5c84eca210
Since commit 07223c5c27 (Tutorial: Update Step 5 to work on Windows,
2020-02-18, v3.18.0-rc1~655^2) the logic does not work on non-Windows
platforms when cmake is re-run on an existing build tree. It is also
more complicated than we'd like for a tutorial example. Avoid the need
to consider the `m` library case by performing the check as C++.
Since `check_cxx_symbol_exists` cannot handle overloaded functions
like `exp` and `log`, check with `check_cxx_source_compiles` instead.
This also presents a more general-purpose example in the tutorial.
Fixes: #23524
60 lines
2.4 KiB
ReStructuredText
60 lines
2.4 KiB
ReStructuredText
Step 5: Adding System Introspection
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===================================
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Let us consider adding some code to our project that depends on features the
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target platform may not have. For this example, we will add some code that
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depends on whether or not the target platform has the ``log`` and ``exp``
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functions. Of course almost every platform has these functions but for this
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tutorial assume that they are not common.
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If the platform has ``log`` and ``exp`` then we will use them to compute the
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square root in the ``mysqrt`` function. We first test for the availability of
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these functions using the :module:`CheckCXXSourceCompiles` module in
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``MathFunctions/CMakeLists.txt``.
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Add the checks for ``log`` and ``exp`` to ``MathFunctions/CMakeLists.txt``,
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after the call to :command:`target_include_directories`:
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.. literalinclude:: Step6/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-check_cxx_source_compiles
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:language: cmake
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:start-after: # to find MathFunctions.h, while we don't.
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:end-before: # add compile definitions
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If available, use :command:`target_compile_definitions` to specify
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``HAVE_LOG`` and ``HAVE_EXP`` as ``PRIVATE`` compile definitions.
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.. literalinclude:: Step6/MathFunctions/CMakeLists.txt
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:caption: MathFunctions/CMakeLists.txt
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:name: MathFunctions/CMakeLists.txt-target_compile_definitions
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:language: cmake
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:start-after: # add compile definitions
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:end-before: # install rules
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If ``log`` and ``exp`` are available on the system, then we will use them to
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compute the square root in the ``mysqrt`` function. Add the following code to
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the ``mysqrt`` function in ``MathFunctions/mysqrt.cxx`` (don't forget the
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``#endif`` before returning the result!):
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.. literalinclude:: Step6/MathFunctions/mysqrt.cxx
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:caption: MathFunctions/mysqrt.cxx
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:name: MathFunctions/mysqrt.cxx-ifdef
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:language: c++
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:start-after: // if we have both log and exp then use them
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:end-before: // do ten iterations
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We will also need to modify ``mysqrt.cxx`` to include ``cmath``.
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.. literalinclude:: Step6/MathFunctions/mysqrt.cxx
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:caption: MathFunctions/mysqrt.cxx
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:name: MathFunctions/mysqrt.cxx-include-cmath
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:language: c++
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:end-before: #include <iostream>
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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 and run the Tutorial executable.
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Which function gives better results now, ``sqrt`` or ``mysqrt``?
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