Step 3: Adding Usage Requirements for a Library

Usage requirements allow for far better control over a library or executable's link and include line while also giving more control over the transitive property of targets inside CMake. The primary commands that leverage usage requirements are:

• target_compile_definitions()

• target_compile_options()

• target_include_directories()

• target_link_libraries()

Let's refactor our code from Adding a Library to use the modern CMake approach of usage requirements. We first state that anybody linking to MathFunctions needs to include the current source directory, while MathFunctions itself doesn't. So this can become an INTERFACE usage requirement.

Remember INTERFACE means things that consumers require but the producer doesn't. Add the following lines to the end of MathFunctions/CMakeLists.txt:

MathFunctions/CMakeLists.txt

target_include_directories(MathFunctions
          INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
          )

Now that we've specified usage requirements for MathFunctions we can safely remove our uses of the EXTRA_INCLUDES variable from the top-level CMakeLists.txt, here:

CMakeLists.txt

if(USE_MYMATH)
  add_subdirectory(MathFunctions)
  list(APPEND EXTRA_LIBS MathFunctions)
endif()

And here:

CMakeLists.txt

target_include_directories(Tutorial PUBLIC
                           "${PROJECT_BINARY_DIR}"
                           )

Once this is done, run the cmake executable or the cmake-gui to configure the project and then build it with your chosen build tool or by using cmake --build . from the build directory.