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