Step 2: Adding a Library ======================== At this point, we have seen how to create a basic project using CMake. In this step, we will learn how to create and use a library in our project. We will also see how to make the use of our library optional. Exercise 1 - Creating a Library ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ To add a library in CMake, use the :command:`add_library` command and specify which source files should make up the library. Rather than placing all of the source files in one directory, we can organize our project with one or more subdirectories. In this case, we will create a subdirectory specifically for our library. Here, we can add a new ``CMakeLists.txt`` file and one or more source files. In the top level ``CMakeLists.txt`` file, we will use the :command:`add_subdirectory` command to add the subdirectory to the build. Once the library is created, it is connected to our executable target with :command:`target_include_directories` and :command:`target_link_libraries`. Goal ---- Add and use a library. Helpful Resources ----------------- * :command:`add_library` * :command:`add_subdirectory` * :command:`target_include_directories` * :command:`target_link_libraries` * :variable:`PROJECT_SOURCE_DIR` Files to Edit ------------- * ``CMakeLists.txt`` * ``tutorial.cxx`` * ``MathFunctions/CMakeLists.txt`` Getting Started --------------- In this exercise, we will add a library to our project that contains our own implementation for computing the square root of a number. The executable can then use this library instead of the standard square root function provided by the compiler. For this tutorial we will put the library into a subdirectory called ``MathFunctions``. This directory already contains the header files ``MathFunctions.h`` and ``mysqrt.h``. Their respective source files ``MathFunctions.cxx`` and ``mysqrt.cxx`` are also provided. We will not need to modify any of these files. ``mysqrt.cxx`` has one function called ``mysqrt`` that provides similar functionality to the compiler's ``sqrt`` function. ``MathFunctions.cxx`` contains one function ``sqrt`` which serves to hide the implementation details of ``sqrt``. From the ``Help/guide/tutorial/Step2`` directory, start with ``TODO 1`` and complete through ``TODO 6``. First, fill in the one line ``CMakeLists.txt`` in the ``MathFunctions`` subdirectory. Next, edit the top level ``CMakeLists.txt``. Finally, use the newly created ``MathFunctions`` library in ``tutorial.cxx`` Build and Run ------------- Run the :manual:`cmake ` executable or the :manual:`cmake-gui ` to configure the project and then build it with your chosen build tool. Below is a refresher of what that looks like from the command line: .. code-block:: console mkdir Step2_build cd Step2_build cmake ../Step2 cmake --build . Try to use the newly built ``Tutorial`` and ensure that it is still producing accurate square root values. Solution -------- In the ``CMakeLists.txt`` file in the ``MathFunctions`` directory, we create a library target called ``MathFunctions`` with :command:`add_library`. The source files for the library are passed as an argument to :command:`add_library`. This looks like the following line: .. raw:: html
TODO 1: Click to show/hide answer .. code-block:: cmake :caption: TODO 1: MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-add_library add_library(MathFunctions MathFunctions.cxx mysqrt.cxx) .. raw:: html
To make use of the new library we will add an :command:`add_subdirectory` call in the top-level ``CMakeLists.txt`` file so that the library will get built. .. raw:: html
TODO 2: Click to show/hide answer .. code-block:: cmake :caption: TODO 2: CMakeLists.txt :name: CMakeLists.txt-add_subdirectory add_subdirectory(MathFunctions) .. raw:: html
Next, the new library target is linked to the executable target using :command:`target_link_libraries`. .. raw:: html
TODO 3: Click to show/hide answer .. code-block:: cmake :caption: TODO 3: CMakeLists.txt :name: CMakeLists.txt-target_link_libraries target_link_libraries(Tutorial PUBLIC MathFunctions) .. raw:: html
Finally we need to specify the library's header file location. Modify :command:`target_include_directories` to add the ``MathFunctions`` subdirectory as an include directory so that the ``MathFunctions.h`` header file can be found. .. raw:: html
TODO 4: Click to show/hide answer .. code-block:: cmake :caption: TODO 4: CMakeLists.txt :name: CMakeLists.txt-target_include_directories-step2 target_include_directories(Tutorial PUBLIC "${PROJECT_BINARY_DIR}" "${PROJECT_SOURCE_DIR}/MathFunctions" ) .. raw:: html
Now let's use our library. In ``tutorial.cxx``, include ``MathFunctions.h``: .. raw:: html
TODO 5: Click to show/hide answer .. literalinclude:: Step3/tutorial.cxx :caption: TODO 5: tutorial.cxx :name: CMakeLists.txt-include-MathFunctions.h :language: cmake :start-after: #include :end-before: #include "TutorialConfig.h" .. raw:: html
Lastly, replace ``sqrt`` with the wrapper function ``mathfunctions::sqrt``. .. raw:: html
TODO 6: Click to show/hide answer .. literalinclude:: Step3/tutorial.cxx :caption: TODO 6: tutorial.cxx :name: CMakeLists.txt-option :language: cmake :start-after: const double inputValue = std::stod(argv[1]); :end-before: std::cout .. raw:: html
Exercise 2 - Adding an Option ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Now let us add an option in the MathFunctions library to allow developers to select either the custom square root implementation or the built in standard implementation. While for the tutorial there really isn't any need to do so, for larger projects this is a common occurrence. CMake can do this using the :command:`option` command. This gives users a variable which they can change when configuring their cmake build. This setting will be stored in the cache so that the user does not need to set the value each time they run CMake on a build directory. Goal ---- Add the option to build without ``MathFunctions``. Helpful Resources ----------------- * :command:`if` * :command:`option` * :command:`target_compile_definitions` Files to Edit ------------- * ``MathFunctions/CMakeLists.txt`` * ``MathFunctions/MathFunctions.cxx`` Getting Started --------------- Start with the resulting files from Exercise 1. Complete ``TODO 7`` through ``TODO 14``. First create a variable ``USE_MYMATH`` using the :command:`option` command in ``MathFunctions/CMakeLists.txt``. In that same file, use that option to pass a compile definition to the ``MathFunctions`` library. Then, update ``MathFunctions.cxx`` to redirect compilation based on ``USE_MYMATH``. Lastly, prevent ``mysqrt.cxx`` from being compiled when ``USE_MYMATH`` is on by making it its own library inside of the ``USE_MYMATH`` block of ``MathFunctions/CMakeLists.txt``. Build and Run ------------- Since we have our build directory already configured from Exercise 1, we can rebuild by simply calling the following: .. code-block:: console cd ../Step2_build cmake --build . Next, run the ``Tutorial`` executable on a few numbers to verify that it's still correct. Now let's update the value of ``USE_MYMATH`` to ``OFF``. The easiest way is to use the :manual:`cmake-gui ` or :manual:`ccmake ` if you're in the terminal. Or, alternatively, if you want to change the option from the command-line, try: .. code-block:: console cmake ../Step2 -DUSE_MYMATH=OFF Now, rebuild the code with the following: .. code-block:: console cmake --build . Then, run the executable again to ensure that it still works with ``USE_MYMATH`` set to ``OFF``. Which function gives better results, ``sqrt`` or ``mysqrt``? Solution -------- The first step is to add an option to ``MathFunctions/CMakeLists.txt``. This option will be displayed in the :manual:`cmake-gui ` and :manual:`ccmake ` with a default value of ``ON`` that can be changed by the user. .. raw:: html
TODO 7: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/CMakeLists.txt :caption: TODO 7: MathFunctions/CMakeLists.txt :name: CMakeLists.txt-option-library-level :language: cmake :start-after: # should we use our own math functions :end-before: if (USE_MYMATH) .. raw:: html
Next, make building and linking our library with ``mysqrt`` function conditional using this new option. Create an :command:`if` statement which checks the value of ``USE_MYMATH``. Inside the :command:`if` block, put the :command:`target_compile_definitions` command with the compile definition ``USE_MYMATH``. .. raw:: html
TODO 8: Click to show/hide answer .. code-block:: cmake :caption: TODO 8: MathFunctions/CMakeLists.txt :name: CMakeLists.txt-USE_MYMATH if (USE_MYMATH) target_compile_definitions(MathFunctions PRIVATE "USE_MYMATH") endif() .. raw:: html
When ``USE_MYMATH`` is ``ON``, the compile definition ``USE_MYMATH`` will be set. We can then use this compile definition to enable or disable sections of our source code. The corresponding changes to the source code are fairly straightforward. In ``MathFunctions.cxx``, we make ``USE_MYMATH`` control which square root function is used: .. raw:: html
TODO 9: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/MathFunctions.cxx :caption: TODO 9: MathFunctions/MathFunctions.cxx :name: MathFunctions-USE_MYMATH-if :language: c++ :start-after: which square root function should we use? :end-before: } .. raw:: html
Next, we need to include ``mysqrt.h`` if ``USE_MYMATH`` is defined. .. raw:: html
TODO 10: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/MathFunctions.cxx :caption: TODO 10: MathFunctions/MathFunctions.cxx :name: MathFunctions-USE_MYMATH-if-include :language: c++ :start-after: include :end-before: namespace mathfunctions .. raw:: html
Finally, we need to include ``cmath`` now that we are using ``std::sqrt``. .. raw:: html
TODO 11: Click to show/hide answer .. code-block:: c++ :caption: TODO 11 : MathFunctions/MathFunctions.cxx :name: tutorial.cxx-include_cmath #include .. raw:: html
At this point, if ``USE_MYMATH`` is ``OFF``, ``mysqrt.cxx`` would not be used but it will still be compiled because the ``MathFunctions`` target has ``mysqrt.cxx`` listed under sources. There are a few ways to fix this. The first option is to use :command:`target_sources` to add ``mysqrt.cxx`` from within the ``USE_MYMATH`` block. Another option is to create an additional library within the ``USE_MYMATH`` block which is responsible for compiling ``mysqrt.cxx``. For the sake of this tutorial, we are going to create an additional library. First, from within ``USE_MYMATH`` create a library called ``SqrtLibrary`` that has sources ``mysqrt.cxx``. .. raw:: html
TODO 12: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/CMakeLists.txt :caption: TODO 12 : MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-add_library-SqrtLibrary :language: cmake :start-after: # library that just does sqrt :end-before: # TODO 7: Link .. raw:: html
Next, we link ``SqrtLibrary`` onto ``MathFunctions`` when ``USE_MYMATH`` is enabled. .. raw:: html
TODO 13: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/CMakeLists.txt :caption: TODO 13 : MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-target_link_libraries-SqrtLibrary :language: cmake :start-after: to tutorial_compiler_flags :end-before: endif() .. raw:: html
Finally, we can remove ``mysqrt.cxx`` from our ``MathFunctions`` library source list because it will be pulled in when ``SqrtLibrary`` is included. .. raw:: html
TODO 14: Click to show/hide answer .. literalinclude:: Step3/MathFunctions/CMakeLists.txt :caption: TODO 14 : MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-remove-mysqrt.cxx-MathFunctions :language: cmake :end-before: # TODO 1: .. raw:: html
With these changes, the ``mysqrt`` function is now completely optional to whoever is building and using the ``MathFunctions`` library. Users can toggle ``USE_MYMATH`` to manipulate what library is used in the build.