why does cmake also adds compile options for imported packages?

Question

I have a project where I need some dependencies and and I use vcpkg to manage them. Now I want to set compile options with add_compile_options( /W4 /WX /std:c++17 ) for my executable and subdirectories but I don't want them to be applied on packages which were loaded with find_package .

This is the code inside my CMakeLists.txt from my test repository:

cmake_minimum_required(VERSION 3.15)
project(test)

set(VCPKG_DEPENDENCIES glm)

# set the build triplet for windows because default is x86-windows
if(WIN32)
    set(VCPKG_TARGET_TRIPLET "x64-windows")
    add_compile_options( /W4 /WX /std:c++17 )
endif()

get_filename_component(ABS_PATH_VCPKG "./vcpkg" REALPATH)
set(VCPKG_ROOT ${ABS_PATH_VCPKG})
set(CMAKE_TOOLCHAIN_FILE "${ABS_PATH_VCPKG}/scripts/buildsystems/vcpkg.cmake")
# additional folder where find_XXXX functions are searching for packages
set(CMAKE_PREFIX_PATH "${VCPKG_ROOT}/installed/${VCPKG_TARGET_TRIPLET}/share")

foreach(DEPENDENCY ${VCPKG_DEPENDENCIES})
    message(STATUS "installing vcpkg dependency: <${DEPENDENCY}>")
    execute_process(COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/vcpkg/vcpkg.exe install ${DEPENDENCY}:${VCPKG_TARGET_TRIPLET} OUTPUT_FILE ${CMAKE_CURRENT_SOURCE_DIR}/vcpkg_install_log.txt)
endforeach()

add_executable(test main.cpp)

find_package(glm CONFIG REQUIRED)
target_link_libraries(test PRIVATE glm)

Ans this is my main:

#include <iostream>
#include <glm/glm.hpp>
#include <glm/gtc/type_ptr.hpp>

int main()
{
    glm::vec3 vec(1.0);
    std::cout << ("Hello World!");
    return 0;
}

The glm package however is calling add_library with the IMPORTED key which should inform about that this package is a system package and should not get the options applied to.

This is the error code:

[build] Starting build
[proc] Executing command: "C:\Program Files\CMake\bin\cmake.EXE" --build <path>/cpp_test_Isystem/build --config Debug --target all -- -j 18
[build] [1/2  50% :: 0.752] Building CXX object CMakeFiles\test.dir\main.cpp.obj
[build] FAILED: CMakeFiles/test.dir/main.cpp.obj 
[build] <path>\VC\Tools\MSVC\1423~1.281\bin\Hostx64\x64\cl.exe  /nologo /TP  -I..\vcpkg\installed\x64-windows\include /DWIN32 /D_WINDOWS /GR /EHsc /Zi /Ob0 /Od /RTC1 -MDd   /W4 /WX /std:c++17 /showIncludes /FoCMakeFiles\test.dir\main.cpp.obj /FdCMakeFiles\test.dir\ /FS -c ..\main.cpp
[build] <path>\cpp_test_Isystem\vcpkg\installed\x64-windows\include\glm\ext\../detail/type_quat.hpp(58): error C2220: the following warning is treated as an error
[build] <path>\cpp_test_Isystem\vcpkg\installed\x64-windows\include\glm\ext\../detail/type_quat.hpp(137): note: see reference to class template instantiation 'glm::qua<T,Q>' being compiled
[build] <path>\cpp_test_Isystem\vcpkg\installed\x64-windows\include\glm\ext\../detail/type_quat.hpp(58): warning C4201: nonstandard extension used: nameless struct/union
[build] ninja: build stopped: subcommand failed.
[build] Build finished with exit code 1

Answer 1

Your main.cpp file includes this header, so any options that apply to it also apply to the header.

Your options:

• Disable the C4201 warning with a #pragma around the #include of GLM headers:

#pragma warning( push )
#pragma warning( disable : 4201 )
#include <glm/glm.hpp>
#include <glm/gtc/type_ptr.hpp>
#pragma warning( pop )

• Disable the warning from CMake:

target_add_compile_options(test /wd4201)

• EDIT: Use a GLM-specific define to silence these warnings ( source ):

#define GLM_FORCE_SILENT_WARNINGS

(or equivalent with target_add_definitions )