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

[Stephen Kelly]

Stephen Kelly wrote:

> I'll try to get a reviewable and first-feature-complete infrastructure
> branch together soon.

I've pushed a first iteration of the target_compiler_features branch to my 
clone.

For the language specification, I added a prefix to each feature. This 
matches the feature tests of clang for standard features, and it is 
extensible to extensions with gnuxx_typeof, msvc_sealed etc.

 http://clang.llvm.org/docs/LanguageExtensions.html

So far, it only supports the REQUIRED signature of the command. 

It will need to get a <PUBLIC|PRIVATE> specifier, so that the 
INTERFACE_COMPILER_FEATURES property can be populated in PUBLIC mode. It 
also needs to learn to process generator expressions and how to consume that 
property.

Optional features with defines are not yet implemented. I was considering 
renaming target_compiler_features to target_required_features and not using 
the same command for optional features. Instead I think it might be a good 
idea to have a separate command for optional features. Something like:

 write_compiler_feature_file(
   FILENAME "${CMAKE_CURRENT_BINARY_DIR}/grantlee_compiler_features.h"
   FEATURES cxx_static_assert gnuxx_typeof cxx_variadic_templates
 )
 install(FILES "${CMAKE_CURRENT_BINARY_DIR}/grantlee_compiler_features.h"
   DESTINATION include
 )

which writes the file with appropriate content. I don't see any need to use 
preprocessor macros for that instead of a file.

I also prototyped a simple way to test clang features as reportedly 
supported by clang itself. This won't get us all of the way there with 
clang, as some features (such as the gnu typeof extension) do not have 
support via __has_extension.

Additionally, I changed my mind on the issue of whether to bump the dialect 
to c++11 if the compiler supports the requested feature in c++98 mode. For 
example, cxx_variadic_templates would be in CMAKE_CXX98_COMPILER_FEATURES 
for GCC and clang, because both support variadic templates even when using 
the -std=c++98 dialect. This will still warn, but that's ok, I think.

Note that adding -std=c++11 is almost, but not quite, entirely source 
compatible.

For example, this will work with c++98, but errors with c++11:

 #define add_prefix(suf) "pre_"suf
 add_prefix("_end");

The feature can be extended for the C standard (89, 99, 11) features too as 
well as extensions.

Any comments so far?

Thanks,

Steve.