path: root/Help/prop_tgt/AUTOMOC.rst

AUTOMOC
-------

Should the target be processed with auto-moc (for Qt projects).

``AUTOMOC`` is a boolean specifying whether CMake will handle the Qt
``moc`` preprocessor automatically, i.e.  without having to use commands like
:module:`QT4_WRAP_CPP() <FindQt4>`, ``QT5_WRAP_CPP()``, etc.
Currently, Qt versions 4 to 6 are supported.

This property is initialized by the value of the :variable:`CMAKE_AUTOMOC`
variable if it is set when a target is created.

When this property is set ``ON``, CMake will scan the header and
source files at build time and invoke ``moc`` accordingly.


Header file processing
^^^^^^^^^^^^^^^^^^^^^^

At configuration time, a list of header files that should be scanned by
``AUTOMOC`` is computed from the target's sources.

- All header files in the target's sources are added to the scan list.
- For all C++ source files ``<source_base>.<source_extension>`` in the
  target's sources, CMake searches for

  - a regular header with the same base name
    (``<source_base>.<header_extention>``) and
  - a private header with the same base name and a ``_p`` suffix
    (``<source_base>_p.<header_extention>``)

  and adds these to the scan list.

At build time, CMake scans each unknown or modified header file from the
list and searches for

- a Qt macro from :prop_tgt:`AUTOMOC_MACRO_NAMES`,
- additional file dependencies from the ``FILE`` argument of a
  ``Q_PLUGIN_METADATA`` macro and
- additional file dependencies detected by filters defined in
  :prop_tgt:`AUTOMOC_DEPEND_FILTERS`.

If a Qt macro is found, then the header will be compiled by the ``moc`` to the
output file ``moc_<base_name>.cpp``.  The complete output file path is
described in the section `Output file location`_.

The header will be ``moc`` compiled again if a file from the additional file
dependencies changes.

Header ``moc`` output files ``moc_<base_name>.cpp`` can be included in source
files.  In the section `Including header moc files in sources`_ there is more
information on that topic.


Source file processing
^^^^^^^^^^^^^^^^^^^^^^

At build time, CMake scans each unknown or modified C++ source file from the
target's sources for

- a Qt macro from :prop_tgt:`AUTOMOC_MACRO_NAMES`,
- includes of header ``moc`` files
  (see `Including header moc files in sources`_),
- additional file dependencies from the ``FILE`` argument of a
  ``Q_PLUGIN_METADATA`` macro and
- additional file dependencies detected by filters defined in
  :prop_tgt:`AUTOMOC_DEPEND_FILTERS`.

If a Qt macro is found, then the C++ source file
``<base>.<source_extension>`` is expected to as well contain an include
statement

.. code-block:: c++

  #include <<base>.moc> // or
  #include "<base>.moc"

The source file then will be compiled by the ``moc`` to the output file
``<base>.moc``.  A description of the complete output file path is in section
`Output file location`_.

The source will be ``moc`` compiled again if a file from the additional file
dependencies changes.

Including header moc files in sources
"""""""""""""""""""""""""""""""""""""

A source file can include the ``moc`` output file of a header
``<header_base>.<header_extension>`` by using an include statement of
the form

.. code-block:: c++

  #include <moc_<header_base>.cpp> // or
  #include "moc_<header_base>.cpp"

If the ``moc`` output file of a header is included by a source, it will
be generated in a different location than if it was not included.  This is
described in the section `Output file location`_.


Output file location
^^^^^^^^^^^^^^^^^^^^

Included moc output files
"""""""""""""""""""""""""

``moc`` output files that are included by a source file will be generated in

- ``<AUTOGEN_BUILD_DIR>/include``
  for single configuration generators or in
- ``<AUTOGEN_BUILD_DIR>/include_<CONFIG>``
  for :prop_gbl:`multi configuration <GENERATOR_IS_MULTI_CONFIG>` generators.

Where ``<AUTOGEN_BUILD_DIR>`` is the value of the target property
:prop_tgt:`AUTOGEN_BUILD_DIR`.

The include directory is automatically added to the target's
:prop_tgt:`INCLUDE_DIRECTORIES`.

Not included moc output files
"""""""""""""""""""""""""""""

``moc`` output files that are not included in a source file will be generated
in

- ``<AUTOGEN_BUILD_DIR>/<SOURCE_DIR_CHECKSUM>``
  for single configuration generators or in,
- ``<AUTOGEN_BUILD_DIR>/include_<CONFIG>/<SOURCE_DIR_CHECKSUM>``
  for :prop_gbl:`multi configuration <GENERATOR_IS_MULTI_CONFIG>` generators.

Where ``<SOURCE_DIR_CHECKSUM>`` is a checksum computed from the relative
parent directory path of the ``moc`` input file.  This scheme allows to have
``moc`` input files with the same name in different directories.

All not included ``moc`` output files will be included automatically by the
CMake generated file

- ``<AUTOGEN_BUILD_DIR>/mocs_compilation.cpp``, or
- ``<AUTOGEN_BUILD_DIR>/mocs_compilation_$<CONFIG>.cpp``,

which is added to the target's sources.


