the following ranges:

~X.Y.Z  [X.Y.Z  X.Y+1.0)

^X.Y.Z  [X.Y.Z  X+1.0.0)  if X >  0
^0.Y.Z  [0.Y.Z  0.Y+1.0)  if X == 0

Zero major version component is customarily used during early development where the minor version effectively becomes major. As a result, the tilde constraint has a special treatment of this case.

Unless you have good reasons not to (for example, a dependency does not use semantic versioning), we suggest that you use the ^ constraint which provides a good balance between compatibility and upgradability with ~ being a more conservative option.

Ok, we've specified where our package comes from (repositories.manifest) and which versions we find acceptable (manifest). The next step is to edit hello/buildfile and import the libhello library into our build:

import libs += libhello%lib{hello}

Finally, we modify our source code to use the library:

#include <libhello/hello.hxx>
...

int main (int argc, char* argv[])
{
  ...
  hello::say_hello (cout, argv[1]);
}

You are probably wondering why we have to specify this repeating information in so many places. Let's start with the source code: we can't specify the version constraint or location there because it will have to be repeated in every source file that uses the dependency.

Moving up, buildfile is also not a good place to specify this information for the same reason (a library can be imported in multiple buildfiles) plus the build system doesn't really know anything about version constraints or repositories which is the purview of the dependency management tools.

Finally, we have to separate the version constraint and the location because the same package can be present in multiple repositories with different policies. For example, when a package from a version control-based repository is published in an archive-based repository, its repositories.manifest file is ignored and all its dependencies should be available from the archive-based repository itself (or its fixed set of prerequisite repositories). In other words, manifest belongs to a package while repositories.manifest – to a repository.

Also note that this is unlikely to become burdensome since adding new dependencies is not something that happens often. There are also ideas to automate this with a bdep-add(1) command in the future.

To summarize, these are the files we had to modify to add a dependency to our project:

repositories.manifest   # add https://pkg.cppget.org/1/stable
manifest                # add 'depends: libhello ^1.0.0'
buildfile               # import libhello
hello.cxx               # use libhello

With a new dependency added, let's check the status of our project:

$ bdep status
fetching pkg:cppget.org/stable (prerequisite of dir:/tmp/hello)
warning: authenticity of the certificate for pkg:cppget.org/stable
         cannot be established
certificate is for cppget.org, "Code Synthesis" <admin@cppget.org>
certificate SHA256 fingerprint:
86:BA:D4:DE:2C:87:1A:EE:38:<...>:5A:EA:F4:F7:8C:1D:63:30:C6
trust this certificate? [y/n] y

hello configured 0.1.0-a.0.19700101000000
      available  0.1.0-a.0.19700101000000#1

The bdep-status(1) command has detected that the dependency information has changed and tells us that a new iteration of our project (that #1) is now available for synchronization with the build configuration.

We've also been prompted to authenticate the prerequisite repository. This will have to happen once for every build configuration we initialize our project in and can quickly become tedious. To overcome this, we can mention the certificate fingerprint that we wish to automatically trust in the repositories.manifest file (replace it with the actual fingerprint from the repository's about page):

role: prerequisite
location: https://pkg.cppget.org/1/stable
trust: 86:BA:D4:DE:2C:87:1A:EE:38:<...>:5A:EA:F4:F7:8C:1D:63:30:C6

To synchronize a project with one or more build configurations we use the bdep-sync(1) command:

$ bdep sync
synchronizing:
  new libhello/1.0.0 (required by hello)
  upgrade hello/0.1.0-a.0.19700101000000#1

Or we could just build the project without an explicit sync – if necessary, it will be automatically synchronized:

$ b
synchronizing:
  new libhello/1.0.0 (required by hello)
  upgrade hello/0.1.0-a.0.19700101000000#1
c++ ../hello-gcc/libhello-1.0.0/libhello/cxx{hello}
ld ../hello-gcc/libhello-1.0.0/libhello/libs{hello}
c++ hello/cxx{hello}@../hello-gcc/hello/hello/
ld ../hello-gcc/hello/hello/exe{hello}
ln ../hello-gcc/hello/hello/exe{hello} -> hello/

The synchronization as performed by the sync command is two-way: dependency packages are first added, removed, upgraded, or downgraded in build configurations according to the project's version constraints and user input. Then the actual versions of the dependencies present in the build configurations are recorded in the project's lockfile so that if desired, the build can be reproduced exactly. The lockfile functionality is not yet implemented. For a new dependency the latest available version that satisfies the version constraint is used.

Synchronization is also the last step in the bdep-init(1) command's logic.

Let's now examine the status in all (--all|-a) the build configurations and include the immediate dependencies (--immediate|-i):

$ bdep status -ai
in configuration @gcc:
hello configured 0.1.0-a.0.19700101000000#1
  libhello ^1.0.0 configured 1.0.0

in configuration @clang:
hello configured 0.1.0-a.0.19700101000000
      available 0.1.0-a.0.19700101000000#1

Since we didn't specify a configuration explicitly, only the default (gcc) was synchronized. Normally, you would try a new dependency in one configuration, make sure everything looks good, then synchronize the rest with --all|-a (or, again, just build what you need directly). Here are a few examples (see bdep-projects-configs(1) for details):

$ bdep sync -a
$ bdep sync @gcc @clang
$ bdep sync -c ../hello-mingw

After adding a new (or upgrading/downgrading existing) dependency, it's a good idea to deep-test our project: run not only our own tests but also of its immediate (--immediate|-i) or even all (--recursive|-r) dependencies. For example:

$ bdep test -ai
in configuration @gcc:
test hello/testscript{testscript} ../hello-gcc/hello/hello/exe{hello}
test ../hello-gcc/libhello-1.0.0/tests/basics/exe{driver}

in configuration @clang:
test hello/testscript{testscript} ../hello-clang/hello/hello/exe{hello}
test ../hello-clang/libhello-1.0.0/tests/basics/exe{driver}

To get rid of a dependency, we simply remove it from the manifest file and synchronize the project. For example, assuming libhello is no longer mentioned as a dependency in our manifests:

$ bdep status
hello configured 0.1.0-a.0.19700101000000#1
      available  0.1.0-a.0.19700101000000#2

$ bdep sync
synchronizing:
  drop libhello/1.0.0 (unused)
  upgrade hello/0.1.0-a.0.19700101000000#2

If instead of building a dependency from source you would prefer to use a version that is installed by your system package manager, see Using System-Installed Dependencies. And for information on using dependencies that are not build2 packages refer to Using Unpackaged Dependencies.

2.4 Upgrading and Downgrading Dependencies

Let's say we would like to try that 1.1.0 version we have seen in the libhello git repository. First, we need to add the repository to the repositories.manifest file:

role: prerequisite
location: https://git.build2.org/hello/libhello.git

Note that we don't need the trust value since git repositories are not authenticated.

