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    <pre><code>template&lt;typename T&gt; struct is_io_type : std::false_type {}; template&lt;&gt; struct is_io_type&lt;int&gt; : std::true_type {}; template&lt;&gt; struct is_io_type&lt;double&gt; : std::true_type {}; template&lt;&gt; struct is_io_type&lt;std::string&gt; : std::true_type {}; </code></pre> <p>Then, instead of your macro, simply type:</p> <pre><code>static_assert( is_io_type&lt;T&gt;::value, "Type not defined for this template." ) </code></pre> <p>which is about as verbose and not nearly as obscure or hard to figure out what the heck is going on.</p> <p>Now, suppose we really need the <code>is_base_of</code> functionality.</p> <pre><code>template&lt;typename T,typename=void&gt; struct is_io_type : std::false_type {}; template&lt;&gt; struct is_io_type&lt;int,void&gt; : std::true_type {}; template&lt;typename T&gt; struct is_io_type&lt;T,typename std::enable_if&lt;std::is_base_of&lt;int, T&gt;::type&gt; : std::true_type {}; template&lt;&gt; struct is_io_type&lt;double,void&gt; : std::true_type {}; template&lt;typename T&gt; struct is_io_type&lt;T,typename std::enable_if&lt;std::is_base_of&lt;double, T&gt;::type&gt; : std::true_type {}; template&lt;&gt; struct is_io_type&lt;std::string,void&gt; : std::true_type {}; template&lt;typename T&gt; struct is_io_type&lt;T,typename std::enable_if&lt;std::is_base_of&lt;std::string, T&gt;::type&gt; : std::true_type {}; </code></pre> <p>which gets pretty verbose.</p> <p>But rather than creating a variardic macro expansion at the point where we use the macro, we can instead use a macro to build the above type trait specializations.</p> <p>So you first create your type trait, then extend it:</p> <pre><code>CREATE_TYPE_CATEGORY( is_io_type ); ADD_BASE_TYPE_CATEGORY( is_io_type, int ); ADD_BASE_TYPE_CATEGORY( is_io_type, double ); ADD_BASE_TYPE_CATEGORY( is_io_type, std::string ); </code></pre> <p>which would write the code above a bit less verbosely. It also supports distributed trait modification: if someone creates a new type that should be <code>is_io_type</code>, they can use the above macro after introducing the type and it (and its descendents) become an io type.</p>
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    1. USYakk - Adam Nevraumont
    1. USYakk - Adam Nevraumont
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    1. CO+1 Traits are a PITA to write, but it's the clearest, easiest thing to read/extend. For your macro fix, you can always write a macro to take a list of types and write the template specialization for you.
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      1. USKitsuneYMG
 

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