How to set a CMake flag cache variable in presets without overwriting it?

Question

Trying to follow best practice as suggested in this post, I'd like to set optional flags in the preset file and required flags in the toolchain/CMakelist. To be specific CMAKE_x_FLAGS, but I think this question applies to any other cache variable.

Toolchain file

set(CMAKE_CXX_FLAGS_INIT "-mthumb -mcpu=cortex-m4")

CMakeLists.txt

set(CMAKE_CXX_FLAGS  "${CMAKE_CXX_FLAGS} -fno-exceptions")

CMakePresets.json

{
    "version": 3,
    "configurePresets": [
        {
            "name": "debug",
            "generator": "Ninja",
            "binaryDir": "${sourceDir}/build/${presetName}",
            "toolchainFile": "${sourceDir}/cmake/toolchain.cmake",
            "cacheVariables": {
                "CMAKE_EXPORT_COMPILE_COMMANDS": "ON",
                "CMAKE_BUILD_TYPE": "Debug",
                "CMAKE_CXX_FLAGS": "-Wall -Wextra"
            }
        }
    ]
}

My expectation, as described in the linked post, was that the preset variable is appended to the already specified variables. However, the flags specified in the Toolchain file are overwritten.
-Wall -Wextra -fno-exceptions

This can be prevented by the following line in the CMakeLists.txt
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS_INIT} ${CMAKE_CXX_FLAGS} -fno-exceptions")

Seems to me this is not the way it should work? So my question is, how should I manage CMAKE_CXX_FLAGS(_INIT) from the various files with the following idea?

• Toolchain specific flags in the toolchain file.
• Project specific flags in the CMakeLists.txt.
• Optional, warning flags in the CMakePresets.json file.

Answer 1

The better practice is using add_compile_options() in CMakeLists.txt.

CMAKE_LANG_FLAGS:

variables will be passed before flags added by commands such as add_compile_options() and target_compile_options().