[CMake] Using generator expression with add_library

[Petr Kmoch]

Hi.

The argument STATIC or SHARED is processed at CMake configure time (that
is, when CMake is executing the add_library() command). However, generator
expressions are only evaluated at generate time, which comes only after all
CMake code is processed.

Fortunately for you, compiler ID is something that is already known at
configure time, meaning you don't need to use a genex to read it. You can
just do this:

if(MSVC)
  set(LibType STATIC)
else()
  set(LibType SHARED)
endif()
add_library(
  ${_star_lib_name}
  ${LibType}
  ...
)

(Feel free to modify the if(), use CMAKE_<LANG>_COMPILER_ID (
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html)
etc. as necessary).

Petr

On Thu, 7 Nov 2019 at 12:28, David Aldrich <david.aldrich.ntml at gmail.com>
wrote:

> I want to build a shared library for Linux and a static library for
> Windows. So I have tried:
>
>  set (_star_lib_name "StdStars")
>
>  add_library(${_star_lib_name}
>              $<$<CXX_COMPILER_ID:GNU>:SHARED>
>              $<$<CXX_COMPILER_ID:MSVC>:STATIC>
>              ""
>  )
>
> but that gives me error:
>
> CMake Error at C:/SVNProj/zodiac/branches/TRY_TML_CMake_3Oct2019/StarLibs/StdStars/CMakeLists.txt:7 (add_library):
> Cannot find source file:
>
>     STATIC
>
>   Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
>   .hpp .hxx .in .txx
>
> What is the correct way of doing this please?
> --
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