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POHow to initialize with multiple return values in c++(0x)
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  1. POHow to initialize with multiple return values in c++(0x)
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    copied!<p><code>tuple</code> in boost and TR1/c++0x provides a convenient (for the writer of the function) method to return two values from a function--however it seems to damage one major feature of the language for the caller: the ability to simply use the function to initialize a variable:</p> <pre><code>T happy(); const auto meaningful_name(happy()); // RVO means no excess copies </code></pre> <p>but for:</p> <pre><code>tuple&lt;T,U&gt; sad(); </code></pre> <p>we either have to surrender the ability to pick a meaningful name for our return values, and use <code>get&lt;n&gt;()</code> everywhere:</p> <pre><code>const auto two_unrelated_things(sad()); </code></pre> <p>or make a temporary:</p> <pre><code>const auto unwanted_named_temporary(sad()); const auto one_name(get&lt;0&gt;(unwanted_named_temporary)); const auto two_name(get&lt;1&gt;(unwanted_named_temporary)); </code></pre> <p>or switch from initialization to assignment, which only works when the types are assignable, and breaks <code>auto</code>:</p> <pre><code>tuple_element&lt;0, decltype(sad())&gt;::type one_mutable; // there might be a less tuple_element&lt;1, decltype(sad())&gt;::type two_mutable; // verbose way tie(one_mutable,two_mutable) = sad(); </code></pre> <p>or do something unnatural to a local class:</p> <pre><code>const struct ugh { ugh( decltype(sad()) rhs ) : one_name(get&lt;0&gt;(rhs)), two_name(get&lt;1&gt;(rhs)) {} const tuple_element&lt;0, decltype(sad())&gt;::type one_name; const tuple_element&lt;1, decltype(sad())&gt;::type two_name; } stuff(sad()); // at least we avoid the temporary and get initialization </code></pre> <p>Is there a better way? I'm using VC10 compatible constructs above, would anything in full c++0x or boost help? </p> <p>Ideally it would:</p> <ul> <li>allow me to use initialization, not just assignment</li> <li>let the <strong>caller</strong> pick the names for the returned-into variables</li> <li>not make extra copies</li> <li>work for both stack variables and class members</li> <li>possibly be a big crazy template library, but have sane syntax for caller and function writer</li> </ul>
 

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