To refresh the list of available dependency versions we use the bdep-fetch(1) command (or the --fetch|-f option to status):

$ bdep fetch
$ bdep status libhello
libhello configured 1.0.0 available [1.1.0]

To upgrade (or downgrade) dependencies we again use the bdep-sync(1) command. We can upgrade one or more specific dependencies by listing them as arguments to sync:

$ bdep sync libhello
synchronizing:
  new libformat/1.0.0 (required by libhello)
  new libprint/1.0.0 (required by libhello)
  upgrade libhello/1.1.0
  upgrade hello/0.1.0-a.0.19700101000000#3

Without an explicit version or the --patch|-p option, sync will upgrade the specified dependencies to the latest available versions. For example, if we don't like version 1.1.0, we can downgrade it back to 1.0.0 by specifying the version explicitly (we pass --old-available|-o to status to see the old versions):

$ bdep status -o libhello
libhello configured 1.1.0 available (1.1.0) [1.0.0]

$ bdep sync libhello/1.0.0
synchronizing:
  drop libprint/1.0.0 (unused)
  drop libformat/1.0.0 (unused)
  downgrade libhello/1.0.0
  reconfigure hello/0.1.0-a.0.19700101000000#3

The available versions are listed in the descending order with [] indicating that the version is only available as a dependency and () marking the current version.

Instead of specific dependencies we can also upgrade (--upgrade|-u) or patch (--patch|-p) immediate (--immediate|-i) or all (--recursive|-r) dependencies of our project.

As a more realistic example, version 1.1.0 of libhello depends on two other libraries: libformat and libprint. Here is our project's dependency tree while we were still using that version:

$ bdep status -r
hello configured 0.1.0-a.0.19700101000000#3
  libhello ^1.0.0 configured 1.1.0
    libformat ^1.0.0 configured 1.0.0
    libprint ^1.0.0 configured 1.0.0

A typical conservative dependency management workflow would look like this:

$ bdep status -fi  # refresh and examine immediate dependencies
hello configured 0.1.0-a.0.19700101000000#3
  libhello configured 1.1.0 available [2.0.0] [1.2.0] [1.1.2] [1.1.1]

$ bdep sync -pi    # upgrade immediate to latest patch version
synchronizing:
  upgrade libhello/1.1.2
  reconfigure hello/0.1.0-a.0.19700101000000#3
continue? [Y/n] y

Notice that in case of such mass upgrades you are prompted for confirmation before anything is actually changed (unless you pass --yes|-y).

In contrast, the following would be a fairly aggressive workflow where we upgrade everything to the latest available version (version constraints permitting; here we assume ^1.0.0 was used for all the dependencies):

$ bdep status -fr  # refresh and examine all dependencies
hello configured 0.1.0-a.0.19700101000000#3
  libhello configured 1.1.0 available [2.0.0] [1.2.0] [1.1.1]
    libprint configured 1.0.0 available [2.0.0] [1.1.0] [1.0.1]
    libformat configured 1.0.0 available [2.0.0] [1.1.0] [1.0.1]

$ bdep sync -ur    # upgrade all to latest available version
synchronizing:
  upgrade libprint/1.1.0
  upgrade libformat/1.1.0
  upgrade libhello/1.2.0
  reconfigure hello/0.1.0-a.0.19700101000000#3
continue? [Y/n] y

We can also have something in between: patch all (sync -pr), upgrade immediate (sync -ui), or even upgrade immediate and patch the rest (sync -ui followed by sync -pr).

2.5 Versioning and Release Management

Let's now discuss versioning and release management and, yes, that strange-looking 0.1.0-a.0.19700101000000 we keep seeing. While a build system project doesn't need a version and a bpkg package can use custom versioning schemes (see Package Version), a project managed by bdep must use standard versioning. A dependency, which is a bpkg package, need not use standard versioning.

Standard versioning (stdver) is a semantic versioning (semver) scheme with a more precisely defined pre-release component and without any build metadata.

If you believe that semver is just major.minor.patch, then in your worldview stdver would be the same as semver. In reality, semver also allows loosely defined pre-release and build metadata components. For example, 1.2.3-beta.1+build.23456 is a valid semver.

A standard version has the following form:

major.minor.patch[-prerel]

The major, minor, and patch components have the same meaning as in semver. The prerel component is used to provide continuous versioning of our project between releases. Specifically, during development of a new version we may want to publish several pre-releases, for example, alpha or beta. In between those we may also want to publish a number of snapshots, for example, for CI. With continuous versioning all these releases, pre-releases, and snapshots are assigned unique, properly ordered versions.

Continuous versioning is a cornerstone of the build2 project dependency management. In case of snapshots, an appropriate version is assigned automatically in cooperation with your VCS.

The prerel component for a pre-release has the following form:

(a|b).num

Here a stands for alpha, b stands for beta, and num is the alpha/beta number. For example:

1.1.0        # final              release  for 1.1.0
1.2.0-a.1    # first  alpha   pre-release  for 1.2.0
1.2.0-a.2    # second alpha   pre-release  for 1.2.0
1.2.0-b.1    # first  beta    pre-release  for 1.2.0
1.2.0        # final              release  for 1.2.0

The prerel component for a snapshot has the following form:

(a|b).num.snapsn[.snapid]

Where snapsn is the snapshot sequence number and snapid is the snapshot id. In case of git, snapsn is the commit timestamp in the YYYYMMDDhhmmss form and UTC timezone while snapid is a 12-character abbreviated commit id. For example:

1.2.3-a.1.20180319215815.26efe301f4a7

Notice also that a snapshot version is ordered after the corresponding pre-release version. That is, 1.2.3-a.1 < 1.2.3-a.1.1. As a result, it is customary to start the development of a new version with X.Y.Z-a.0.z, that is, a snapshot after the (non-existent) zero'th alpha release. We will explain the meaning of z in this version momentarily. The following chronologically-ordered versions illustrate a typical release flow of a project that uses git as its VCS:

0.1.0-a.0.19700101000000               # snapshot (no commits yet)
0.1.0-a.0.20180319215815.26efe301f4a7  # snapshot (first commit)
...                                    # more commits/snapshots
0.1.0-a.1                              # pre-release (first alpha)
0.1.0-a.1.20180319221826.a6f0f41205b8  # snapshot
...                                    # more commits/snapshots
0.1.0-a.2                              # pre-release (second alpha)
0.1.0-a.2.20180319231937.b701052316c9  # snapshot
...                                    # more commits/snapshots
0.1.0-b.1                              # pre-release (first beta)
0.1.0-b.1.20180319242038.c812163417da  # snapshot
...                                    # more commits/snapshots
0.1.0                                  # release
0.2.0-a.0.20180319252139.d923274528eb  # snapshot (first in 0.2.0)
...

