1 Step 8: Adding a Custom Command and Generated File
2 ==================================================
4 Suppose, for the purpose of this tutorial, we decide that we never want to use
5 the platform ``log`` and ``exp`` functions and instead would like to
6 generate a table of precomputed values to use in the ``mysqrt`` function.
7 In this section, we will create the table as part of the build process,
8 and then compile that table into our application.
10 First, let's remove the check for the ``log`` and ``exp`` functions in
11 ``MathFunctions/CMakeLists.txt``. Then remove the check for ``HAVE_LOG`` and
12 ``HAVE_EXP`` from ``mysqrt.cxx``. At the same time, we can remove
13 :code:`#include <cmath>`.
15 In the ``MathFunctions`` subdirectory, a new source file named
16 ``MakeTable.cxx`` has been provided to generate the table.
18 After reviewing the file, we can see that the table is produced as valid C++
19 code and that the output filename is passed in as an argument.
21 The next step is to add the appropriate commands to the
22 ``MathFunctions/CMakeLists.txt`` file to build the MakeTable executable and
23 then run it as part of the build process. A few commands are needed to
26 First, at the top of ``MathFunctions/CMakeLists.txt``, the executable for
27 ``MakeTable`` is added as any other executable would be added.
29 .. literalinclude:: Step9/MathFunctions/CMakeLists.txt
30 :caption: MathFunctions/CMakeLists.txt
31 :name: MathFunctions/CMakeLists.txt-add_executable-MakeTable
33 :start-after: # first we add the executable that generates the table
34 :end-before: # add the command to generate the source code
36 Then we add a custom command that specifies how to produce ``Table.h``
39 .. literalinclude:: Step9/MathFunctions/CMakeLists.txt
40 :caption: MathFunctions/CMakeLists.txt
41 :name: MathFunctions/CMakeLists.txt-add_custom_command-Table.h
43 :start-after: # add the command to generate the source code
44 :end-before: # add the main library
46 Next we have to let CMake know that ``mysqrt.cxx`` depends on the generated
47 file ``Table.h``. This is done by adding the generated ``Table.h`` to the list
48 of sources for the library MathFunctions.
50 .. literalinclude:: Step9/MathFunctions/CMakeLists.txt
51 :caption: MathFunctions/CMakeLists.txt
52 :name: MathFunctions/CMakeLists.txt-add_library-Table.h
54 :start-after: # add the main library
55 :end-before: # state that anybody linking
57 We also have to add the current binary directory to the list of include
58 directories so that ``Table.h`` can be found and included by ``mysqrt.cxx``.
60 .. literalinclude:: Step9/MathFunctions/CMakeLists.txt
61 :caption: MathFunctions/CMakeLists.txt
62 :name: MathFunctions/CMakeLists.txt-target_include_directories-Table.h
64 :start-after: # state that we depend on our bin
65 :end-before: # install libs
67 Now let's use the generated table. First, modify ``mysqrt.cxx`` to include
68 ``Table.h``. Next, we can rewrite the ``mysqrt`` function to use the table:
70 .. literalinclude:: Step9/MathFunctions/mysqrt.cxx
71 :caption: MathFunctions/mysqrt.cxx
72 :name: MathFunctions/mysqrt.cxx
74 :start-after: // a hack square root calculation using simple operations
76 Run the :manual:`cmake <cmake(1)>` executable or the
77 :manual:`cmake-gui <cmake-gui(1)>` to configure the project and then build it
78 with your chosen build tool.
80 When this project is built it will first build the ``MakeTable`` executable.
81 It will then run ``MakeTable`` to produce ``Table.h``. Finally, it will
82 compile ``mysqrt.cxx`` which includes ``Table.h`` to produce the
83 ``MathFunctions`` library.
85 Run the Tutorial executable and verify that it is using the table.