[SOLVED] Why is CMAKE_BUILD_TYPE pre-set with single-config generators on Windows?

Issue

I was trying to set the default build type to Release with CMake, using the method described here (straight from the horse's mouth). This method checks if CMAKE_BUILD_TYPE is already set, and sets it to Release only if its value was empty. This works fine on macOS and Linux. However, on Windows, CMAKE_BUILD_TYPE is already set to Debug from the default. Why does this happen and how can I work around it, so that I can reliably set the default build type to Release?

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To try this out yourself, create a trivial CMake file:

# CMakeLists.txt

project(Foo)
message(STATUS "Build type is '${CMAKE_BUILD_TYPE}'")

Then use with with a single-configuration generator, such as cmake . -G"NMake Makefiles" or cmake . -GNinja. In both cases, on Windows I see: Build type is 'Debug'. On other platforms I see Build type is ''.

Solution

CMAKE_BUILD_TYPE is getting set on the project line. Before that it is empty. Thus, checking whether CMAKE_BUILD_TYPE is empty and setting a default value can be done before this line.

Here is a discussion about this: https://gitlab.kitware.com/cmake/cmake/-/issues/19401

Answered By - Szabolcs