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

There is an example of const_cast usage in c++ primer(5th edition) book. Below function returns reference to const string

// return a reference to the shorter of two strings
const string &shorterString(const string &s1, const string
&s2)
{
    return s1.size() <= s2.size() ? s1 : s2;
}

The book then mentions the case when we want a non const reference.

We can call the function on a pair of nonconst string arguments, but we’ll get a reference to a const string as the result. We might want to have a version of shorterString that, when given nonconst arguments, would yield a plain reference. We can write this version of our function using a const_cast:

string &shorterString(string &s1, string &s2)
{
    auto &r = shorterString(const_cast(s1),
                            const_cast(s2));
    return const_cast(r);
}

This version calls the const version of shorterString by casting its arguments to references to const. That function returns a reference to a const string, which we know is bound to one of our original, nonconst arguments. Therefore, we know it is safe to cast that string back to a plain string& in the return.

According to the book, it should be used if we know it is safe to cast.