Correct usage(s) of const_cast<>

Question

As a common rule, it is very often considered a bad practice to use const_cast<>() in C++ code as it reveals (most of the time) a flaw in the design.

While I totally agree with this, I however wonder what are the cases were using const_cast<>() is ok and the only solution.

Could you guys please give me some examples you know/you encountered ?

Thank you very much.

Answer 1

it is pretty much designed to be only used with legacy APIs that are not const correct i.e. with a function you can't change that has non const interface but doesn't actually mutate anything on the interface

Answer 2

Like others have said, its primary purpose is to remove const from objects in order to pass to non-const correct functions you know won't modify the argument.

There is a trick (by Meyers?) to avoid code duplication, and it goes like this:

struct foo {     const return_type& get(void) const     {         // fancy pants code that you definitely         // don't want to repeat          return theValue; // and got it     }      return_type& get(void)     {         // well-defined: Add const to *this,         // call the const version, then         // const-cast to remove const (because         // *this is non-const, this is ok)         return const_cast<return_type&>(static_cast<const foo&>(*this).get());     } };