89 lines
3.3 KiB
Markdown
89 lines
3.3 KiB
Markdown
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# Useful Modules
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There are a ton of [modules] in CMake; but some of them are more useful than others. Here are a few highlights.
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## [CMakeDependentOption]
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This adds a command `cmake_dependent_option` that sets an option based on another set of variables being true. It looks like this:
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```cmake
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include(CMakeDependentOption)
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cmake_dependent_option(BUILD_TESTS "Build your tests" ON "VAL1;VAL2" OFF)
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```
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which is just a shortcut for this:
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```cmake
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if(VAL1 AND VAL2)
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set(BUILD_TESTS_DEFAULT ON)
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else()
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set(BUILD_TESTS_DEFAULT OFF)
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endif()
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option(BUILD_TESTS "Build your tests" ${BUILD_TESTS_DEFAULT})
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if(NOT BUILD_TESTS_DEFAULT)
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mark_as_advanced(BUILD_TESTS)
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endif()
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```
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## [CheckCXXCompilerFlag]
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This checks to see if a flag is supported. For example:
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```cmake
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include(CheckCXXCompilerFlag)
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check_cxx_compiler_flag(-someflag OUPUT_VARIABLE)
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```
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Note that `OUTPUT_VARIABLE` will also appear in the configuration printout, so choose a good name.
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This is just one of many similar modules, such as `CheckIncludeFileCXX`, `CheckStructHasMember`, `TestBigEndian`, and `CheckTypeSize` that allow you
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to check for information about the system (and you can communicate that to your source code).
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## [WriteCompilerDetectionHeader]
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This is an amazing module similar to the ones listed above, but special enough to deserve its own section. It allows you
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to look for a list of features that some compilers support, and write out a C++ header file that lets you know whether that
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feature is available. It even can provide compatibility macros for features that have changed names!
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To use:
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```cmake
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write_compiler_detection_header(
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FILE myoutput.h
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PREFIX My
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COMPILERS GNU Clang MSVC Intel
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FEATURES cxx_variadic_templates
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)
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```
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This supports compiler features (defined to 0 or 1), symbols (defined to empty or the symbol), and macros that
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support different names. They will be prefixed with the PREFIX you provide. You can separate compilers into different
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files using `OUTPUT_FILES_DIR
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The downside is that you do have to list the compilers you expect to support. If you use the `ALLOW_UNKNOWN_COMPILERS` flag(s),
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you can keep this from erroring on unknown compilers, but it will still leave all features empty.
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## [try_compile](https://cmake.org/cmake/help/latest/command/try_compile.html)/[try_run](https://cmake.org/cmake/help/latest/command/try_run.html)
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This is not exactly a module, but is crutial to many of the modules listed above. You can attept to compile (and possibly run) a bit of code at configure time. This can allow you to get information about the capabilities of your system. The basic syntax is:
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```cmake
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try_compile(
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RESULT_VAR
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bindir
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SOURCES
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source.cpp
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)
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```
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There are lots of options you can add, like `COMPILE_DEFINITIONS`. In CMake 3.8+, this will honor the CMake C/C++/CUDA standard settings. If you use `try_run` instead, it will run the resulting program and give you the output in `RUN_OUTPUT_VARIABLE`.
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[modules]: https://cmake.org/cmake/help/latest/manual/cmake-modules.7.html
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[CMakeDependentOption]: https://cmake.org/cmake/help/latest/module/CMakeDependentOption.html
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[CheckCXXCompilerFlag]: https://cmake.org/cmake/help/latest/module/CheckCXXCompilerFlag.html
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[WriteCompilerDetectionHeader]: https://cmake.org/cmake/help/latest/module/WriteCompilerDetectionHeader.html
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