How to make cMake use Debug compilation mode for only a subset of directories in a project?

Question

Rephrased the question.

I have the following problem:

My project has several binaries and libraries that reside in distinct sub-directories under the main project folder. It is useful to be able to debug only a subset of them, without recompiling the whole project in Debug mode.

I want to be able to only change the compilation mode for this subset in a semi-automatic fashion.

How can I accomplish this using CMake?

Answer 1

If you change the build type, the whole project will be recompiled from scratch. Usually you keep 2 separated build tree, one configured debug and one configured release.

Note that CMAKE_BUILD_TYPE can be set from command line or from cmake-gui, you shouldn't set it in the CMakeLists.txt file.

---

To compile only some part of your project in debug mode, you can proceed as follow. In your main CMakeLists.txt, before including any subdirectory, define the following macro:

macro (CHECK_IF_DEBUG)
  if (NOT (CMAKE_BUILD_TYPE MATCHES Debug))
    get_filename_component(THIS_DIR ${CMAKE_CURRENT_SOURCE_DIR} NAME)
    STRING(REGEX REPLACE " " "_" THIS_DIR ${THIS_DIR}) #Replace spaces with underscores
    if (DEFINED DEBUG_${THIS_DIR})
      message(STATUS "Note: Targets in directory ${THIS_DIR} will be built Debug") #A reminder
      set (CMAKE_BUILD_TYPE Debug)
    endif()
  endif()
endmacro()

Then, in each subdirectory add (at the beginning of the CMakelists.txt) the macro call

 CHECK_IF_DEBUG()

When you need to temporarily debug a part (subdirectory) of your project, open your project in cmake-gui, and define a variable ("Add Entry") with name DEBUG_<DirectoryName>. You can define multiple ones. If your directory name contains spaces, in the variable name replace them with underscores. Don't forget to click Configure and Generate from cmake-gui to make the change effective. The value assigned to the variable is not important, it can be left empty.

When you are finished debugging, go back to cmake-gui and remove the corresponding entries. Don't forget to Configure and Generate.

I have tested it in a small project and it seems to work properly.

Note: If you create more than one target (add_library or add_executable) in the same CMakeLists.txt (=in the same subdirectory), I haven't found a way to have one target built in one way and one target in another: the only thing that seems to count is the value of the CMAKE_BUILD_TYPE directory when CMake is finished parsing the file.

---

In answer to your comment, you can have a reminder printed at build time by adding in the block the following line:

add_custom_target(info_${THIS_DIR} ALL COMMAND echo Targets in directory ${THIS_DIR} are built Debug)

See add_custom_target.