How to properly set CMake flags in CMakeLists.txt

Question

I'm having trouble setting up options and CMake variables and getting them propagated to the CMake GUI.

For example, the following lines are in my CMakeLists.txt file:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -tR -DPOCO_DLL")
message("CMake flags: " ${CMAKE_CXX_FLAGS}) 

When running configure in the CMake GUI it prints "CMake flags: -tM -tR -DPOCO_DLL" indicating that setting the CMAKE_CXX_FLAGS "works".

The GUI however don't get updated and only shows "-tM" on the CMAKE_CXX_FLAGS line.

What is the proper way of setting up these CMAKE options in the CMakeLists file so they get propagated to the CMake GUI?

Answer 1

The trick is to set CMAKE_CXX_FLAGS before project(...) statement. But this will not be enough; you will also need to put it into the cache.

Also, if you plan to support setting it initially from the command line interface, and/or -C cmake option (locad initial cache), you will need to force put it into the cache, as shown:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -tR -DPOCO_DLL" CACHE STRING "Default CXX options" FORCE)
...
...
project(MyProject)
....

On the other hand, be very careful, because you are setting command line options at the moment you know nothing about the compiler, that is by default determined during execution of project statement.

Implicitly, this renders your CMakeLists.txt compiler dependant.

At the end, here is documentation about set cmake command