CMake CMAKE_CXX_FLAGS enabled optimization unexpectly

Question

In one of my CMakeLists.txt, I have the follow instructions:

IF ( MSVC )
    SET ( CMAKE_CXX_FLAGS_DEBUG "/MDd" )
)

After the MSVC 10.0 solution is created, the optimization (/O2) is unexpectedly enabled by the code above. I'm sure I didn't enable it somewhere else.

Why is that?

Answer 1

With the code in your question you are hiding the default parameters - including the optimization level - that CMake does apply.

Please try appending your options with

SET ( CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /MDd" )

or with the use of generator expressions and add_compile_options():

add_compile_options("$<$<CONFIG:Debug>:/MDd>")

But you may not need this particular option, because /MDd is already part of CMake's default MSVC debug flag's setting.

Background

If you look at CMake's Windows-MSVC.cmake you'll see the following initialization settings:

set(CMAKE_${lang}_FLAGS_DEBUG_INIT "/D_DEBUG /MDd /Zi /Ob0 /Od ${_RTC1}")

Without changing any flags in your CMakeLists.txt you will see in your CMakeCache.txt:

//Flags used by the compiler during debug builds.
CMAKE_CXX_FLAGS_DEBUG:STRING=/D_DEBUG /MDd /Zi /Ob0 /Od /RTC1

With your code you are hiding this cached variable and you will end up with just /MDd.

References

• What's the CMake syntax to set and use variables?
• cmake - Global linker flag setting (for all targets in directory)
• CMAKE - setting compile flags for libraries
• Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake