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.

Answer 1

One very legitimate use of this is when you have both a const and non const api (for const and non const objects respectively) as in

class Bar {
   const SomeType& foo() const; 
   SomeType& foo();
}

Then since we don't want to duplicate the code in both functions we often use

class Bar {
   SomeType& foo() {
      //Actual implementation 
   }
   const SomeType& foo() const {
        return const_cast<Bar*>(this)->foo();
   }
};

This is of course assuming that foo does not do something that violates the const semantics.

Answer 2

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