Step 8: Adding a Custom Command and Generated File ================================================== Suppose, for the purpose of this tutorial, we decide that we never want to use the platform ``log`` and ``exp`` functions and instead would like to generate a table of precomputed values to use in the ``mysqrt`` function. In this section, we will create the table as part of the build process, and then compile that table into our application. First, let's remove the check for the ``log`` and ``exp`` functions in ``MathFunctions/CMakeLists.txt``. Then remove the check for ``HAVE_LOG`` and ``HAVE_EXP`` from ``mysqrt.cxx``. At the same time, we can remove :code:`#include <cmath>`. In the ``MathFunctions`` subdirectory, a new source file named ``MakeTable.cxx`` has been provided to generate the table. After reviewing the file, we can see that the table is produced as valid C++ code and that the output filename is passed in as an argument. The next step is to add the appropriate commands to the ``MathFunctions/CMakeLists.txt`` file to build the MakeTable executable and then run it as part of the build process. A few commands are needed to accomplish this. First, at the top of ``MathFunctions/CMakeLists.txt``, the executable for ``MakeTable`` is added as any other executable would be added. .. literalinclude:: Step9/MathFunctions/CMakeLists.txt :caption: MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-add_executable-MakeTable :language: cmake :start-after: # first we add the executable that generates the table :end-before: # add the command to generate the source code Then we add a custom command that specifies how to produce ``Table.h`` by running MakeTable. .. literalinclude:: Step9/MathFunctions/CMakeLists.txt :caption: MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-add_custom_command-Table.h :language: cmake :start-after: # add the command to generate the source code :end-before: # add the main library Next we have to let CMake know that ``mysqrt.cxx`` depends on the generated file ``Table.h``. This is done by adding the generated ``Table.h`` to the list of sources for the library MathFunctions. .. literalinclude:: Step9/MathFunctions/CMakeLists.txt :caption: MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-add_library-Table.h :language: cmake :start-after: # add the main library :end-before: # state that anybody linking We also have to add the current binary directory to the list of include directories so that ``Table.h`` can be found and included by ``mysqrt.cxx``. .. literalinclude:: Step9/MathFunctions/CMakeLists.txt :caption: MathFunctions/CMakeLists.txt :name: MathFunctions/CMakeLists.txt-target_include_directories-Table.h :language: cmake :start-after: # state that we depend on our bin :end-before: # install libs Now let's use the generated table. First, modify ``mysqrt.cxx`` to include ``Table.h``. Next, we can rewrite the ``mysqrt`` function to use the table: .. literalinclude:: Step9/MathFunctions/mysqrt.cxx :caption: MathFunctions/mysqrt.cxx :name: MathFunctions/mysqrt.cxx :language: c++ :start-after: // a hack square root calculation using simple operations 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. When this project is built it will first build the ``MakeTable`` executable. It will then run ``MakeTable`` to produce ``Table.h``. Finally, it will compile ``mysqrt.cxx`` which includes ``Table.h`` to produce the ``MathFunctions`` library. Run the Tutorial executable and verify that it is using the table.
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