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    <p>The following is taken from the talk <a href="https://www.youtube.com/watch?v=_zgq6_zFNGY" rel="noreferrer">"Variadic Templates are Funadic"</a> by Andrei Alexandrescu at GoingNative 2012. I can recommend it for a good introduction on variadic templates.</p> <hr> <p>There are two things one can do with a variadic pack. It's possible to apply <code>sizeof...(vs)</code> to get the number of elements and expand it.</p> <h1>Expansion rules</h1> <pre><code>Use Expansion Ts... T1, ..., Tn Ts&amp;&amp;... T1&amp;&amp;, ..., Tn&amp;&amp; x&lt;Ts,Y&gt;::z... x&lt;T1,Y&gt;::z, ..., x&lt;Tn,Y&gt;::z x&lt;Ts&amp;,Us&gt;... x&lt;T1&amp;,U1&gt;, ..., x&lt;Tn&amp;,Un&gt; func(5,vs)... func(5,v1), ..., func(5,vn) </code></pre> <p>Expansion proceeds inwards outwards. When expanding two lists in lock-step, they have to have the same size.</p> <h3>More examples:</h3> <pre><code>gun(A&lt;Ts...&gt;::hun(vs)...); </code></pre> <p>Expands all <code>Ts</code> in the template argument list of <code>A</code> and then the function <code>hun</code> gets expanded with all <code>vs</code>.</p> <pre><code>gun(A&lt;Ts...&gt;::hun(vs...)); </code></pre> <p>Expands all <code>Ts</code> in the template argument list of <code>A</code> and all <code>vs</code> as the function arguments for <code>hun</code>.</p> <pre><code>gun(A&lt;Ts&gt;::hun(vs)...); </code></pre> <p>Expands the function <code>hun</code> with <code>Ts</code> and <code>vs</code> in lock-step.</p> <h3>Note:</h3> <p><code>Ts</code> is not a type and <code>vs</code> is not a value! They are aliases for a list of types/values. Either list may be potentially empty. Both obey only specific actions. So the following is not possible:</p> <pre><code>typedef Ts MyList; // error! Ts var; // error! auto copy = vs; // error! </code></pre> <h1>Expansion loci</h1> <h3>Function arguments</h3> <pre><code>template &lt;typename... Ts&gt; void fun(Ts... vs) </code></pre> <h3>Initializer lists</h3> <pre><code>any a[] = { vs... }; </code></pre> <h3>Base specifiers</h3> <pre><code>template &lt;typename... Ts&gt; struct C : Ts... {}; template &lt;typename... Ts&gt; struct D : Box&lt;Ts&gt;... { /**/ }; </code></pre> <h3>Member initializer lists</h3> <pre><code>// Inside struct D template &lt;typename... Us&gt; D(Us... vs) : Box&lt;Ts&gt;(vs)... {} </code></pre> <h3>Tempate argument lists</h3> <pre><code>std::map&lt;Ts...&gt; m; </code></pre> <p>Will only compile if there is a possible match for the arguments.</p> <h3>Capture lists</h3> <pre><code>template &lt;class... Ts&gt; void fun(Ts... vs) { auto g = [&amp;vs...] { return gun(vs...); } g(); } </code></pre> <h3>Attribute lists</h3> <pre><code>struct [[ Ts... ]] IAmFromTheFuture {}; </code></pre> <p>It is in the specification, but there is no attribute that can be expressed as a type, yet.</p>
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    1. POWhat are the rules for the "..." token in the context of variadic templates?
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    1. CONice. Function arguments are left out of the loci, though :P
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    2. CO@Potatoswatter Thanks. The expansion in the function argument list was not mentioned.
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