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POCan type to value meta-function be used as variable alias in C++14?
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  1. POCan type to value meta-function be used as variable alias in C++14?
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    copied!<p>While looking at C++14 the meta-function alias proposals (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3655.pdf" rel="nofollow">TransformationTraits Redux, v2,N3655</a>), I noticed that, not only type to type transformations (such as <code>add_const</code>), type to value meta-functions (such as <code>is_void</code> ) are also type aliased. (Which are not present in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf" rel="nofollow">N3797</a>). </p> <p>Is there any advantage of aliasing type to value meta-functions? I think, it is possible to use them without those aliases, such as <code>enable_if_t&lt;is_void&lt;T&gt;::value,T&gt;</code> or <code>enable_if_t&lt;is_void&lt;T&gt;{}(),T&gt;</code> when the conversion operation is present. (I guess <code>is_void&lt;T&gt;::type::value</code> is same as <code>is_void&lt;T&gt;::value</code>)</p> <p>If the type to value meta-functions need to be aliases, wont it be better to alias them as variable template ( I do not have C++14 compiler and never used variable template. So the syntax may be wrong)? e.g. alias <code>is_void</code> as </p> <pre><code>template &lt;class T&gt; constexpr bool is_void_t = is_void&lt;T&gt;::value; </code></pre> <p>Instead of </p> <pre><code>template &lt;class T&gt; using is_void_t = typename is_void&lt;T&gt;::type; </code></pre> <p>Then one can write <code>enable_if_t&lt;is_void_t&lt;T&gt;,T&gt;</code> without boost style <code>enable_if</code>,and composing expression will be easier (e.g. <code>enable_if_t&lt;(is_void_t&lt;T&gt; || is_integral_t&lt;T&gt;),T&gt;</code></p>
 

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