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104 lines
4.3 KiB
104 lines
4.3 KiB
Step 8: Adding a Custom Command and Generated File
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==================================================
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Suppose, for the purpose of this tutorial, we decide that we never want to use
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the platform ``log`` and ``exp`` functions and instead would like to
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generate a table of precomputed values to use in the ``mysqrt`` function.
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In this section, we will create the table as part of the build process,
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and then compile that table into our application.
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First, let's remove the check for the ``log`` and ``exp`` functions in
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``MathFunctions/CMakeLists.txt``. Then remove the check for ``HAVE_LOG`` and
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``HAVE_EXP`` from ``mysqrt.cxx``. At the same time, we can remove
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:code:`#include <cmath>`.
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In the ``MathFunctions`` subdirectory, a new source file named
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``MakeTable.cxx`` has been provided to generate the table.
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After reviewing the file, we can see that the table is produced as valid C++
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code and that the output filename is passed in as an argument.
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The next step is to create ``MathFunctions/MakeTable.cmake``. Then, add the
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appropriate commands to the file to build the ``MakeTable`` executable and
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then run it as part of the build process. A few commands are needed to
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accomplish this.
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First, we add an executable for ``MakeTable``.
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.. literalinclude:: Step9/MathFunctions/MakeTable.cmake
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:caption: MathFunctions/MakeTable.cmake
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:name: MathFunctions/MakeTable.cmake-add_executable-MakeTable
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:language: cmake
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:start-after: # first we add the executable that generates the table
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:end-before: target_link_libraries
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After creating the executable, we add the ``tutorial_compiler_flags`` to our
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executable using :command:`target_link_libraries`.
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.. literalinclude:: Step9/MathFunctions/MakeTable.cmake
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:caption: MathFunctions/MakeTable.cmake
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:name: MathFunctions/MakeTable.cmake-link-tutorial-compiler-flags
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:language: cmake
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:start-after: add_executable
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:end-before: # add the command to generate
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Then we add a custom command that specifies how to produce ``Table.h``
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by running MakeTable.
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.. literalinclude:: Step9/MathFunctions/MakeTable.cmake
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:caption: MathFunctions/MakeTable.cmake
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:name: MathFunctions/MakeTable.cmake-add_custom_command-Table.h
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:language: cmake
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:start-after: # add the command to generate the source code
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Next we have to let CMake know that ``mysqrt.cxx`` depends on the generated
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file ``Table.h``. This is done by adding the generated ``Table.h`` to the list
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of sources for the library ``SqrtLibrary``.
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.. literalinclude:: Step9/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-add_library-Table.h
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:language: cmake
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:start-after: # library that just does sqrt
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:end-before: # state that we depend on
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We also have to add the current binary directory to the list of include
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directories so that ``Table.h`` can be found and included by ``mysqrt.cxx``.
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.. literalinclude:: Step9/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-target_include_directories-Table.h
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:language: cmake
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:start-after: # state that we depend on our bin
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:end-before: target_link_libraries
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As the last step, we need to include
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``MakeTable.cmake`` at the top of the ``MathFunctions/CMakeLists.txt``.
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.. literalinclude:: Step9/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-include-MakeTable.cmake
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:language: cmake
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:start-after: # generate Table.h
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:end-before: # library that just does sqrt
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Now let's use the generated table. First, modify ``mysqrt.cxx`` to include
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``Table.h``. Next, we can rewrite the ``mysqrt`` function to use the table:
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.. literalinclude:: Step9/MathFunctions/mysqrt.cxx
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:caption: MathFunctions/mysqrt.cxx
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:name: MathFunctions/mysqrt.cxx
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:language: c++
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:start-after: // a hack square root calculation using simple operations
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Run the :manual:`cmake <cmake(1)>` executable or the
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:manual:`cmake-gui <cmake-gui(1)>` to configure the project and then build it
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with your chosen build tool.
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When this project is built it will first build the ``MakeTable`` executable.
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It will then run ``MakeTable`` to produce ``Table.h``. Finally, it will
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compile ``mysqrt.cxx`` which includes ``Table.h`` to produce the
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``MathFunctions`` library.
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Run the Tutorial executable and verify that it is using the table.
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