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    <p>In the context of variadic template, the ellipsis <code>...</code> is used to unpack the template parameter pack if it appears on the right side of an expression (call this expression <em>pattern</em> for a moment). The rule is that whatever <em>pattern</em> is on the left side of <code>...</code> is repeated &mdash; the unpacked patterns (call them <em>expressions</em> now) are separated by comma <code>,</code>. </p> <p>It can be best understood by some examples. Suppose you have this function template:</p> <pre><code>template&lt;typename ...T&gt; void f(T ... args) { g( args... ); //pattern = args h( x(args)... ); //pattern = x(args) m( y(args...) ); //pattern = args (as argument to y()) n( z&lt;T&gt;(args)... ); //pattern = z&lt;T&gt;(args) } </code></pre> <p>Now if I call this function passing <code>T</code> as <code>{int, char, short}</code>, then each of the function call is expanded as:</p> <pre><code>g( arg0, arg1, arg2 ); h( x(arg0), x(arg1), x(arg2) ); m( y(arg0, arg1, arg2) ); n( z&lt;int&gt;(arg0), z&lt;char&gt;(arg1), z&lt;short&gt;(arg2) ); </code></pre> <p>In the code you posted, <code>std::forward</code> follows the fourth pattern illustrated by <code>n()</code> function call.</p> <p><strong>Note the difference between <code>x(args)...</code> and <code>y(args...)</code> above!</strong></p> <hr> <p>You can use <code>...</code> to initialize an array also as:</p> <pre><code>struct data_info { boost::any data; std::size_t type_size; }; std::vector&lt;data_info&gt; v{{args, sizeof(T)}...}; //pattern = {args, sizeof(T)} </code></pre> <p>which is expanded to this:</p> <pre><code>std::vector&lt;data_info&gt; v { {arg0, sizeof(int)}, {arg1, sizeof(char)}, {arg2, sizeof(short)} }; </code></pre> <hr> <p>I just realized a pattern could even include access specifier such as <code>public</code>, as shown in the following example:</p> <pre><code>template&lt;typename ... Mixins&gt; struct mixture : public Mixins ... //pattern = public Mixins { //code }; </code></pre> <p>In this example, the pattern is expanded as:</p> <pre><code>struct mixture__instantiated : public Mixin0, public Mixin1, .. public MixinN </code></pre> <p>That is, <code>mixture</code> derives <em>publicly</em> from all the base classes.</p> <p>Hope that helps.</p>
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    1. POWhat are the rules for the "..." token in the context of variadic templates?
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    1. USNawaz
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    1. POWhat are the rules for the "..." token in the context of variadic templates?
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    1. CORegarding the expression that is matched; It should be the *largest possible* expression, should it not? For instance `x+args...` should be expanded to `x+arg0,x+arg1,x+arg2`, not `x+arg0,arg1,arg2`.
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    2. COSo the `...` applies to every expandable entity in the pattern.
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      1. USLightness Races in Orbit
    3. CO@bitmask: Yes. `x+args...` should expand to `x+arg0,x+arg1,x+arg2`, **not** `x+arg0,arg1,arg2`.
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