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    copied!<p><em>“Is there any advantage of aliasing type to value meta-functions?”</em></p> <p>Quoting from the linked N3655, before the specifications of <code>is_void_t</code> and siblings (page 4):</p> <blockquote> <h3>4 &nbsp; Supplementary proposed wording</h3> <p>The following wording is provided in response to LWG’s request that aliases for <b><code>::type</code></b> members be consistently provided for all the type traits, not only for those classified as <i>TransformationTraits</i>. Accordingly, this section provides the specifications needed in order to complete the set.</p> </blockquote> <p>That explains why <em>“type to value meta-functions (such as <code>is_void</code> ) are also type aliased”</em> in the proposal: for consistency.</p> <p>Moreover, it would be wrong to use the name “<code>is_void_<b>t</b></code>” to alias <code>is_void&lt;T&gt;::<b>value</b></code>. The “<code>_t</code>” suffix always denotes a <strong>type</strong>. For the value, maybe one could use a “<strong><code>_v</code></strong>” suffix. So we would then have both:</p> <pre><code>template &lt;class T&gt; using is_void_t = typename is_void&lt;T&gt;::type; template &lt;class T&gt; constexpr bool is_void_v = is_void&lt;T&gt;::value; </code></pre> <p>That should compile in C++14, and then you could write things like <code>enable_if_t&lt;(is_void_v&lt;T&gt; || is_integral_v&lt;T&gt;),T&gt;</code>. But I feel that the value alias is “less needed” than the type one: it doesn't save as much typing, and, as you said, you can use the short <code>is_void&lt;T&gt;{}()</code> with the same effect (and for C++11, <code>is_void&lt;T&gt;{}</code> is often enough, thanks to its <code>operator bool()</code>).</p>
 

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