How to set compiler options with CMake in Visual Studio 2017

Question

Visual Studio 2017 comes with full CMake integration. To learn about this combination, I was starting with this basic sample:

# CMakeLists.txt
cmake_minimum_         


        

Answer 1

The default settings for the compiler are picked up from standard module files located in the Modules directory of the CMake installation. The actual module file used depends on both the platform and the compiler. E.g., for Visual Studio 2017, CMake will load the default settings from the file Windows-MSVC.cmake and language specific settings from Windows-MSVC-C.cmake or Windows-MSVC-CXX.cmake.

To inspect the default settings, create a file CompilerOptions.cmake in the project directory with the following contents:

# log all *_INIT variables
get_cmake_property(_varNames VARIABLES)
list (REMOVE_DUPLICATES _varNames)
list (SORT _varNames)
foreach (_varName ${_varNames})
    if (_varName MATCHES "_INIT$")
        message(STATUS "${_varName}=${${_varName}}")
    endif()
endforeach()

Then initialize the CMAKE_USER_MAKE_RULES_OVERRIDE variable in your CMakeLists.txt:

# CMakeLists.txt
cmake_minimum_required(VERSION 3.8)
set (CMAKE_USER_MAKE_RULES_OVERRIDE "${CMAKE_CURRENT_LIST_DIR}/CompilerOptions.cmake")
project(foo)
add_executable(foo foo.cpp)

When the project is configured upon opening the directory in Visual Studio 2017, the following information will be show in the IDE's output window:

 ...
 -- CMAKE_CXX_FLAGS_DEBUG_INIT= /MDd /Zi /Ob0 /Od /RTC1
 -- CMAKE_CXX_FLAGS_INIT= /DWIN32 /D_WINDOWS /W3 /GR /EHsc
 -- CMAKE_CXX_FLAGS_MINSIZEREL_INIT= /MD /O1 /Ob1 /DNDEBUG
 -- CMAKE_CXX_FLAGS_RELEASE_INIT= /MD /O2 /Ob2 /DNDEBUG
 -- CMAKE_CXX_FLAGS_RELWITHDEBINFO_INIT= /MD /Zi /O2 /Ob1 /DNDEBUG
 ...

So the warning setting /W3 is picked up from the CMake variable CMAKE_CXX_FLAGS_INIT which then applies to all CMake targets generated in the project.

To control the warning level on the CMake project or target level, one can alter the CMAKE_CXX_FLAGS_INIT variable in the CompilerOptions.cmake by adding the following lines to the file:

if (MSVC)
    # remove default warning level from CMAKE_CXX_FLAGS_INIT
    string (REGEX REPLACE "/W[0-4]" "" CMAKE_CXX_FLAGS_INIT "${CMAKE_CXX_FLAGS_INIT}")
endif()

The warning flags can then be controlled by setting the target compile options in CMakeLists.txt:

...
add_executable(foo foo.cpp)
target_compile_options(foo PRIVATE "/W4")

For most CMake projects it makes sense to control the default compiler options in an override file instead of manually tweaking variables like CMAKE_CXX_FLAGS.

When making changes to the CompilerOptions.cmake file, it is necessary to recreate the build folder. When using Visual Studio 2017 in Open Folder mode, choose the command Cache ... -> Delete Cache Folders from the CMake menu and then Cache ... -> Generate from the CMake menu to recreate the build folder.