How to do it

In this recipe, we will show how the build type can be set for an example project:

1. We start out by defining the minimum CMake version, project name, and supported languages:

cmake_minimum_required(VERSION 3.5 FATAL_ERROR)

project(recipe-07 LANGUAGES C CXX)

2. Then, we set a default build type (in this case, Release) and print it in a message for the user. Note that the variable is set as a CACHE variable, so that it can be subsequently edited through the cache:

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release CACHE STRING "Build type" FORCE)
endif()

message(STATUS "Build type: ${CMAKE_BUILD_TYPE}")

3. Finally, we print corresponding compile flags set by CMake as a function of the build type:

message(STATUS "C flags, Debug configuration: ${CMAKE_C_FLAGS_DEBUG}")
message(STATUS "C flags, Release configuration: ${CMAKE_C_FLAGS_RELEASE}")
message(STATUS "C flags, Release configuration with Debug info: ${CMAKE_C_FLAGS_RELWITHDEBINFO}")
message(STATUS "C flags, minimal Release configuration: ${CMAKE_C_FLAGS_MINSIZEREL}")

message(STATUS "C++ flags, Debug configuration: ${CMAKE_CXX_FLAGS_DEBUG}")
message(STATUS "C++ flags, Release configuration: ${CMAKE_CXX_FLAGS_RELEASE}")
message(STATUS "C++ flags, Release configuration with Debug info: ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
message(STATUS "C++ flags, minimal Release configuration: ${CMAKE_CXX_FLAGS_MINSIZEREL}")

4. Let us now verify the output of a default configuration:

$ mkdir -p build
$ cd build
$ cmake ..

...
-- Build type: Release
-- C flags, Debug configuration: -g
-- C flags, Release configuration: -O3 -DNDEBUG
-- C flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C flags, minimal Release configuration: -Os -DNDEBUG
-- C++ flags, Debug configuration: -g
-- C++ flags, Release configuration: -O3 -DNDEBUG
-- C++ flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C++ flags, minimal Release configuration: -Os -DNDEBUG

5. And now, let us switch the build type:

$ cmake -D CMAKE_BUILD_TYPE=Debug ..

-- Build type: Debug
-- C flags, Debug configuration: -g
-- C flags, Release configuration: -O3 -DNDEBUG
-- C flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C flags, minimal Release configuration: -Os -DNDEBUG
-- C++ flags, Debug configuration: -g
-- C++ flags, Release configuration: -O3 -DNDEBUG
-- C++ flags, Release configuration with Debug info: -O2 -g -DNDEBUG
-- C++ flags, minimal Release configuration: -Os -DNDEBUG