For a more detailed discussion of standard versioning and its support in build2 refer to version Module.

Let's now see how this works in practice by publishing a couple of versions for our hello project. By now it should be clear what that 0.1.0-a.0.19700101000000 means – it is the first snapshot version of our project. Since there are no commits yet, it has the UNIX epoch as its commit timestamp. Let's see what changes after we've made our first commit:

$ git add .
$ git commit -m "Initial implementation"

$ bdep status
hello configured 0.1.0-a.0.19700101000000
      available  0.1.0-a.0.20180507062614.ee006880fc7e

Just like with changes to dependency information, status has detected that a new (snapshot) version of our project is available for synchronization.

Another way to view the project's version (which works even if we are not using bdep) is with the build system's info operation:

$ b info
project: hello
version: 0.1.0-a.0.20180507062614.ee006880fc7e
summary: hello executable project
...

Let's synchronize with the default build configuration:

$ bdep sync
synchronizing:
  upgrade hello/0.1.0-a.0.20180507062614.ee006880fc7e

$ bdep status
hello configured 0.1.0-a.0.20180507062614.ee006880fc7e

Notice that we didn't have to manually change the version anywhere. All we had to do was commit our changes and a new snapshot version was automatically derived by build2 from the new git commit. Without this automation continuous versioning would hardly be practical.

If we now make another commit, we will see a similar picture:

$ bdep status
hello configured 0.1.0-a.0.20180507062614.ee006880fc7e
      available  0.1.0-a.0.20180507062615.8fb9de05b38f

Note that you don't need to manually run sync after every commit. As discussed earlier, you can simply run the build system to update your project and things will get automatically synchronized if necessary.

Ok, time for our first release. Let's start with 0.1.0-a.1. Unlike snapshots, for pre-releases as well as final releases we have to update the version in the manifest file manually:

version: 0.1.0-a.1

The manifest file is the singular place where we specify the package version. The build system's version module makes it available in various forms in buildfiles and even source code.

To ensure continuous versioning, this change to version must be the last commit for this (pre-)release which itself must be immediately followed by a second change to the version starting the development of the next (pre-)release. We also recommend that you tag the release commit with a tag name in the vX.Y.Z form.

Having regular release tag names with the v prefix allows one to distinguish them from other tags, for example, with wildcard patterns.

Here is the release workflow for our example:

$ git commit -a -m "Release version 0.1.0-a.1"
$ git tag -a v0.1.0-a.1 -m "Tag version 0.1.0-a.1"
$ git push --follow-tags

# Version 0.1.0-a.1 is now public.

$ edit manifest  # change 'version: 0.1.0-a.1.z'
$ git commit -a -m "Change version to 0.1.0-a.1.z"
$ git push

# Master is now open for business.

Notice also that when specifying a snapshot version in manifest we use the special z snapshot value (for example, 0.1.0-a.1.z) which is recognized and automatically replaced by build2 with, in case of git, a commit timestamp and id (refer to version Module for details).

Publishing the final release to the version control repository is exactly the same. This time, however, let's also see how we can publish it to an archive-based repository. The first step is again to change the version, commit, tag, and push:

$ edit manifest  # change 'version: 0.1.0'
$ git commit -a -m "Release version 0.1.0"
$ git tag -a v0.1.0 -m "Tag version 0.1.0"
$ git push --follow-tags

To publish our project to an archive-based repository we use the bdep-publish(1) command. For example:

$ bdep publish
publishing:
  to:      https://cppget.org
  as:      John Doe <john@example.org>
  package: hello
  version: 0.1.0
  project: hello
  section: alpha
  control: https://github.com/john-doe/hello.git
continue? [y/n] y
pushing build2-control
submitting hello-0.1.0.tar.gz
############################################################# 100.0%
package submission is queued: https://queue.cppget.org/hello/0.1.0
reference: 0c596fca2017

Let's see what's going on here. By default publish submits to the cppget.org repository. On cppget.org package names are assigned on a first come first serve basis. But instead of using logins or emails to authenticate package ownership, cppget.org uses your version control repository as a proxy. In a nutshell, when we submit a package for the first time, its control repository is associated with its name and all subsequent submissions have to use the same control repository (the authentication part). When submitting a package, publish also adds a file to the build2-control branch of the control repository with the package archive checksum. On the other side, cppget.org checks for the presence of this file to make sure that whomever is making this submission has write access to the control repository (the authorization part). See bdep-publish(1) for details.

The rest should be pretty straightforward: publish prepares and uploads a distribution of our package which goes into the alpha section of the repository (because it has 0 major version). In response we get a URL which we can use to check the status of our submission on queue.cppget.org. And after some basic testing and verification, our package should appear on cppget.org (the exact steps are described in Submission Policies). Note also that package submissions to cppget.org are public and permanent and cannot be removed under any circumstances.

Finally, we also shouldn't forget to increment the version for the next development cycle:

$ edit manifest  # change 'version: 0.2.0-a.0.z'
$ git commit -a -m "Change version to 0.2.0-a.0.z"
$ git push

One sticky point of continuous versioning is choosing the next version. For example, above should we continue with 0.1.1-a.0, 0.2.0-a.0, or 1.0.0-a.0? The important rule to keep in mind is that we can jump forward to any further version at any time and without breaking continuous versioning. But we can never jump backwards.

For example, we can start with 0.2.0-a.0 but if we later realize that this will actually be a new major release, we can easily change it to 1.0.0-a.0. As a result, the general recommendation is to start conservatively by either incrementing the patch or the minor version component. The recommended strategy is to increment the minor component and, if required, release patch versions from a separate branch (created by branching off from the release commit).

Note also that you don't have to make any pre-releases if you don't need them. While during development you would still keep the version as X.Y.Z-a.0, at release you simply change it directly to the final X.Y.Z.

When publishing the final release you may also want to clean up now obsolete pre-release tags. For example:

$ git tag -l 'v0.1.0-*' | xargs git push --delete origin
$ git tag -l 'v0.1.0-*' | xargs git tag --delete

While at first removing such tags may seem like a bad idea, pre-releases are by nature temporary and their use only makes sense until the final release is published.

Also note that having a git repository with a large number of published but unused version tags may result in a significant download overhead.

Let's also briefly discuss in which situations we should increment each of the version components. While semver gives basic guidelines, there are several ways to apply them in the context of C/C++ where there is a distinction between binary and source compatibility. We recommend that you reserve patch releases for specific bug fixes and security issues that you can guarantee with a high level of certainty to be binary-compatible. Otherwise, if the changes are source-compatible, increment minor. And if they are breaking (that is, the user code likely will need adjustments), increment major. During early development, when breaking changes are frequent, it is customary to use the 0.Y.Z versions where Y effectively becomes the major component. Again, refer to the version Module for a more detailed discussion of this topic.

