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UNITY_BUILD
-----------
Should the target source files be processed into batches for
faster compilation. This feature is known as "Unity build",
or "Jumbo build".
The ``C`` and ``CXX`` source files are grouped separately.
This property is initialized by the value of the
:variable:`CMAKE_UNITY_BUILD` variable if it is set when
a target is created.
.. note::
It's not recommended to directly set :prop_tgt:`UNITY_BUILD`
to ``ON``, but to instead set :variable:`CMAKE_UNITY_BUILD` from
the command line. However, it IS recommended to set
:prop_tgt:`UNITY_BUILD` to ``OFF`` if you need to ensure that a
target doesn't get a unity build.
The batch size can be specified by setting
:prop_tgt:`UNITY_BUILD_BATCH_SIZE`.
The batching of source files is done by adding new sources files
which will ``#include`` the source files, and exclude them from
building by setting :prop_sf:`HEADER_FILE_ONLY` to ``ON``.
.. note::
Marking the original sources with :prop_sf:`HEADER_FILE_ONLY`
is considered an implementation detail that may change in the
future because it does not work well in combination with
the :variable:`CMAKE_EXPORT_COMPILE_COMMANDS` variable.
ODR (One definition rule) errors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Since multiple source files are included into one source file,
it can lead to ODR errors. This section contains properties
which help fixing these errors.
The source files marked by :prop_sf:`GENERATED` will be skipped
from unity build. This applies also for the source files marked
with :prop_sf:`SKIP_UNITY_BUILD_INCLUSION`.
The source files that have :prop_sf:`COMPILE_OPTIONS`,
:prop_sf:`COMPILE_DEFINITIONS`, :prop_sf:`COMPILE_FLAGS`, or
:prop_sf:`INCLUDE_DIRECTORIES` will also be skipped.
With the :prop_tgt:`UNITY_BUILD_CODE_BEFORE_INCLUDE` and
:prop_tgt:`UNITY_BUILD_CODE_AFTER_INCLUDE` one can specify code
to be injected in the unity source file before and after every
``#include`` statement.
.. note::
The order of source files defined in the ``CMakeLists.txt`` will
be preserved into the generated unity source files. This can
be used to manually enforce a specific grouping based on the
:prop_tgt:`UNITY_BUILD_BATCH_SIZE` target property.
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