[cmake-developers] Please review CXXFeatures.cmake

[Stephen Kelly]

Stephen Kelly wrote:

> Stephen Kelly wrote:
>> I still think you should revisit my review mail on point 2 too.
> 
> Something that becomes possible when thinking about the above and target
> properties is interface requirements.

I've implemented a prototype which has better API than your module. 

In my concept:

* Applying the -std=c++11 flag is the responsibility of CMake, not the user. 

* CMake determines the -std= flag to use (GCC 4.9 has -std=c++1y by now). It 
won't add '-std=c++11 -std=c++14', but just '-std=c++14'

* Required compiler features are scoped to targets.

* Target properties are set, exported and transitively evaluated track the 
compiler features required.

* Compiler requirements can be optional, in which case -D defines are 
generated in the (PUBLIC) COMPILE_DEFINITIONS

* The -D defines are for scoping or replacement, depending on what the 
feature calls for.

* Required compiler features are listed in the INTERFACE of exported 
targets. CMake evaluates whether the compiler is capable of using the 
IMPORTED targets it is supposed to use transitively, in a similar way to the 
POSITION_INDEPENDENT_CODE feature.

* A new command target_compiler_feature is introduced with two signatures:

 target_compiler_feature(<target> <PUBLIC|PRIVATE>
   REQUIRED <feature1> [<feature2> ...])
 target_compiler_feature(<target> <PUBLIC|PRIVATE>
   OPTIONAL <feature> DEFINE <define_name>)

* Another non-target-scoped command could be introduced, similar to 
target_compile_options -> add_compile_options.


This is better in many noteworthy ways to the cxx11 topic in the stage.

Brad, do you have any response to any part of anything I wrote about that 
topic?

Thanks,

Steve.
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