[cmake-developers] C++11 and target_compiler_feature proposal

[Stephen Kelly]

Brad King wrote:

> Steve, please explain your proposal in more detail.  How does the list of
> requested features get mapped to the proper -std=cxx11 or equivalent flag?
> 

In my branch that is determined by which list the feature appears in. Eg, 
from GNU-CXX.cmake:

 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
   list(APPEND CMAKE_CXX11_COMPILER_FEATURES
     final
     override
   )
 endif()
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9)
   list(APPEND CMAKE_CXX14_COMPILER_FEATURES
     generalized_lambda_capture
     return_type_deduction
   )
 endif()

Then, in the implementation of target_compiler_feature, 

 list(FIND CMAKE_CXX11_COMPILER_FEATURES ${FEATURE_NAME} needs11)
 list(FIND CMAKE_CXX14_COMPILER_FEATURES ${FEATURE_NAME} needs14)
 list(FIND CMAKE_CXX17_COMPILER_FEATURES ${FEATURE_NAME} needs17)

 if(NOT needs17 EQUAL -1)
   set(standard 17)
 elseif(NOT needs14 EQUAL -1)
   set(standard 14)
 elseif(NOT needs11 EQUAL -1)
   set(standard 11)
 endif()

 # ...
 set_property(TARGET ${TARGET_NAME} PROPERTY CXX_STANDARD ${standard})

Then, in cmLocalGenerator:

 const char *standard = target->GetProperty("CXX_STANDARD");
 std::string compile_option =
           "CMAKE_CXX" + std::string(standard) + "_STANDARD_COMPILE_OPTION";
 if (const char *opt =
               target->GetMakefile()->GetDefinition(compile_option.c_str()))
   {
   this->AppendFlags(flags, opt);
   }


This is where I have an open question in the branch:

 
 # TODO: Gnu extensions supported by -std=gnu++98 ?
 # And others. http://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/C-Dialect-Options.html#C-Dialect-Options
 # TODO: Maybe instead we should define like this:
 #
 #   set(CMAKE_C_COMPILE_OPTIONS_STD "-std=")
 #   set(CMAKE_CXX_COMPILE_OPTIONS_STD "-std=")
 #
 # so that the CXX_STANDARD target property can contain the argument string 
 # (including possibly extensions).
 # That might call for some kind of mapping though because XL uses different 
 # values (eg extended0x)

 if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
   set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++11")
 endif()

 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9
 #       AND VERSION_LESS 4.11 # Speculative
     )
   set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
 endif()

 # if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.11) # Speculative
 #   set(CMAKE_CXX14_STANDARD_COMPILE_OPTION -std=c++14)
 # endif()