Qt version detection
^^^^^^^^^^^^^^^^^^^^

``AUTOMOC`` enabled targets need to know the Qt major and minor
version they're working with.  The major version usually is provided by the
``INTERFACE_QT_MAJOR_VERSION`` property of the ``Qt[456]Core`` library,
that the target links to.  To find the minor version, CMake builds a list of
available Qt versions from

- ``Qt6Core_VERSION_MAJOR`` and ``Qt6Core_VERSION_MINOR`` variables
  (usually set by ``find_package(Qt6...)``)
- ``Qt6Core_VERSION_MAJOR`` and ``Qt6Core_VERSION_MINOR`` directory properties
- ``Qt5Core_VERSION_MAJOR`` and ``Qt5Core_VERSION_MINOR`` variables
  (usually set by ``find_package(Qt5...)``)
- ``Qt5Core_VERSION_MAJOR`` and ``Qt5Core_VERSION_MINOR`` directory properties
- ``QT_VERSION_MAJOR`` and ``QT_VERSION_MINOR``  variables
  (usually set by ``find_package(Qt4...)``)
- ``QT_VERSION_MAJOR`` and ``QT_VERSION_MINOR``  directory properties

in the context of the :command:`add_executable` or :command:`add_library` call.

Assumed  ``INTERFACE_QT_MAJOR_VERSION`` is a valid number, the first
entry in the list with a matching major version is taken.  If no matching major
version was found, an error is generated.
If  ``INTERFACE_QT_MAJOR_VERSION`` is not a valid number, the first
entry in the list is taken.

A ``find_package(Qt[456]...)`` call sets the ``QT/Qt[56]Core_VERSION_MAJOR/MINOR``
variables.  If the call is in a different context than the
:command:`add_executable` or :command:`add_library` call, e.g. in a function,
then the version variables might not be available to the ``AUTOMOC``
enabled target.
In that case the version variables can be forwarded from the
``find_package(Qt[456]...)`` calling context to the :command:`add_executable`
or :command:`add_library` calling context as directory properties.
The following Qt5 example demonstrates the procedure.

.. code-block:: cmake

  function (add_qt5_client)
    find_package(Qt5 REQUIRED QUIET COMPONENTS Core Widgets)
    ...
    set_property(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
      PROPERTY Qt5Core_VERSION_MAJOR "${Qt5Core_VERSION_MAJOR}")
    set_property(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
      PROPERTY Qt5Core_VERSION_MINOR "${Qt5Core_VERSION_MAJOR}")
    ...
  endfunction ()
  ...
  add_qt5_client()
  add_executable(myTarget main.cpp)
  target_link_libraries(myTarget Qt5::QtWidgets)
  set_property(TARGET myTarget PROPERTY AUTOMOC ON)


Modifiers
^^^^^^^^^

:prop_tgt:`AUTOMOC_EXECUTABLE`:
The ``moc`` executable will be detected automatically, but can be forced to
a certain binary using this target property.

:prop_tgt:`AUTOMOC_MOC_OPTIONS`:
Additional command line options for ``moc`` can be set in this target property.

:prop_tgt:`AUTOMOC_MACRO_NAMES`:
This list of Qt macro names can be extended to search for additional macros in
headers and sources.

:prop_tgt:`AUTOMOC_DEPEND_FILTERS`:
``moc`` dependency file names can be extracted from headers or sources by
defining file name filters in this target property.

:prop_tgt:`AUTOMOC_COMPILER_PREDEFINES`:
Compiler pre definitions for ``moc`` are written to the ``moc_predefs.h`` file.
The generation of this file can be enabled or disabled in this target property.

:prop_sf:`SKIP_AUTOMOC`:
Sources and headers can be excluded from ``AUTOMOC`` processing by
setting this source file property.

:prop_sf:`SKIP_AUTOGEN`:
Source files can be excluded from ``AUTOMOC``,
:prop_tgt:`AUTOUIC` and :prop_tgt:`AUTORCC` processing by
setting this source file property.

:prop_gbl:`AUTOGEN_SOURCE_GROUP`:
This global property can be used to group files generated by
``AUTOMOC`` or :prop_tgt:`AUTORCC` together in an IDE, e.g.  in MSVS.

:prop_gbl:`AUTOGEN_TARGETS_FOLDER`:
This global property can be used to group ``AUTOMOC``,
:prop_tgt:`AUTOUIC` and :prop_tgt:`AUTORCC` targets together in an IDE,
e.g.  in MSVS.

:variable:`CMAKE_GLOBAL_AUTOGEN_TARGET`:
A global ``autogen`` target, that depends on all ``AUTOMOC`` or
:prop_tgt:`AUTOUIC` generated ``<ORIGIN>_autogen`` targets in the project,
will be generated when this variable is ``ON``.

:prop_tgt:`AUTOGEN_PARALLEL`:
This target property controls the number of ``moc`` or ``uic`` processes to
start in parallel during builds.

:prop_tgt:`AUTOGEN_COMMAND_LINE_LENGTH_MAX`:
This target property controls the limit when to use response files for
``moc`` or ``uic`` processes on Windows.

See the :manual:`cmake-qt(7)` manual for more information on using CMake
with Qt.