What is the difference between static_cast and Implicit_cast?

Question

What is implicit_cast? when should I prefer implicit_cast rather than static_cast?

Answer 1

I'm copying over from a comment i made to answer this comment at another place.

You can down-cast with static_cast. Not so with implicit_cast. static_cast 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 implicit_cast will only accept implicit conversions. no down-cast, no void*->T*, no U->T if T has only explicit constructors for U.

Note that it's important to note the difference between a cast and a conversion. In the following no cast is going on

int a = 3.4;

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 boost::implicit_cast is a lovely combination of "cast using implicit conversions". Now the whole implementation of boost::implicit_cast is this (explained here):

template struct identity { typedef T type; };
template Dst implicit_cast(typename identity::type t)
{ return t; }

The idea is to use a non-deduced context for the parameter t. That will avoid pitfalls like the following:

call_const_version(implicit_cast(this)); // oops, wrong!

What was desired is to write it out like this

call_const_version(implicit_cast(this)); // right!

The compiler can't deduce what type the template parameter Dst should name, because it first must know what identity is, since it is part of the parameter used for deduction. But it in turn depends on the parameter Dst (identity 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.