2.6 Developing Multiple Packages and Projects

How does a library like libhello get developed? It's possible someone woke up one day and realized that they were going to build a useful library that everyone was going to use. But somehow this doesn't feel like how it really works. In the real world things start organically: someone had a project like hello and then needed the same functionality in another project. Or someone else needed it and asked the author to factor it out into a library. For this approach to work, however, moving such common functionality into a library and then continue its parallel development must be a simple, frictionless process. Let's see how this works in build2.

First, we need to decide whether to make libhello another package in our hello project (that is, in the same git repository) or a separate project (with a separate repository). Both arrangements are equally well supported.

A multi-package project works best if all the packages have the same version and are released together. While the packages themselves can have different versions (since each has its own manifest), in this scenario following the release tagging recommendations discussed earlier will be problematic.

Let's start with a separate project since it is simpler. As the first step we use bdep-new(1) to create a new library project next to our hello:

$ bdep new -t lib -l c++ libhello
created new library project libhello in /tmp/libhello/

$ ls
hello/
libhello/
hello-gcc/
hello-clang/

Let's also edit the generated manifest file and add the project value (customarily after version) to indicate that our library belongs to the same overall project as our executable:

$ cat libhello/manifest
: 1
name: libhello
version: 0.1.0-a.0.z
project: hello
summary: hello library
...

The project value is used to group related packages together in order to help with their organization and discovery. For example, if later we create libhello2 or libhello-extra, then it would make sense for them to also belong to the hello project. See the project value documentation for details.

Our two projects will be sharing the same set of build configurations, so next we initialize libhello in hello-gcc and hello-clang:

$ cd libhello

$ bdep init -A ../hello-gcc @gcc
initializing in project /tmp/libhello/
added configuration @gcc /tmp/hello-gcc/ default,auto-synchronized
synchronizing:
  new libhello/0.1.0-a.0.19700101000000

$ bdep init -A ../hello-clang @clang
initializing in project /tmp/libhello/
added configuration @clang /tmp/hello-clang/ auto-synchronized
synchronizing:
  new libhello/0.1.0-a.0.19700101000000

If two or more projects share the same build configuration, then all of them are always synchronized at once, regardless of the originating project. It also makes sense to have the same default configuration and use identical configuration names in all the projects.

The last step is to move the desired functionality from hello to libhello and at the same time add a dependency on libhello, just as we did earlier (add a depends entry to manifest, then import the library in buildfile, and so on). One interesting question is what to put as a prerequisite repository in repositories.manifest. Our own setup will work even if we don't put anything there – the dependency will be automatically resolved to our local version of libhello since we have initialized it in all our build configurations. However, in case our hello repository is used by someone else, it's a good idea to add the remote git repository for libhello as a prerequisite.

By now you have probably realized that our project directory is just another type of package repository. See bpkg-repository-types(1) for more information.

And that's it, now we can build and test our new arrangement:

$ cd ../hello   # back to hello project root
$ bdep test -i
c++ ../libhello/libhello/cxx{hello}
c++ ../libhello/tests/basics/cxx{driver}
c++ hello/cxx{hello}
ld ../hello-gcc/libhello/libhello/libs{hello}
ld ../hello-gcc/libhello/tests/basics/exe{driver}
ld ../hello-gcc/hello/hello/exe{hello}
test ../hello-gcc/libhello/tests/basics/exe{driver}
test hello/testscript{testscript} ../hello-gcc/hello/hello/exe{hello}

This is also the approach we would use if we wanted to fix a bug in someone else's library. That is, we would clone their project repository and initialize it in the build configurations of our project which will "upgrade" the dependency to use the local version. Then we make the fix, submit it upstream, and continue using the local version until our fix is merged/published, at which point we deinitialize the project and switch back to using the upstream version.

Let's now examine the second option: making libhello a package inside hello. Here is the original structure of our hello project:

hello/
├── .git/
├── build/
├── hello/
│   ├── hello.cxx
│   └── buildfile
├── buildfile
├── manifest
└── repositories.manifest

As the first step, we move the hello program into its own subdirectory:

hello/
├── .git/
├── hello/
│   ├── build/
│   ├── hello/
│   │   ├── hello.cxx
│   │   └── buildfile
│   ├── buildfile
│   └── manifest
└── repositories.manifest

Next we again use bdep-new(1) to create a new library but this time as a package inside an already existing project:

$ cd hello
$ bdep new --package -t lib -l c++ libhello
created new library package libhello in /tmp/hello/libhello/

Let's see what our project looks like now:

hello/
├── .git/
├── hello/
│   ├── ...
│   └── manifest
├── libhello/
│   ├── ...
│   └── manifest
├── packages.manifest
└── repositories.manifest

Notice that, as discussed earlier, repositories.manifest belongs to the project (repository) while manifest – to the package.

Besides the libhello directory the new command also created the packages.manifest file in the root directory of our project. Let's take a look inside:

$ cat packages.manifest
: 1
location: libhello/

Up until now our hello was a simple, single-package project that didn't need this file – manifest in its root directory was sufficient (see bpkg-repository-types(1) for details on the project repository structure). But now it contains several packages and we need to specify where they are located within the project. So let's go ahead and add the location of the hello package:

$ cat packages.manifest
: 1
location: libhello/
:
location: hello/

Packages in a project can reside next to each other or in subdirectories but they cannot nest. When published to an archive-based repository, each such package will be placed into its own archive.

Next we initialize the new package in all our build configurations:

$ cd libhello
$ bdep init -a
initializing in project /tmp/hello/
in configuration @gcc:
synchronizing:
  upgrade hello/0.1.0-a.0.19700101000000#1
  new libhello/0.1.0-a.0.19700101000000

in configuration @clang:
synchronizing:
  upgrade hello/0.1.0-a.0.19700101000000#1
  new libhello/0.1.0-a.0.19700101000000

Notice that the hello package has been "upgraded" to reflect its new location.

Finally, as before, we move the desired functionality from hello to libhello and at the same time add a dependency on libhello. Note, however, that in this case we don't need to add anything to repositories.manifest since both packages are in the same project (repository). And that's it, now we can build and test our new arrangement:

$ cd ..      # back to hello project root
$ bdep test
c++ libhello/libhello/cxx{hello}
c++ libhello/tests/basics/cxx{driver}
c++ hello/hello/cxx{hello}
ld ../hello-gcc/libhello/libhello/libs{hello}
ld ../hello-gcc/libhello/tests/basics/exe{driver}
ld ../hello-gcc/hello/hello/exe{hello}
test ../hello-gcc/libhello/tests/basics/exe{driver}
test hello/hello/testscript{testscript} ../hello-gcc/hello/hello/exe{hello}

2.7 Package Consumption

Ok, now that we have published a few releases of hello, how would the users of our project get them? While they could clone the repository and use bdep just like we did, this is more of a development rather than consumption workflow. For consumption it is much easier to use the package dependency manager, bpkg(1), directly.

