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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:
- :command:`target_compile_definitions`
- :command:`target_compile_options`
- :command:`target_include_directories`
- :command:`target_link_libraries`
Let's refactor our code from :guide:`tutorial/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``:
.. literalinclude:: Step4/MathFunctions/CMakeLists.txt
:caption: MathFunctions/CMakeLists.txt
:name: MathFunctions/CMakeLists.txt-target_include_directories-INTERFACE
:language: cmake
:start-after: # to find MathFunctions.h
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:
.. literalinclude:: Step4/CMakeLists.txt
:caption: CMakeLists.txt
:name: CMakeLists.txt-remove-EXTRA_INCLUDES
:language: cmake
:start-after: # add the MathFunctions library
:end-before: # add the executable
And here:
.. literalinclude:: Step4/CMakeLists.txt
:caption: CMakeLists.txt
:name: CMakeLists.txt-target_include_directories-remove-EXTRA_INCLUDES
:language: cmake
:start-after: # so that we will find TutorialConfig.h
Once this is done, run the :manual:`cmake <cmake(1)>` executable or the
:manual:`cmake-gui <cmake-gui(1)>` to configure the project and then build it
with your chosen build tool or by using ``cmake --build .`` from the build
directory.
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