Question

Normally, if I need to detect whether a type is const I just use boost::is_const. However, I ran into trouble when trying to detect the const-ness of a nested type. Consider the following traits template, which is specialized for const types:

template <class T>
struct traits
{
    typedef T& reference;
};

template <class T>
struct traits<const T>
{
    typedef T const& reference;
};

The problem is that boost::is_const doesn’t seem to detect that traits<const T>::reference is a const type.

For example:

std::cout << std::boolalpha;
std::cout << boost::is_const<traits<int>::reference>::value << " ";
std::cout << boost::is_const<traits<const int>::reference>::value << std::endl;

This outputs: false false

Why doesn’t it output false true?

Answer 1

Because the reference is not const, it’s the type it’s referencing that is const. Right, there are no const references. So imagine that the reference is a pointer, then the difference is easier to understand: int const* not const, int *const is const.

Use remove_reference to get the actual const type:

cout << boost::is_const<
            boost::remove_reference<int const&>::type>::value << '\n';