Note that this approach also works for libraries in case you wish to use them in a project with a build system other than build2. See Using Unpackaged Dependencies for background on cross-build system library consumption.

First, we create a suitable build configuration with the bpkg-cfg-create(1) command. We can use the same place for building all our tools so let's call the directory tools. Seeing that we are only interested in using (rather than developing) such tools, let's build them optimized and also configure a suitable installation location:

$ bpkg create -d tools cc        \
  config.cxx=g++                 \
  config.cc.coptions=-O3         \
  config.install.root=/usr/local \
  config.install.sudo=sudo
created new configuration in /tmp/tools/

$ cd tools

The same step on Windows using Visual Studio would look like this (again, remember to run this from the Visual Studio development command prompt):

$ bpkg create -d tools cc ^
  config.cxx=cl           ^
  config.cc.coptions=/O2  ^
  config.install.root= C:\install

To fetch and build packages (as well as all their dependencies) we use the bpkg-pkg-build(1) command. We can use either an archive-based repository like cppget.org or build directly from git:

$ bpkg build hello@https://git.build2.org/hello/hello.git
fetching from https://git.build2.org/hello/hello.git
  new libformat/1.0.0 (required by libhello)
  new libprint/1.0.0 (required by libhello)
  new libhello/1.1.0 (required by hello)
  new hello/1.0.0
continue? [Y/n] y
configured libformat/1.0.0
configured libprint/1.0.0
configured libhello/1.1.0
configured hello/1.0.0
c++ libprint-1.0.0/libprint/cxx{print}
c++ hello-1.0.0/hello/cxx{hello}
c++ libhello-1.1.0/libhello/cxx{hello}
c++ libformat-1.0.0/libformat/cxx{format}
ld libprint-1.0.0/libprint/libs{print}
ld libformat-1.0.0/libformat/libs{format}
ld libhello-1.1.0/libhello/libs{hello}
ld hello-1.0.0/hello/exe{hello}
updated hello/1.0.0

Passing a repository URL to the build command is a shortcut to the following sequence of commands:

$ bpkg add https://git.build2.org/hello/hello.git  # add repository
$ bpkg fetch                             # fetch package list
$ bpkg build hello                       # build package by name

Once built, we can install the package to the location that we have specified with config.install.root using the bpkg-pkg-install(1) command:

$ bpkg install hello
...
install libformat-1.0.0/libformat/libs{format}
install libprint-1.0.0/libprint/libs{print}
install libhello-1.1.0/libhello/libs{hello}
install hello-1.0.0/hello/exe{hello}

$ hello World
Hello, World!

If on your system the installed executables don't run from /usr/local because of the unresolved shared libraries (or if you are installing somewhere else, such as /opt), then the easiest way to fix this is with rpath. Simply add the following configuration variable when creating the build configuration (or as an argument to the install command):

config.bin.rpath=/usr/local/lib

Note to Windows users: this is not an issue on this platform since executables and shared (DLL) libraries are installed into the same subdirectory (bin) of the installation directory.

The installation contents and layout under config.install.root would be along these lines:

/usr/local/
├── bin/
│   └── hello
├── include/
│   ├── libformat/
│   │   ├── export.hxx
│   │   ├── format.hxx
│   │   └── version.hxx
│   ├── libhello/
│   │   ├── export.hxx
│   │   ├── hello.hxx
│   │   └── version.hxx
│   └── libprint/
│       ├── export.hxx
│       ├── print.hxx
│       └── version.hxx
├── lib/
│   ├── libformat-1.0.so
│   ├── libformat.so -> libformat-1.0.so
│   ├── libhello-1.1.so
│   ├── libhello.so -> libhello-1.1.so
│   ├── libprint-1.0.so
│   ├── libprint.so -> libprint-1.0.so
│   └── pkgconfig/
│       ├── libformat.shared.pc
│       ├── libhello.shared.pc
│       └── libprint.shared.pc
└── share/
    └── doc/
        ├── libformat/
        │   └── manifest
        ├── libhello/
        │   └── manifest
        └── libprint/
            └── manifest

The installation locations of various types of files (executables, libraries, headers, documentation, etc) can be customized using a number of the config.install.* variables with the most commonly used ones and their defaults (relative to config.install.root) listed below (see the install build system module documentation for the complete list).

config.install.bin     = root/bin/
config.install.lib     = root/lib/
config.install.doc     = root/share/doc/
config.install.man     = root/share/man/
config.install.include = root/include/

If we need to uninstall a previously installed package, there is the bpkg-pkg-uninstall(1) command:

$ bpkg uninstall hello
uninstall hello-1.0.0/hello/exe{hello}
uninstall libhello-1.1.0/libhello/libs{hello}
uninstall libprint-1.0.0/libprint/libs{print}
uninstall libformat-1.0.0/libformat/libs{format}
...

To upgrade or downgrade packages we again use the build command. Here is a typical upgrade workflow:

$ bpkg fetch              # refresh available package list
$ bpkg status             # see if new versions are available

$ bpkg uninstall hello    # uninstall old version
$ bpkg build     hello    # upgrade to the latest version
$ bpkg install   hello    # install new version

Similar to bdep, to downgrade we have to specify the desired version explicitly. There are also the --upgrade|-u and --patch|-p as well as --immediate|-i and --recursive|-r options that allow us to upgrade or patch packages that we have built and/or their immediate or all dependencies (see bpkg-pkg-build(1) for details). For example, to make sure everything is patched, run:

$ bpkg fetch
$ bpkg build -pr

If a package is no longer needed, we can remove it from the configuration with bpkg-pkg-drop(1):

$ bpkg drop hello
following dependencies were automatically built but
will no longer be used:
  libhello
  libformat
  libprint
drop unused packages? [Y/n] y
  drop hello
  drop libhello
  drop libformat
  drop libprint
continue? [Y/n] y
purged hello
purged libhello
purged libformat
purged libprint

2.8 Using System-Installed Dependencies

Our operating system might already have a package manager (which we will refer to as system package manager) and for various reasons we may want to use the system-installed version of a dependency rather than building one from source.

Using system-installed versions works best for mature rather than rapidly-developed packages since for the latter you often need to track the latest version (which may not yet be available from the system repository) and/or test with multiple versions (which is not something that many system package managers support).

We can instruct build2 to configure a dependency package as available from the system rather than building it from source. Let's see how this works in an example. Say, we want to use libsqlite3 in our hello project.

The first step is to add it as a dependency, just like we did for libhello. That is, add another depends entry to manifest, then import it in buildfile, and so on.

