How to convert `const std::vector<T*>` into `std::span<const T*>`

Question

Title.

I am implementing this class:

#include <span>
#include <vector>

class MyClass {
public:
    std::span<int *> numbers(void) {
        return m_numbers;
    }

    std::span<const int *> cnumbers(void) const {
        // What to do here?
    }

private:
    std::vector<int *> m_numbers;
};

My first attempt was to use the automatic conversion exhibited in the non-const function (I believe this has something to do with std:decay? I don't really understand it).

return m_numbers

This fails to compile, with no known conversion.

One implementation I've found is this:

return std::span(m_numbers.begin(), m_numbers.end());

this returns a std::span<int * const> though, which is not exactly what I want. Variations of this using cbegin do not seem to help.

I'm targeting c++20

Answer 1

You're trying to break const-correctness. Returning std::span<const int *> would've allowed modification of those elements (pointers themselves, which won't be const).

The correct return type should be std::span<const int * const>, note the additional const at the right.