How do I make conditional cxxflags feature in my boost jamfile?

Question

I am using Boost.Build for a small project and want to enable C++17 support. According to this answer, I can pass the compiler flag -std=c++17 in the following way and the flag is propagated to all executables and libraries:

import feature ;
feature.feature cpp17 : on : composite optional propagated ;
feature.compose <cpp17>on : <cxxflags>"-std=c++17" ;

project myproject : requirements <cpp17>on ;

This works fine when I use toolset=gcc with gcc7. However, on other compilers the C++17 mode is enabled in another way. Clang 3.9 for example expects -std=c++1z instead of -std=c++17. In MSVC, the C++ version depends on the MSVC version.

Is there a way to set the C++ version (C++11 / C++14 / C++17) in the jamfile in a (more or less) toolset independent way?

Answer 1

You can set properties to apply in specific circumstances (conditional requirements in b2 parlance) such that they only get added when those requirements are met. One of the possible requirements you can use is the toolset specification (all the way down to the version if needed). In your case you probably want something like this:

import feature ;
feature.feature cpp17 : on : composite optional propagated ;
feature.compose <cpp17>on :
  <toolset>gcc:<cxxflags>"-std=c++17"
  <toolset>clang:<cxxflags>"-std=c++1z" ;

project myproject : requirements <cpp17>on ;

As for a "toolset independent" way of setting the C++ Standard version.. We are working on adding that. Expect that some time this Fall (2017).