Note that the dependency still has to be packaged and available from one of the project's prerequisite repositories. However, it can be a stub – a package that does not contain any source code and that can only be "obtained" from the system (see Package Version for details). See also Using Unpackaged Dependencies for how to deal with dependencies that are not packaged.

Now, if we just run sync or try to build our project, build2 will download and build the new dependency from source, just like it did for libhello. Instead, we can issue an explicit sync command that configures the libsqlite3 package as coming from the system:

$ bdep sync ?sys:libsqlite3
synchronizing:
  configure sys:libsqlite3/*
  upgrade hello/0.1.0-a.0.19700101000000#3

Here ? is a package flag that instructs build2 to treat it as a dependency and sys is a package scheme that tells build2 it comes from the system. See bpkg-pkg-build(1) for details.

We can have some build configurations using a system-installed version of a dependency while others building it from source, for example, for testing.

The system-installed dependency doesn't really have to come from the system package manager. It can also be manually installed and, as discussed in Using Unpackaged Dependencies, not necessarily into the system-default location like /usr/local.

Currently, unless we specify the installed version explicitly, a system-installed package is assumed to satisfy any dependency constraint. In the future, build2 will automatically query commonly used system package managers for the installed version and maybe even request installation of the absent packages. To support this functionality, the package manifest may need to specify package name mappings for various system package managers (which is the rationale behind stub packages).

2.9 Using Unpackaged Dependencies

Generally, we will have a much better time if all our dependencies come as build2 packages. Unfortunately, this won't always be the case in the real world and some libraries that you may need will use other build systems.

There is also the opposite problem: you may want to consume a library that uses build2 in a project that uses a different build system. For that refer to Package Consumption.

The standard way to consume such unpackaged libraries is to install them (not necessarily into a system-default location like /usr/local) so that we have a single directory with their headers and a single directory with their libraries. We can then configure our builds to use these directories when searching for imported libraries.

Needless to say, none of the build2 dependency management mechanisms such as version constraints or upgrade/downgrade will work on such unpackaged libraries. You will have to manage all these yourself manually.

Let's see how this all works in an example. Say, we want to use libextra that uses a different build system in our hello project. The first step is to manually build and install this library for each build configuration that we have. For example, we can install all such unpackaged libraries into unpkg-gcc and unpkg-clang, next to our hello-gcc and hello-clang build configurations:

$ ls
hello/
hello-gcc/
unpkg-gcc/
hello-clang/
unpkg-clang/

If you would like to try this out but don't have a suitable libextra, you can create and install one with these commands:

$ bdep new -t lib -l c++ libextra -C libextra-gcc cc config.cxx=g++
$ b install: libextra-gcc/ config.install.root=/tmp/unpkg-gcc

If we look inside one of these unpkg-* directories, we should see something like this:

$ tree unpkg-gcc
unpkg-gcc/
├── include/
│   └── libextra/
│       └── extra.hxx
└── lib/
    ├── libextra.a
    ├── libextra.so
    └── pkgconfig/
        └── libextra.pc

Notice that libextra.pc – it's a pkg-config(1) file that contains any extra compile and link options that may be necessary to consume this library. This is the de facto standard for build systems to communicate library build information to each other and is today supported by most commonly used implementations. Speaking of build2, it both recognizes .pc files when consuming third-party libraries and automatically produces them when installing its own.

While this may all seem foreign to Windows users, there is nothing platform-specific about this approach, including support for pkg-config, which, at least in case of build2, works equally well on Windows.

Next, we create a build configuration and configure it to use one of these unpkg-* directories (replace ... with the absolute path):

$ bdep init -C ../hello-gcc @gcc cc config.cxx=g++ \
  config.cc.poptions=-I.../unpkg-gcc/include       \
  config.cc.loptions=-L.../unpkg-gcc/lib

If using Visual Studio, replace -I with /I and -L with /LIBPATH:.

Alternatively, if you want to reconfigure one of the existing build configurations, then simply edit the build/config.build file (that is, hello-gcc/build/config.build in our case) and adjust the poptions and loptions values. Or you can use the build system directly to reconfigure the build configuration (see b(1) for details):

b configure: ../hello-gcc/                    \
  config.cc.poptions+=-I.../unpkg-gcc/include \
  config.cc.loptions+=-L.../unpkg-gcc/lib

If all the unpackaged libraries included .pc files, then the -L alone would have been sufficient. However, it doesn't hurt to also add -I, for good measure.

Once this is done, adjust your buildfile to import the library:

import libs += libextra%lib{extra}

And your source code to use it:

#include <libextra/extra.hxx>

Notice that we don't add the corresponding depends value to the project's manifest since this library is not a package. However, it is a good idea to instead add a requires entry as a documentation to users of our project.

3 Canonical Project Structure

The goal of establishing a canonical project structure is to create an ecosystem of packages that can coexist, are easy to comprehend by both humans and tools, scale to complex, real-world requirements, and, last but not least, are pleasant to work with.

The canonical structure is primarily meant for a package – a single library or program (or, sometimes, a collection of related programs) with a specific and well-defined function. While it may be less suitable for more elaborate, multi-library/program end-products that are not meant to be packaged, most of the recommendations discussed below would still apply. Oftentimes, you would start with a canonical project and expand from there. Note also that while the discussion below focuses on C++, most of it applies equally to C projects.

We often find ourselves factoring common functionality out of such end-products and into separate packages, for example, in order to be reused in another end-product). In this light, it can be helpful to start a new end-product project as a composition of individual packages that follow the canonical structure.

Projects created by the bdep-new(1) command have the canonical structure. The overall layouts for executable (-t exe) and library (-t lib) projects are presented below.

<name>/
├── build/
├── <name>/
│   ├── <name>.cxx
│   ├── <name>.test.cxx
│   ├── testscript
│   └── buildfile
├── buildfile
└── manifest
lib<name>/
├── build/
├── lib<name>/
│   ├── <name>.hxx
│   ├── <name>.cxx
│   ├── <name>.test.cxx
│   ├── export.hxx
│   ├── version.hxx.in
│   └── buildfile
├── tests/
├── buildfile
└── manifest

The canonical structure for both project types is discussed in detail in the following sections with a short summary of the key points presented below.

Let's start with naming our projects: A project name should only contain ASCII alphabetic characters ([a-zA-Z]), digits ([0-9]), underscores (_), plus/minus (+-), and dots (.) as well as be at least two characters long (see Package Name for additional restrictions and recommendations).

If a project consists of a library and an executable, then they should be split into separate packages (see Developing Multiple Packages and Projects for some common arrangements). In this case, by convention, the library name should start with the lib prefix, for example, libhello and hello. It is also strongly recommended (but not required) to follow this convention in new projects, even if there are no plans to have a related executable.

