Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake

Question

I would like to change the default values for CMAKE_CXX_FLAGS_RELEASE or CMAKE_CXX_FLAGS_DEBUG in CMake. Basically, I have some project defaults th

Answer 1

I just wanted to add the four possibilities I see:

1. Having your own toolchain files containing the presets for each compiler you support like:

   GNUToolchain.cmake

   set(CMAKE_CXX_FLAGS_DEBUG "-ggdb3 -O0" CACHE STRING "")
   

   And then use it with

   cmake -DCMAKE_TOOLCHAIN_FILE:string=GNUToolchain.cmake ...
   

2. You can try to determine the compiler by checking CMAKE_GENERATOR (which is valid before the project() command):

   CMakeLists.txt

   if("${CMAKE_GENERATOR}" MATCHES "Makefiles" OR 
      ("${CMAKE_GENERATOR}" MATCHES "Ninja" AND NOT WIN32))
       set(CMAKE_CXX_FLAGS_DEBUG "-ggdb3 -O0" CACHE STRING "")
   endif()
   
   project(your_project C CXX)
   

3. You can use CMAKE_USER_MAKE_RULES_OVERRIDE to give a script with your own ..._INIT values:

   It is loaded after CMake’s builtin compiler and platform information modules have been loaded but before the information is used. The file may set platform information variables to override CMake’s defaults.

   MyInitFlags.cmake

   # Overwrite the init values choosen by CMake
   if (CMAKE_COMPILER_IS_GNUCXX)
       set(CMAKE_CXX_FLAGS_DEBUG_INIT "-ggdb3 -O0")
   endif()
   

   CMakeLists.txt

   set(CMAKE_USER_MAKE_RULES_OVERRIDE "MyInitFlags.cmake")
   
   project(your_project C CXX)
   

4. You can simplify your solution from March 1st by checking against the ..._INIT variants of the compiler flag variables:

   CMakeLists.txt

   project(your_project C CXX)
   
   if (DEFINED CMAKE_CXX_FLAGS_DEBUG_INIT AND  
       "${CMAKE_CXX_FLAGS_DEBUG_INIT}" STREQUAL "${CMAKE_CXX_FLAGS_DEBUG}")
       # Overwrite the init values choosen by CMake
       if (CMAKE_COMPILER_IS_GNUCXX)
           set(CMAKE_CXX_FLAGS_DEBUG "-ggdb3 -O0" CACHE STRING "" FORCE)
       endif()
   endif()
   

Comments:

I prefer and use the toolchain variant. But I admit it has the disadvantage of having to give the toolchain file manually (if you are not calling cmake via a script/batch file).

References:

• CMake: In which Order are Files parsed (Cache, Toolchain, …)?
• cmake - Global linker flag setting (for all targets in directory)
• Switching between GCC and Clang/LLVM using CMake