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125 lines
5.3 KiB
125 lines
5.3 KiB
target_precompile_headers
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-------------------------
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Add a list of header files to precompile.
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Precompiling header files can speed up compilation by creating a partially
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processed version of some header files, and then using that version during
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compilations rather than repeatedly parsing the original headers.
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Main Form
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^^^^^^^^^
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.. code-block:: cmake
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target_precompile_headers(<target>
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<INTERFACE|PUBLIC|PRIVATE> [header1...]
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[<INTERFACE|PUBLIC|PRIVATE> [header2...] ...])
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The command adds header files to the :prop_tgt:`PRECOMPILE_HEADERS` and/or
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:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` target properties of ``<target>``.
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The named ``<target>`` must have been created by a command such as
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:command:`add_executable` or :command:`add_library` and must not be an
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:ref:`ALIAS target <Alias Targets>`.
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The ``INTERFACE``, ``PUBLIC`` and ``PRIVATE`` keywords are required to
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specify the scope of the following arguments. ``PRIVATE`` and ``PUBLIC``
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items will populate the :prop_tgt:`PRECOMPILE_HEADERS` property of
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``<target>``. ``PUBLIC`` and ``INTERFACE`` items will populate the
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:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` property of ``<target>``
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(:ref:`IMPORTED targets <Imported Targets>` only support ``INTERFACE`` items).
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Repeated calls for the same ``<target>`` will append items in the order called.
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Projects should generally avoid using ``PUBLIC`` or ``INTERFACE`` for targets
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that will be :ref:`exported <install(EXPORT)>`, or they should at least use
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the ``$<BUILD_INTERFACE:...>`` generator expression to prevent precompile
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headers from appearing in an installed exported target. Consumers of a target
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should typically be in control of what precompile headers they use, not have
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precompile headers forced on them by the targets being consumed (since
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precompile headers are not typically usage requirements). A notable exception
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to this is where an :ref:`interface library <Interface Libraries>` is created
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to define a commonly used set of precompile headers in one place and then other
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targets link to that interface library privately. In this case, the interface
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library exists specifically to propagate the precompile headers to its
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consumers and the consumer is effectively still in control, since it decides
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whether to link to the interface library or not.
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The list of header files is used to generate a header file named
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``cmake_pch.h|xx`` which is used to generate the precompiled header file
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(``.pch``, ``.gch``, ``.pchi``) artifact. The ``cmake_pch.h|xx`` header
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file will be force included (``-include`` for GCC, ``/FI`` for MSVC) to
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all source files, so sources do not need to have ``#include "pch.h"``.
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Header file names specified with angle brackets (e.g. ``<unordered_map>``) or
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explicit double quotes (escaped for the :manual:`cmake-language(7)`,
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e.g. ``[["other_header.h"]]``) will be treated as is, and include directories
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must be available for the compiler to find them. Other header file names
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(e.g. ``project_header.h``) are interpreted as being relative to the current
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source directory (e.g. :variable:`CMAKE_CURRENT_SOURCE_DIR`) and will be
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included by absolute path.
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Arguments to ``target_precompile_headers()`` may use "generator expressions"
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with the syntax ``$<...>``.
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See the :manual:`cmake-generator-expressions(7)` manual for available
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expressions. See the :manual:`cmake-compile-features(7)` manual for
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information on compile features and a list of supported compilers.
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The ``$<COMPILE_LANGUAGE:...>`` generator expression is particularly
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useful for specifying a language-specific header to precompile for
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only one language (e.g. ``CXX`` and not ``C``). For example:
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.. code-block:: cmake
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target_precompile_headers(myTarget
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PUBLIC
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project_header.h
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"$<$<COMPILE_LANGUAGE:CXX>:cxx_only.h>"
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PRIVATE
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[["other_header.h"]]
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<unordered_map>
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)
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When specifying angle brackets inside a :manual:`generator expression
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<cmake-generator-expressions(7)>`, be sure to encode the closing ``>``
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as ``$<ANGLE-R>``. For example:
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.. code-block:: cmake
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target_precompile_headers(mylib PRIVATE
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"$<$<COMPILE_LANGUAGE:C>:<stddef.h$<ANGLE-R>>"
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"$<$<COMPILE_LANGUAGE:CXX>:<cstddef$<ANGLE-R>>"
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)
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Reusing Precompile Headers
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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The command also supports a second signature which can be used to specify that
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one target re-uses a precompiled header file artefact from another target
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instead of generating its own:
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.. code-block:: cmake
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target_precompile_headers(<target> REUSE_FROM <other_target>)
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This form sets the :prop_tgt:`PRECOMPILE_HEADERS_REUSE_FROM` property to
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``<other_target>`` and adds a dependency such that ``<target>`` will depend
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on ``<other_target>``. CMake will halt with an error if the
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:prop_tgt:`PRECOMPILE_HEADERS` property of ``<target>`` is already set when
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the ``REUSE_FROM`` form is used.
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.. note::
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The ``REUSE_FROM`` form requires the same set of compiler options,
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compiler flags and compiler definitions for both ``<target>`` and
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``<other_target>``. Some compilers (e.g. GCC) may issue a warning if the
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precompiled header file cannot be used (``-Winvalid-pch``).
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See Also
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^^^^^^^^
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To disable precompile headers for specific targets, see the
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:prop_tgt:`DISABLE_PRECOMPILE_HEADERS` target property.
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To prevent precompile headers from being used when compiling a specific
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source file, see the :prop_sf:`SKIP_PRECOMPILE_HEADERS` source file property.
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