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UNITY_BUILD_UNIQUE_ID
---------------------
.. versionadded:: 3.20
The name of a valid C-identifier which is set to a unique per-file
value during unity builds.
When this property is populated and when :prop_tgt:`UNITY_BUILD`
is true, the property value is used to define a compiler definition
of the specified name. The value of the defined symbol is unspecified,
but it is unique per file path.
Given:
.. code-block:: cmake
set_target_properties(myTarget PROPERTIES
UNITY_BUILD "ON"
UNITY_BUILD_UNIQUE_ID "MY_UNITY_ID"
)
the ``MY_UNITY_ID`` symbol is defined to a unique per-file value.
One known use case for this identifier is to disambiguate the
variables in an anonymous namespace in a limited scope.
Anonymous namespaces present a problem for unity builds because
they are used to ensure that certain variables and declarations
are scoped to a translation unit which is approximated by a
single source file. When source files are combined in a unity
build file, those variables in different files are combined in
a single translation unit and the names clash. This property can
be used to avoid that with code like the following:
.. code-block:: cpp
// Needed for when unity builds are disabled
#ifndef MY_UNITY_ID
#define MY_UNITY_ID
#endif
namespace { namespace MY_UNITY_ID {
// The name 'i' clashes (or could clash) with other
// variables in other anonymous namespaces
int i = 42;
}}
int use_var()
{
return MY_UNITY_ID::i;
}
The pseudononymous namespace is used within a truly anonymous namespace.
On many platforms, this maintains the invariant that the symbols within
do not get external linkage when performing a unity build.
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