Notes on Visual Studio 15

The following notes are based on our experience adding support for the Visual Studio 15 C/C++ compiler in build2.

Normally, when it comes to Visual Studio (referred to as VC from now on), there are three numbers that can be used to identify it: the release year, the VC version, and the cl.exe compiler version. For example, here is the mapping table we keep as a comment in the build2 source code:

// year   ver cl.exe  crt/dll
// 2015   14  19.00  14.0/140
// 2013   12  18.00  12.0/120
// 2012   11  17.00  11.0/110
// 2010   10  16.00  10.0/100
// 2008    9  15.00   9.0/90
// 2005    8  14.00   8.0/80
// 2003  7.1  13.10   7.1/71

The last column lists the runtime version and its representation in the DLL name (e.g., msvcp140.dll). Notice that the runtime versions match the VC versions (though 13 was conspicuously skipped). The year is just a marketing gimmick so we ignore in preference to the version, for example VC14. The compiler version is available as the _MSC_VER preprocessor macro and you sometimes need to map between the two.

While confusing initially (why three versions?), everyone got used to it. With VC15, however, Microsoft decided to shake things up a bit. Take a look at this path from the RC1 installation, it sets up the expectations nicely:

...\Microsoft Visual Studio\2017\Professional\VC\Redist\MSVC\14.10.24629\x64\Microsoft.VC150.CRT\msvcp140.dll

The year (2017) and the VC version (15) are as expected. But look at the runtime – it's still 14.0! Or is it? What's that 14.10 earlier in the path? Checking the msvcp140.dll's embedded manifest confirms this – it is 14.10. According to Microsoft it is backwards-compatible with 14.0 and, presumably, they had to still call the DLLs *140.dll to allow upgrading existing programs without a re-link.

Ok, what about the compiler version?

> cl.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.10.24629 for x64

Another deviation from the established scheme: it's not 20, it's 19.10. Fun fact: up to Preview 4 the compiler version was 19.00 which means it was indistinguishable from VC14 via _MSC_VER. Thankfully, that got fixed.

Here is the new line for VC15 in our mapping table (VC14 is shown for comparison):

// year   ver cl.exe  crt/dll
// 2017   15  19.10  14.1/140
// 2015   14  19.00  14.0/140

BTW, if you ever need to determine the runtime version at compile time, there is the crtversion.h header. Here are its contents (again from VC15 RC1):

#define _VC_CRT_BUILD_VERSION 24629

Another notable change is the addition of the /std: compiler option. Up until now (or, more precisely, up until VC14 Update 3 where this option was added), C++ standard-wise, you got what you got. Now the default is c++14 and to enable C++17 (and beyond) one has to pass the /std:c++latest. There is also the _MSVC_LANG macro that one can use to determine the enabled standard.

The RC1 and RC2 installers only create one Visual Studio Command Prompt shortcut in the Start menu. While one can appreciate the simplification, unfortunately, on 64-bit Windows the shortcut is for the 32-bit environment.

We've reported this and hopefully it will be fixed before the final release. In the meantime (or if this isn't fixed), the good news is that the batch files are still there and we can create the necessary shortcuts ourselves. The batch files are located in this directory:

...\Microsoft Visual Studio\2017\Professional\VC\Auxiliary\Build\

For example, to create a shortcut for the native 64-bit environment, right-click on vcvars64.bat and select "Create shortcut". Then edit the shortcut's "Target" field and add the %comspec% /k prefix.

Also see the discussion on r/cpp.