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template.html 4.3 KB
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  3. <title>Constrained template macros - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">Constrained template macros</h1></div></div></div>
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  14.   <p><em>Overridable</em>: All of the following macros are overridable, define before inclusion.</p>
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  15. 

  16. <p><em>Header</em>: <code>&lt;boost/outcome/config.hpp&gt;</code></p>
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  17. 

  18. <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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  19. <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>
    20. 

  20.   <span class="n">BOOST_OUTCOME_TREQUIRES</span><span class="p">(</span>
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  21.     <span class="c1">// If this is a valid expression
    22. 

  22. </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>
    23. 

  23.     <span class="c1">// If this predicate is true
    24. 

  24. </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">&lt;</span><span class="n">ErrorCondEnum</span><span class="o">&gt;</span><span class="p">)</span>
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  25.     <span class="c1">// Any additional requirements follow here ...
    26. 

  26. </span><span class="c1"></span>  <span class="p">)</span>
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  27.   <span class="k">constexpr</span> <span class="n">basic_result</span><span class="p">(</span><span class="n">ErrorCondEnum</span> <span class="o">&amp;&amp;</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>
    28. 

  28. </code></pre></div>
    29. 

  29. <p>Be aware that slightly different semantics occur for real C++ 20 constrained templates than for the SFINAE emulation.</p>
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  30. 

  31. <ul>
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  32. <li><p><a name="template"></a><code>BOOST_OUTCOME_TEMPLATE(template args ...)</code></p>
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  33. 

  34. <p>Begins a constrained template declaration.</p></li>
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  35. 

  36. <li><p><a name="trequires"></a><code>BOOST_OUTCOME_TREQUIRES(requirements ...)</code></p>
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  37. 

  38. <p>Specifies the requirements for the constrained template to be available for selection by the compiler.</p></li>
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  39. 

  40. <li><p><a name="texpr"></a><code>BOOST_OUTCOME_TEXPR(expression)</code></p>
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  41. 

  42. <p>A requirement that the given expression is valid.</p></li>
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  43. 

  44. <li><p><a name="tpred"></a><code>BOOST_OUTCOME_TPRED(boolean)</code></p>
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  45. 

  46. <p>A requirement that the given constant time expression is true.</p></li>
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  47. </ul>
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  48. 

  49. 

  50.         </div><p><small>Last revised: December 12, 2018 at 22:01:42 UTC</small></p>
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