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<title>Constrained template macros - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both">Constrained template macros</h1></div></div></div>
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<p><em>Overridable</em>: All of the following macros are overridable, define before inclusion.</p>
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<p><em>Header</em>: <code><boost/outcome/config.hpp></code></p>
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<p>These macros expand into either the syntax for directly specifying constrained templates in C++ 20, or into a SFINAE based emulation for earlier C++ versions. Form of usage looks as follows:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">BOOST_OUTCOME_TEMPLATE</span><span class="p">(</span><span class="k">class</span><span class="err"> </span><span class="nc">ErrorCondEnum</span><span class="p">)</span>
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<span class="n">BOOST_OUTCOME_TREQUIRES</span><span class="p">(</span>
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<span class="c1">// If this is a valid expression
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</span><span class="c1"></span> <span class="n">BOOST_OUTCOME_TEXPR</span><span class="p">(</span><span class="n">error_type</span><span class="p">(</span><span class="n">make_error_code</span><span class="p">(</span><span class="n">ErrorCondEnum</span><span class="p">()))),</span>
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<span class="c1">// If this predicate is true
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</span><span class="c1"></span> <span class="n">BOOST_OUTCOME_TPRED</span><span class="p">(</span><span class="n">predicate</span><span class="o">::</span><span class="k">template</span> <span class="n">enable_error_condition_converting_constructor</span><span class="o"><</span><span class="n">ErrorCondEnum</span><span class="o">></span><span class="p">)</span>
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<span class="c1">// Any additional requirements follow here ...
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</span><span class="c1"></span> <span class="p">)</span>
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<span class="k">constexpr</span> <span class="n">basic_result</span><span class="p">(</span><span class="n">ErrorCondEnum</span> <span class="o">&&</span><span class="n">t</span><span class="p">,</span> <span class="n">error_condition_converting_constructor_tag</span> <span class="cm">/*unused*/</span> <span class="o">=</span> <span class="p">{});</span>
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</code></pre></div>
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<p>Be aware that slightly different semantics occur for real C++ 20 constrained templates than for the SFINAE emulation.</p>
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<ul>
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<li><p><a name="template"></a><code>BOOST_OUTCOME_TEMPLATE(template args ...)</code></p>
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<p>Begins a constrained template declaration.</p></li>
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<li><p><a name="trequires"></a><code>BOOST_OUTCOME_TREQUIRES(requirements ...)</code></p>
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<p>Specifies the requirements for the constrained template to be available for selection by the compiler.</p></li>
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<li><p><a name="texpr"></a><code>BOOST_OUTCOME_TEXPR(expression)</code></p>
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<p>A requirement that the given expression is valid.</p></li>
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<li><p><a name="tpred"></a><code>BOOST_OUTCOME_TPRED(boolean)</code></p>
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<p>A requirement that the given constant time expression is true.</p></li>
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</ul>
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</div><p><small>Last revised: December 12, 2018 at 22:01:42 UTC</small></p>
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