I'm having a bit of punt into the murky world of metaprogramming, so this
<div>might be a bit naive...!</div><div><br></div><div>I&#39;ve been using Boost.enable_if for a while, and now I&#39;ve just encountered Boost.MPL.if_</div><div>and wonder if enable_if is simply a special case of MPL.if_.</div>

<div><br></div><div>Given</div><div><br></div><div><font face="&#39;courier new&#39;, monospace">struct empty { };</font></div><div><font face="&#39;courier new&#39;, monospace">template &lt;typename T&gt; struct my_condition : /* boost true_ or false_ */;</font></div>

<div><br></div><div>is</div><div><br></div><div><font face="&#39;courier new&#39;, monospace">template &lt;typename T&gt; typename enable_if&lt;my_condition&lt;T&gt;, int&gt;::type f( ) { /* .... */ }</font></div><div><br>

</div><div>the same as</div><div><br></div><div><font face="&#39;courier new&#39;, monospace">template &lt;typename T&gt; typename eval_if&lt;my_condition&lt;T&gt;, identity&lt;int&gt;, empty&gt;::type f( ) { /* ... */ }</font></div>

<div><br></div><div>Or is there some subtlety I&#39;m missing (which is quite likely!)</div><div><br></div><div>Thx</div><div><br></div><div>- Rob.</div><div><br></div>