How do I remove code duplication between similar const and non-const member functions?

Question

Let\'s say I have the following class X where I want to return access to an internal member:

class Z
{
    // details
};

class X
{
    std::vec         


        

Answer 1

C++17 has updated the best answer for this question:

T const & f() const {
    return something_complicated();
}
T & f() {
    return const_cast(std::as_const(*this).f());
}

This has the advantages that it:

• Is obvious what is going on
• Has minimal code overhead -- it fits in a single line
• Is hard to get wrong (can only cast away volatile by accident, but volatile is a rare qualifier)

If you want to go the full deduction route then that can be accomplished by having a helper function

template
constexpr T & as_mutable(T const & value) noexcept {
    return const_cast(value);
}
template
constexpr T * as_mutable(T const * value) noexcept {
    return const_cast(value);
}
template
constexpr T * as_mutable(T * value) noexcept {
    return value;
}
template
void as_mutable(T const &&) = delete;

Now you can't even mess up volatile, and the usage looks like

decltype(auto) f() const {
    return something_complicated();
}
decltype(auto) f() {
    return as_mutable(std::as_const(*this).f());
}