Re: [boost] Boost CMake support - Request for Comment

Subject: Re: [boost] Boost CMake support - Request for Comment
From: Roger Leigh (rleigh_at_[hidden])
Date: 2018-10-10 08:44:58

On 09/10/2018 23:42, Edward Diener via Boost wrote:
> On 10/9/2018 7:09 PM, mike via Boost wrote:

>> As Richard pointed out, such features exist in cmake too and have been in
>> common use for years. E.g. It has a built-in mechanism that lets you
>> directly query for most of the post c++03 compiler features:
>> (https://cmake.org/cmake/help/v3.13/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html) 
>>
>> and as a more general feature, you can always check if a particular 
>> source
>> file using some feature or type compiles or not:
>> (https://cmake.org/cmake/help/v3.13/module/CheckCXXSourceCompiles.html)
>>
>> Could you maybe point me to some examples where this feature is used 
>> so one
>> could estimate how difficult a conversion is?
> 
> The feature is essentially programmed by config and predef to provide 
> build-time testing of compiler support and testing of such things like 
> the OS versions and compiler versions etc. See 
> https://www.boost.org/doc/libs/1_68_0/libs/config/doc/html/boost_config/build_config.html 
> for the explanation in config and 
> https://www.boost.org/doc/libs/1_68_0/doc/html/predef/check_utilities.html 
> for checking predef values at build time. The basics of the feature are 
> offered by Boost Build as explained at 
> https://boostorg.github.io/build/manual/develop/index.html#bbv2.reference.rules 
> in reference to the check-target-builds rule. Plenty of libraries use 
> the feature during testing and I imagine that some of them may use the 
> feature during building. Asking individual libraries to convert their 
> tests or build to use some CMake equivalent, for every possible setting 
> in config and predef which they use, is not realistic. I understand that 
> CMake may have an equivalent technique, but who is going to program that 
> technique to offer all the equivalents which config and predef currently 
> offer ?

This is all supported by CMake for years as mike said, and it's utterly 
trivial to write such feature tests.  If you would like some examples, 
here's some I wrote for Xerces-C++:

https://github.com/apache/xerces-c/tree/trunk/cmake

For example, here's a simple test for const [it's a port of Autoconf 
logic] with CheckCXXSourceCompiles:
   https://github.com/apache/xerces-c/blob/trunk/cmake/XercesConst.cmake
this could be adapted for any feature you care to check, from constexpr 
to user-defined literals, or any other feature which can be checked by 
running the compiler.

This is a set of checks for integer type sizes: 
https://github.com/apache/xerces-c/blob/trunk/cmake/XercesIntTypes.cmake

This is a more complex set of checks for SSE intrinsics: 
https://github.com/apache/xerces-c/blob/trunk/cmake/XercesSSE2.cmake

Here's some checks to verify Boost is working properly:
https://gitlab.com/codelibre/ome-common-cpp/blob/master/cmake/BoostChecks.cmake

Testing of linker version scripts: 
https://gitlab.com/libtiff/libtiff/blob/master/CMakeLists.txt#L198

Checking include presence: 
https://gitlab.com/libtiff/libtiff/blob/master/CMakeLists.txt#L212

Checking compiler keyword presence and fallbacks: 
https://gitlab.com/libtiff/libtiff/blob/master/CMakeLists.txt#L226

Checking structure presence: 
https://gitlab.com/libtiff/libtiff/blob/master/CMakeLists.txt#L226

Checking library symbol exports: 
https://gitlab.com/libtiff/libtiff/blob/master/CMakeLists.txt#L226

I don't think any of this is difficult.  If anything, you can likely 
reuse the existing source code used by the Boost.Build checks, maybe 
even sourced directly without any changes--just wrap it with 
check_cxx_source_compiles or other feature test macros.

Regards,
Roger