1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 3<title>Constrained template macros - Boost.Outcome documentation</title> 4<link rel="stylesheet" href="../../css/boost.css" type="text/css"> 5<meta name="generator" content="Hugo 0.52 with Boostdoc theme"> 6<meta name="viewport" content="width=device-width,initial-scale=1.0"/> 7 8<link rel="icon" href="../../images/favicon.ico" type="image/ico"/> 9<body><div class="spirit-nav"> 10<a accesskey="p" href="../../reference/macros.html"><img src="../../images/prev.png" alt="Prev"></a> 11 <a accesskey="u" href="../../reference/macros.html"><img src="../../images/up.png" alt="Up"></a> 12 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../reference/macros/version.html"><img src="../../images/next.png" alt="Next"></a></div><div id="content"> 13 <div class="titlepage"><div><div><h1 style="clear: both">Constrained template macros</h1></div></div></div> 14 <p><em>Overridable</em>: All of the following macros are overridable, define before inclusion.</p> 15 16<p><em>Header</em>: <code><boost/outcome/config.hpp></code></p> 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> 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 <span class="n">BOOST_OUTCOME_TREQUIRES</span><span class="p">(</span> 21 <span class="c1">// If this is a valid expression 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 <span class="c1">// If this predicate is true 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"><</span><span class="n">ErrorCondEnum</span><span class="o">></span><span class="p">)</span> 25 <span class="c1">// Any additional requirements follow here ... 26</span><span class="c1"></span> <span class="p">)</span> 27 <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> 28</code></pre></div> 29<p>Be aware that slightly different semantics occur for real C++ 20 constrained templates than for the SFINAE emulation.</p> 30 31<ul> 32<li><p><a name="template"></a><code>BOOST_OUTCOME_TEMPLATE(template args ...)</code></p> 33 34<p>Begins a constrained template declaration.</p></li> 35 36<li><p><a name="trequires"></a><code>BOOST_OUTCOME_TREQUIRES(requirements ...)</code></p> 37 38<p>Specifies the requirements for the constrained template to be available for selection by the compiler.</p></li> 39 40<li><p><a name="texpr"></a><code>BOOST_OUTCOME_TEXPR(expression)</code></p> 41 42<p>A requirement that the given expression is valid.</p></li> 43 44<li><p><a name="tpred"></a><code>BOOST_OUTCOME_TPRED(boolean)</code></p> 45 46<p>A requirement that the given constant time expression is true.</p></li> 47</ul> 48 49 50 </div><p><small>Last revised: December 12, 2018 at 22:01:42 UTC</small></p> 51<hr> 52<div class="spirit-nav"> 53<a accesskey="p" href="../../reference/macros.html"><img src="../../images/prev.png" alt="Prev"></a> 54 <a accesskey="u" href="../../reference/macros.html"><img src="../../images/up.png" alt="Up"></a> 55 <a accesskey="h" href="../../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../../reference/macros/version.html"><img src="../../images/next.png" alt="Next"></a></div></body> 56</html> 57
