Re: Adding CMakeLists.txt files

From: Edward Diener (eldiener_at_[hidden])
Date: 2021-05-31 12:13:13

On 5/31/2021 3:24 AM, Alexander Grund via Boost wrote:
> 
>> OK, I will bite <g>.
>>
>> // Beginning of rant
>>
>> The difficulty of specifying a compiler to use with CMake, compared 
>> with the elegance of b2's toolset definitions, makes me really wonder 
>> why the majority of the C++ world considers CMake anything but 
>> software gone terribly wrong. I know b2 has some abstruse weaknesses, 
>> but specifying a toolset/definition is not one of them. Maybe I have 
>> missed something but I certainly do not see how some CMake generator 
>> equates to a particular compiler other than the Visual C++ generators, 
>> where each generator name equates to a particular version of VC++. 
>> There must be more I am missing but there is not anything in the CMake 
>> documentation I can find which explains to me how I can specify, for 
>> instance, a particular version of mingw-w64 gcc or clang on Windows 
>> for CMake to use. The explanation for the CMake generators at 
>> https://cmake.org/cmake/help/v3.20/manual/cmake-generators.7.html#manual:cmake-generators(7) 
>> is so pathetic that if I did not know the majority of C++ programmers 
>> swear by CMake I would really have thought that such software 
>> documentation would have doomed such software for popular use eons ago.
>>
>> // End of rant
> 
> I'll bite back ;-)
> 
> CMake has 2 levels of things you specify because CMake is not a build 
> system (as B2, make, ninja, ...) but a buildsystem generator.
> In short:
> 
> - With "-G" you choose the build system to generate files for, e.g. VS 
> Solutions, Makefiles, Ninja configs, ...
> - With CMAKE_<lang>_COMPILER you choose the compiler to use, e.g. `cmake 
> -DCMAKE_CXX_COMPILER=g++-10 -DCMAKE_C_COMPILER=gcc-10 ...`
> 
> The defaults if unset are chosen reasonably. E.g. for make/ninja (make 
> is the default) it chooses $CC/$CXX (and maybe some fallbacks such es 
> `cc`) and for the VS generators it chooses the VS compiler for that 
> version.
> 
> What do you gain from this? By switching the generator (and nothing 
> else) you can develop your project in whatever you want. E.g. VS, 
> VScode, vim (with a language server), ...
> I find that quite powerful, especially if you need to develop on 
> Windows. So for that CMake is better than B2.
> 
> However I agree that for building/testing alone B2 is clearly superior.
> 
> So yeah, CMake is a tool, it has strengths and weaknesses and has some 
> historic backage due to the need to be backwards compatible (just as C++ 
> has...)

My beef is that the CMake documentation does not tell you what compilers 
a particular generator supports, except for the Visual Studio 
generators. Period. It is not the process but the lack of documentation. 
The lack of documentation is noticeable because I think the average 
programmer will first ask: What compiler can I use to do X, Y, and Z, 
and how do I specify commands to that compiler. That seems perfectly 
natural to me and the fact that one uses -DCMAKE_CXX_COMPILER with the 
path to the compiler and options separated by semicolons, is lost amidst 
the welter of basic explanations about generators.