Using the lib prefix consistently offers several benefits:

  1. It is clear from the name to both humans and tools what kind of project it is.
  2. All libraries are consistently named (as opposed to some with the lib prefix and some without).
  3. All library names are future-proofed to co-exist with executables. If one starts with a library without the lib prefix but later decides to add an executable, renaming the library would unlikely be an option. And there is no need to spend mental energy on thinking whether it's possible that an executable will be added later.

The project's root directory should contain the root buildfile and package manifest file. Other recommended top-level subdirectory names are examples/ (for libraries it is normally a subproject like tests/, as discussed below), doc/, and etc/ (sample configurations, scripts, third-party contributions, etc). See also build system Project Structure for details on the build-related files (buildfile) and subdirectories (build/).

3.1 Source Directory

The project's source code is placed into a subdirectory of the root directory named the same as the project, for example, hello/hello/ or libhello/libhello/. It is called the project's source subdirectory.

There are several reasons for this layout: It implements the canonical inclusion scheme (discussed below) where each header is prefixed with its project name. It also has a predictable name where users (and tools) can expect to find our project's source code. Finally, this layout prevents clutter in the project's root directory which usually contains various other files (like README, LICENSE) and directories (like doc/, tests/, examples/).

Another popular approach is to place public headers into the include/ subdirectory and source files as well as private headers into src/. The cited advantage of this layout is the predictable location (include/) that contains only the project's public headers (that is, its API). This can make the project easier to navigate and understand while harder to misuse, for example, by including a private header.

However, this split layout is not without drawbacks:

Also, the stated advantage of this layout – separation of public headers from private – is not as clear cut as it may seem at first. The common assumption of the split layout is that only headers from include/ are installed and, conversely, to use the headers in-place, all one has to do is add -I pointing to include/. On the other hand, it is common for public headers to include private, for example, to call an implementation detail function in inline or template code (note that the same applies to private modules imported in public module interfaces). Which means such private, (or, probably now more accurately called implementation detail) headers have to be placed in the include/ directory as well, perhaps into a subdirectory (such as details/) or with a file name suffix (such as -impl) to signal to the user that they are still "private". Needless to say, in an actively developed project, keeping track of which private headers can still stay in src/ and which have to be moved to include/ (and vice versa) is a tedious, error-prone task. As a result, practically, the split layout quickly degrades into the "all headers in include/" arrangement which negates its main advantage.

It is also not clear how the split layout will translate to modularized projects. With modules, both the interface and implementation (including non-inline/template function definitions) can reside in the same file with a substantial number of C++ developers finding this arrangement appealing. If a project consists of only such single-file modules, then include/ and src/ have effectively become the same thing (note that there couldn't be any "private" modules in src/ since there would be nobody to import them). In a sense, we already have this situation with header-only libraries except that in the case of modules calling the directory include/ would be an anachronism.

To summarize, the split directory arrangement offers little benefit over the single directory layout, has a number of real drawbacks, and does not fit modularized projects well. In practice, private headers are placed into include/, often either in a subdirectory or with a special file name suffix, a mechanism that is readily available in the single directory layout.

All headers within a project should be included using the <> style inclusion and contain the project name as a directory prefix. And all headers means all headers – public, private, or implementation detail, in executables or in libraries.

As an example, let's say we've added utility.hxx to our hello project. This is how it should be included in hello.cxx:

// #include "utility.hxx"           // Wrong.
// #include <utility.hxx>           // Wrong.
// #include "../hello/utility.hxx"  // Wrong.

#include <hello/utility.hxx>

Similarly, if we want to include hello.hxx from libhello, then the inclusion should look like this:

#include <libhello/hello.hxx>

The problem with the "" style inclusion is if the header is not found relative to the including file, most compilers will continue looking for it in the include search paths, the same as for <>. As a result, if the header is not present in the right place (for example, because it was mistakenly not listed as to be installed), chances are that a completely unrelated header with the same name will be found and included. Needless to say, debugging situations like these is unpleasant.

Prefixing all inclusions with the project name also makes sure that headers with common names (for example, utility.hxx) can coexist (for example, when installed into a system-wide directory, such as /usr/include). The prefix also plays an important role in supporting auto-generated headers.

Note also that this header inclusion scheme is consistent with the module importation, for example:

import hello.utility;

Finally, note that while adding the project prefix to the "" style inclusion (for example, "libhello/hello.hxx") will make finding an unrelated header unlikely, there is still a possibility. And it is not clear why take the chance when there are no benefits. So let's imagine the "" style inclusion does not exist and we will all have a much better time.

If you have to disregard every rule and recommendation in this section but one, for example, because you are working on an existing library, then insist on this: public header inclusions must use the library name as a directory prefix.

The project's source subdirectory can have subdirectories of its own, for example, to organize the code into components. Naturally, header inclusions will need to contain such subdirectories, for example <libhello/core/hello.hxx>. When the project's headers are installed (for example, into /usr/include), this subdirectory hierarchy is automatically recreated.

If you would like to separate public API headers/modules from implementation details, the convention is to place them into the details/ subdirectory. For example:

libhello/
└── libhello/
    ├── details/
    │   └── utility.hxx
    └── ...

If a project has truly private headers (for example, proprietary code) that must be clearly separated from public and implementation detail headers, then they can be placed into the private/ subdirectory, next to details/. In a sense, this arrangement mimics the C++ public/protected/private member access.

It is recommended that you still install the implementation detail headers and modules for the reasons discussed above. If, however, you would like to disable their installation, you can add the following line to your source subdirectory buildfile:

details/hxx{*}: install = false

If you are creating a family of libraries with a common name prefix, then it may make sense to use a nested source directory layout with a common top-level directory. As an example, let's say we have the libstud-path and libstud-url libraries that belong to the same libstud family. Their source subdirectory layouts could look like this:

libstud-path/
└── libstud/
    └── path/
        ├── path.hxx
        ├── path-io.hxx
        ├── ...
        └── buildfile

libstud-url/
└── libstud/
    └── url/
        ├── url.hxx
        ├── url-io.hxx
        ├── ...
        └── buildfile

With the header inclusion paths adjusted accordingly:

#include <libstud/path/path.hxx>
#include <libstud/url/url.hxx>

3.2 Source Naming

When naming source files, only use ASCII alphabetic characters, digits, as well as _ (underscore) and - (minus). Use . (dot) only for extensions, that is, trailing parts of the name that classify your files. Examples of good names:

SmallVector.hxx
small-vector.hxx
small_vector.hxx
small-vector.test.cxx

Examples of bad names:

small+vector.hxx
small.vector.hxx

If you are using _ or - as word separators in filesystem names, pick one and use it consistently throughout the project.

