[SOLVED] CMake: output the result of a generator expression that cannot be used with add_custom_target or file()?

Issue

I found both this question and href="https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html#debugging" rel="nofollow noreferrer">this documentation entry advising users to "debug" generator expressions by either using the add_custom_target() command or the file(GENERATE) command.

However, say you have a CMakeLists.txt like this:

cmake_minimum_required(VERSION 3.15)

project(Toy VERSION 1.0)

set(gcc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>")
set(msvc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,MSVC>")

and you now want to check for the value of gcc_like_cxx. Well, according to the documentation, something like

file(GENERATE OUTPUT hahaha CONTENT ${gcc_like_cxx})

or

add_custom_target(genexdebug COMMAND ${CMAKE_COMMAND} -E echo "${gcc_like_cxx}")

could be done, but either of the two produces the same error:

  $<COMPILE_LANG_AND_ID:lang,id> may only be used with binary targets to
  specify include directories, compile definitions, and compile options.  It
  may not be used with the add_custom_command, add_custom_target, or
  file(GENERATE) commands.

Is there a way to get the value of the generator expression if the expression may not be used with the "standard" debugging options?

Solution

1. You need to use the target argument in file(GENERATE) in CMake version 3.19.
2. You need to add variable prefix to the output file or there will be following error:

CMake Error in CMakeLists.txt:
  Evaluation file to be written multiple times with different content.  This
  is generally caused by the content evaluating the configuration type,
  language, or location of object files:
    
    /home/user/build/hahaha

I run the following CMake file on my server and it works:

cmake_minimum_required(VERSION 3.19)

project(Toy VERSION 1.0)

set(gcc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>")
set(msvc_like_cxx "$<COMPILE_LANG_AND_ID:CXX,MSVC>")

file(GENERATE OUTPUT "${gcc_like_cxx}/log.txt" CONTENT "${gcc_like_cxx}\n" TARGET ${PROJECT_NAME})

add_executable(Toy toy.c)

CMake Gitlab -- file(GENERATE): Add TARGET argument

GitHub issue -- As a result, I added the TARGET argument to file(GENERATE) in CMake 3.19

Answered By - J. Doe

Answer Checked By - Katrina (WPSolving Volunteer)