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<section id="unity-build-unique-id">
<span id="prop_tgt:UNITY_BUILD_UNIQUE_ID"></span><h1>UNITY_BUILD_UNIQUE_ID<a class="headerlink" href="#unity-build-unique-id" title="Permalink to this heading">ΒΆ</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.20.</span></p>
</div>
<p>The name of a valid C-identifier which is set to a unique per-file
value during unity builds.</p>
<p>When this property is populated and when <span class="target" id="index-0-prop_tgt:UNITY_BUILD"></span><a class="reference internal" href="UNITY_BUILD.html#prop_tgt:UNITY_BUILD" title="UNITY_BUILD"><code class="xref cmake cmake-prop_tgt docutils literal notranslate"><span class="pre">UNITY_BUILD</span></code></a>
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.</p>
<p>Given:</p>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">set_target_properties(</span><span class="nb">myTarget</span><span class="w"> </span><span class="no">PROPERTIES</span><span class="w"></span>
<span class="w"> </span><span class="no">UNITY_BUILD</span><span class="w"> </span><span class="s">"ON"</span><span class="w"></span>
<span class="w"> </span><span class="no">UNITY_BUILD_UNIQUE_ID</span><span class="w"> </span><span class="s">"MY_UNITY_ID"</span><span class="w"></span>
<span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
<p>the <code class="docutils literal notranslate"><span class="pre">MY_UNITY_ID</span></code> symbol is defined to a unique per-file value.</p>
<p>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:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Needed for when unity builds are disabled</span>
<span class="cp">#ifndef MY_UNITY_ID</span>
<span class="cp">#define MY_UNITY_ID</span>
<span class="cp">#endif</span>
<span class="k">namespace</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">MY_UNITY_ID</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="c1">// The name 'i' clashes (or could clash) with other</span>
<span class="w"> </span><span class="c1">// variables in other anonymous namespaces</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">42</span><span class="p">;</span><span class="w"></span>
<span class="p">}}</span><span class="w"></span>
<span class="kt">int</span><span class="w"> </span><span class="n">use_var</span><span class="p">()</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">MY_UNITY_ID</span><span class="o">::</span><span class="n">i</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>The pseudonymous 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.</p>
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