The C source file extensions are always .h/.c. The two alternative C++ source file extension schemes are .?pp and .?xx:

file        .?pp  .?xx

header      .hpp  .hxx
module      .mpp  .mxx
inline      .ipp  .ixx
template    .tpp  .txx
source      .cpp  .cxx

The use of inline and template files is a matter of taste. If used, they are included at the end of the header/module files and contain definitions of inline and non-inline template functions, respectively. The .?xx/.?pp files with the same name (or, sometimes, name prefix) are assumed to be related and are collectively called a module. This term is meant to correspond directly to a C++ module.

By default the bdep-new(1) command uses the naming .?xx scheme. To use .?pp instead, pass -t c++,cpp.

There are several reasons not to "reuse" the .h C header extension for C++ files:

The last two reasons are also why headers without extensions are probably not worth the trouble.

Source files corresponding to C++ modules need to embed a sufficient amount of "module name tail" in their names to unambiguously resolve all the modules used in a project. When deriving file names from C++ module names, . (dot) should be replaced with either _ (underscore), - (minus), a case change, or a directory separator, according to your project's file naming scheme. For example, if our libhello had two modules, hello.core and hello.extra, then their interface units could be named as follows:

hello-core.mxx
hello-extra.mxx

hello_core.mxx
hello_extra.mxx

HelloCore.mxx
HelloExtra.mxx

hello/core.mxx
hello/extra.mxx

core.mxx
extra.mxx

As discussed in the next section, public module names should start with the project name and for such modules it is customary to omit this first component from file names (the last variant in the above example). See also Building Modules for a more detailed discussion of the module name to file name mapping.

3.3 Source Contents

Let's now move inside our source files. All macros defined by a project, such as include guards, version and symbol export macros, etc., must all start with the project name (including the lib prefix for libraries), for example LIBHELLO_VERSION. Similarly, the library's namespace and module names (both public and implementation detail) should all start with the library name but without the lib prefix. For example:

// libhello/hello.mxx

export module hello.core;

namespace hello
{
  ...
}

An executable project may use a namespace (in which case it is natural to call it after the project) and its (private) modules shouldn't be qualified with the project name (in order not to clash with similarly named modules from the corresponding library, if any). A library may also have private modules in which case they shouldn't be qualified either.

Hopefully by now the recommendation for the lib prefix should be easy to understand: oftentimes executables and libraries come in pairs, for example hello and libhello, with the reusable functionality being factored out from the executable into the library. It is natural to want to use the same name stem (hello in our case) for both.

The above naming scheme (with the lib prefix present in some names but not others) is carefully chosen to allow such library/executable pairs to coexist and be used together without too much friction. For example, both the library and executable can have a header called utility.hxx with the executable being able to include both and even get the "merged" functionality without extra effort (since they use the same namespace):

// hello/hello.cxx

#include <hello/utility.hxx>
#include <libhello/utility.hxx>

namespace hello
{
  // Contains names from both utilities.
}

A canonical library project contains two special headers: export.hxx (or export.hpp) that defines the library's symbol exporting macro as well as version.hxx (or version.hpp) that defines the library's version macros (see version Module for details).

3.4 Tests

A project may have unit and/or functional/integration tests. Unit tests exercise each module's (potentially private) functionality in isolation. In contrast, functional/integration tests exercise the project via its public API, just like the real users of the project would.

A source file that implements a module's unit tests should be placed next to that module's files and be called with the module's name plus the .test second-level extension. It is expected to implement an executable (that is, define main()). If a module uses Testscript for unit testing, then the corresponding file should be called with the module's name plus the .test.testscript extension. For example:

libhello/
└── libhello/
    ├── hello.hxx
    ├── hello.cxx
    ├── hello.test.cxx
    └── hello.test.testscript

All source files (that is, headers, modules, etc) with the .test second-level extension are assumed to belong to unit tests and are automatically excluded from the library/executable sources.

A library's functional/integration tests should go into the tests/ subdirectory. Each such test should reside in a separate subdirectory, potentially organized into nested subdirectories (for instance, to correspond to the source directory components). For example, if we were creating an XML parsing and serialization library, then our tests/ could have the following layout:

tests/
├── basics/
│   ├── driver.cxx
│   └── buildfile
├── parser/
│   ├── pull/
│   │   ├── driver.cxx
│   │   └── buildfile
│   └── push/
│       ├── driver.cxx
│       └── buildfile
└── serializer/
    └── ...

In the canonical library project created by bdep-new the tests/ subdirectory is an unnamed subproject (in the build system terms). This allows us to build and run tests against an installed version of the library (see Testing for more information on the contents of this directory).

The build2 CI implementation will automatically perform the installation test if a project contains the tests/ subproject. See bbot Worker Logic for details.

By default executable projects do not have the tests/ subprojects instead placing integration tests next to the source code (the testscript file; see The build2 Testscript Language for details). However, if desired, executable projects can have the tests/ subproject, the same as libraries.

By default projects created by bdep-new include support for functional/integration testing but exclude support for unit testing. These defaults, however, can be overridden with no-tests and unit-tests options, respectively. For example:

$ bdep new -t lib,unit-tests -l c++ libhello

The rationale behind these defaults is that if a functionality can be tested through the public API, then we should generally prefer integration to unit testing. And in simple projects the entire functionality is often exposed through the public API. At the same time, support for unit testing adds extra complexity to the build infrastructure. Note also that it is fairly straightforward to add support for unit testing at a later stage. The relevant build logic is localized in the source subdirectory buildfile so you can simply generate a new project with unit tests enabled and copy over the relevant parts.

3.5 Build Output

There are no bin/ or obj/ subdirectories: build output (object files, libraries, executables, etc) go into a parallel directory structure (in case of an out of source build) or next to the sources (in case of an in source build). See Output Directories and Scopes for details on in and out of source builds.

Projects managed with bdep(1) are always built out-of-source. However, by default, the source directory is configured as forwarded to one of the out-of-source builds. This has two effects: we can run the build system driver b(1) directly in the source directory and certain "interesting" targets (such as executables, documentation, test results, etc) will be automatically backlinked to the source directory (see Configuration for details on forwarded configurations). The following listing illustrates this setup for our hello project (executables are marked with *):

                 hello-gcc/
hello/    ~~>    └── hello/
├── build/    ~~>    ├── build/
└── hello/    ~~>    └── hello/
    ├── hello.cxx        ├── hello.o
    └── hello     -->    └── *hello

The result is an as-if in-source build with all the benefits (such as having both source and relevant output in the same directory) but without any of the drawback (such as the inability to have multiple builds or source directory cluttered with object files).

The often cited motivation for placing executables into bin/ is that in many build systems it is the only way to make things runnable in a reasonably cross-platform manner. The major drawback of this arrangement is the need for unique executable names which is especially constraining when writing tests where it is convenient to call the executable just driver or test.

In build2 there is no such restriction and all executables can run in-place. This is achieved with rpath which is emulated with DLL assemblies on Windows.