https://stackoverflow.com/questions/24460486/cmake-build-type-is-not-being-used-in-cmakelists-txt

There are two types of generators: single-configurations and multi-configurations.

1. Single configurations

Make-like generators: Unix Makefiles, NMake Makefiles, MinGW Makefiles, ...

You set the configuration type in the generate step:

cmake -H. -B_builds/Debug -DCMAKE_BUILD_TYPE=Debug "-GUnix Makefiles"

In this case, the build step is always Debug:

> cmake --build _builds/Debug
/usr/bin/c++ -g ...
> cmake --build _builds/Debug --config Debug # `--config` ignored
/usr/bin/c++ -g ...
> cmake --build _builds/Debug --config Release # yep, ignored
/usr/bin/c++ -g ...

2. Multi-configuration

IDE generators: Visual Studio, Xcode

CMAKE_BUILD_TYPE on generate step is ignored,

CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_BUILD_TYPE

both:

> cmake -H. -B_builds -DCMAKE_BUILD_TYPE=Debug "-GVisual Studio 12 2013 Win64"

and

> cmake -H. -B_builds -DCMAKE_BUILD_TYPE=Release "-GVisual Studio 12 2013 Win64"

will have the same effect:

This is because all the configurations is internal (i.e., _builds/msvc-opaque/Release and _builds/msvc-opaque/Debug or something, doesn't matter).

You can use --config options to switch:

> cmake --build _builds --config Release
cl /O2 ...
> cmake --build _builds --config Debug
cl /Od ...

3. Control

Yes, you can. Just define CMAKE_CONFIGURATION_TYPES:

# Somewhere in CMakeLists.txt
message("Generated with config types: ${CMAKE_CONFIGURATION_TYPES}")

Default output:

-- Detecting CXX compiler ABI info - done
Generated with config types: Debug;Release;MinSizeRel;RelWithDebInfo
-- Configuring done

Rewrite it:

> cmake -H. -B_builds -DCMAKE_CONFIGURATION_TYPES="Debug;Release" "-GVisual Studio 12 2013 Win64"
-- Detecting CXX compiler ABI info - done
Generated with config types: Debug;Release
-- Configuring done

You can even define your own configuration type:

> cmake -H. -B_builds -DCMAKE_CONFIGURATION_TYPES="Debug;MyRelease" -DCMAKE_CXX_FLAGS_MYRELEASE="/My-Rel-flag" -DCMAKE_EXE_LINKER_FLAGS_MYRELEASE="/My-Linker-flags" "-GVisual Studio 12 2013 Win64"

And build:

    cmake --build _builds --config MyRelease

4. Messy (?)

Not at all if you know the trick