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85 lines
3.6 KiB
85 lines
3.6 KiB
Step 10: Adding Generator Expressions
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=====================================
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:manual:`Generator expressions <cmake-generator-expressions(7)>` are evaluated
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during build system generation to produce information specific to each build
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configuration.
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:manual:`Generator expressions <cmake-generator-expressions(7)>` are allowed in
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the context of many target properties, such as :prop_tgt:`LINK_LIBRARIES`,
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:prop_tgt:`INCLUDE_DIRECTORIES`, :prop_tgt:`COMPILE_DEFINITIONS` and others.
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They may also be used when using commands to populate those properties, such as
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:command:`target_link_libraries`, :command:`target_include_directories`,
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:command:`target_compile_definitions` and others.
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:manual:`Generator expressions <cmake-generator-expressions(7)>` may be used
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to enable conditional linking, conditional definitions used when compiling,
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conditional include directories and more. The conditions may be based on the
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build configuration, target properties, platform information or any other
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queryable information.
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There are different types of
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:manual:`generator expressions <cmake-generator-expressions(7)>` including
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Logical, Informational, and Output expressions.
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Logical expressions are used to create conditional output. The basic
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expressions are the ``0`` and ``1`` expressions. A ``$<0:...>`` results in the
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empty string, and ``<1:...>`` results in the content of ``...``. They can also
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be nested.
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A common usage of
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:manual:`generator expressions <cmake-generator-expressions(7)>` is to
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conditionally add compiler flags, such as those for language levels or
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warnings. A nice pattern is to associate this information to an ``INTERFACE``
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target allowing this information to propagate. Let's start by constructing an
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``INTERFACE`` target and specifying the required C++ standard level of ``11``
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instead of using :variable:`CMAKE_CXX_STANDARD`.
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So the following code:
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.. literalinclude:: Step10/CMakeLists.txt
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:caption: CMakeLists.txt
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:name: CMakeLists.txt-CXX_STANDARD-variable-remove
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:language: cmake
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:start-after: project(Tutorial VERSION 1.0)
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:end-before: # control where the static and shared libraries are built so that on windows
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Would be replaced with:
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.. literalinclude:: Step11/CMakeLists.txt
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:caption: CMakeLists.txt
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:name: CMakeLists.txt-cxx_std-feature
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:language: cmake
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:start-after: project(Tutorial VERSION 1.0)
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:end-before: # add compiler warning flags just when building this project via
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**Note**: This upcoming section will require a change to the
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:command:`cmake_minimum_required` usage in the code. The Generator Expression
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that is about to be used was introduced in `3.15`. Update the call to require
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that more recent version:
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.. code-block:: cmake
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:caption: CMakeLists.txt
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:name: CMakeLists.txt-version-update
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cmake_minimum_required(VERSION 3.15)
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Next we add the desired compiler warning flags that we want for our project. As
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warning flags vary based on the compiler we use the ``COMPILE_LANG_AND_ID``
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generator expression to control which flags to apply given a language and a set
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of compiler ids as seen below:
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.. literalinclude:: Step11/CMakeLists.txt
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:caption: CMakeLists.txt
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:name: CMakeLists.txt-target_compile_options-genex
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:language: cmake
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:start-after: # the BUILD_INTERFACE genex
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:end-before: # control where the static and shared libraries are built so that on windows
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Looking at this we see that the warning flags are encapsulated inside a
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``BUILD_INTERFACE`` condition. This is done so that consumers of our installed
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project will not inherit our warning flags.
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**Exercise**: Modify ``MathFunctions/CMakeLists.txt`` so that all targets have
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a :command:`target_link_libraries` call to ``tutorial_compiler_flags``.
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