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    <p><a href="http://en.cppreference.com/w/cpp/types/enable_if" rel="noreferrer"><code>std::enable_if</code></a> works through the principle of <a href="http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error" rel="noreferrer">substitution failure is not an error</a> (SFINAE), which states that when certain types of error occur in instantiating a function template, the program continues to compile with that function template not participating in overload resolution.</p> <p>For SFINAE to kick in, (a) it has to be used on a function (or method) template and (b) it has to be dependent on a template parameter. Your program fails on both counts.</p> <p>To make the <code>enable_if</code> dependent on a template parameter, the easiest thing is to add a default parameter:</p> <pre><code>template&lt;typename T = void&gt; typename enable_if&lt;is_64_bit::value, T&gt;::type my_memcpy(void* target, const void* source, size_t n) </code></pre> <p>However, this is not in general a sensible use of <code>enable_if</code>; since it depends on intercepting compile errors it tends to be expensive. In your case a template specialization would be a much better idea:</p> <pre><code>#include &lt;iostream&gt; template&lt;int = sizeof(void *)&gt; void my_memcpy(void* target, const void* source, size_t n); template&lt;&gt; void my_memcpy&lt;8&gt;(void* target, const void* source, size_t n) { std::cout &lt;&lt; "64 bit memcpy" &lt;&lt; std::endl; } template&lt;&gt; void my_memcpy&lt;4&gt;(void* target, const void* source, size_t n) { std::cout &lt;&lt; "32 bit memcpy" &lt;&lt; std::endl; } </code></pre>
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