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copied!<p>I'm copying over from a comment i made to <a href="https://stackoverflow.com/questions/385556/how-does-c-pick-which-overloaded-function-to-call/394742#394742">answer this comment</a> at another place.</p> <blockquote> <p>You can down-cast with <code>static_cast</code>. Not so with <code>implicit_cast</code>. <code>static_cast</code> basically allows you to do any implicit conversion, and in addition the reverse of any implicit conversion (up to some limits. you can't downcast if there is a virtual base-class involved). But <code>implicit_cast</code> will <em>only</em> accept implicit conversions. no down-cast, no <code>void*->T*</code>, no <code>U->T</code> if T has only explicit constructors for U.</p> </blockquote> <p>Note that it's important to note the difference between a cast and a conversion. In the following no cast is going on</p> <pre><code>int a = 3.4; </code></pre> <p>But an implicit conversion happens from double to int. Things like an "implicit cast" don't exist, since a cast is always an explicit conversion request. The name construct for <code>boost::implicit_cast</code> is a lovely combination of "cast using implicit conversions". Now the whole implementation of <code>boost::implicit_cast</code> is this (explained <a href="https://stackoverflow.com/questions/862858/what-other-useful-casts-can-be-used-in-c/863761#863761">here</a>):</p> <pre><code>template<typename T> struct identity { typedef T type; }; template<typename Dst> Dst implicit_cast(typename identity<Dst>::type t) { return t; } </code></pre> <p>The idea is to use a non-deduced context for the parameter <code>t</code>. That will avoid pitfalls like the following:</p> <pre><code>call_const_version(implicit_cast(this)); // oops, wrong! </code></pre> <p>What was desired is to write it out like this</p> <pre><code>call_const_version(implicit_cast<MyClass const*>(this)); // right! </code></pre> <p>The compiler can't deduce what type the template parameter <code>Dst</code> should name, because it first must know what <code>identity<Dst></code> is, since it is part of the parameter used for deduction. But it in turn depends on the parameter <code>Dst</code> (<code>identity</code> could be explicitly specialized for some types). Now, we got a circular dependency, for which the Standard just says such a parameter is a non-deduced context, and an explicit template-argument must be provided. </p>

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