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POconstruction helper make_XYZ allowing RVO and type deduction even if XZY has noncopy constraint
 

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  1. POconstruction helper make_XYZ allowing RVO and type deduction even if XZY has noncopy constraint
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    <p><strong>UPDATE1:</strong> C++17 added type deduction for constructors - which does not imply that the free function is an inferior solution.</p> <p><strong>UPDATE2:</strong> C++17 added guaranteed copy elision (the copy does not even take place conceptually). So with C++17 my code actually works and with optimal performance. But Martinho's code using brace initialisation for the return value is still the cleaner solution I believe. But checkout <a href="https://stackoverflow.com/a/48533239/2712726">this answer from Barry</a> and the comment from T.C.</p> <p><strong>OLD POST:</strong> Type deduction does not work for constructors (at least until and including C++11). The common solution is to rely on RVO (Return Value Optimisation) and write a make_XYZ template function that forwards its parameters to the constructor. An example is <code>std::make_tuple</code>.</p> <p>Any template acrobat who knows a workaround to make this work when nocopy policy is in the way? A valid solution must still allow RVO to happen.</p> <p>Additionally, will the requirement for any make_XYZ disappear with C++14?</p> <pre><code>#include &lt;iostream&gt; template &lt;typename T&gt; struct XYZ { // remove following two lines to make the code compile XYZ (XYZ const &amp; rhs) = delete; XYZ (XYZ &amp;&amp; rhs) = delete; T i; XYZ (T i):i(i) { } }; template &lt;typename T&gt; XYZ&lt;T&gt; make_XYZ (T &amp;&amp; i) { return XYZ&lt;T&gt;(std::forward&lt;T&gt;(i)); } int main () { auto x = make_XYZ(1); std::cout &lt;&lt; x.i &lt;&lt; std::endl; } </code></pre>
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    1. COWhat if you change the return type of make_XYZ to const-reference? I'm not sure if you'll also need to change to `auto const&` at the call site.
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    2. CO@JohnZwinck, no, that would crash
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    3. COAre you sure? There are special rules for lifetime promotion of the referent of const references. You can read more about that here: http://stackoverflow.com/questions/2615162/return-value-not-a-reference-from-the-function-bound-to-a-const-reference-in/2615181#2615181
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