What does "cannot convert 'this' pointer from 'const hand' to 'hand &' mean? (C++)

Question

The error occurs when I try to do this

friend std::ostream& operator<<(std::ostream& os, const hand& obj)
{
    return obj.show(os, obj);
}

where hand is a class I've created, and show is

std::ostream& hand::show(std::ostream& os, const hand& obj)
{
    return os<<obj.display[0]<<obj.display[1]<<obj.display[2]<<obj.display[3]<<obj.display[4];
}

where display is declared as char display[6].

Does anyone know what the error means?

Answer 1

You need the function itself to also be const (notice the "const" at the end of the first line):

std::ostream& hand::show(std::ostream& os, const hand& obj) const
{
    return os<<obj.display[0]<<obj.display[1]<<obj.display[2]<<obj.display[3]<<obj.display[4];
}

Answer 2

You need to make hand::show(...) a const method; and it doesn't make sense to pass it obj reference -- it already receives that as the 'this' pointer.

This should work:

class hand {
public:
  std::ostream& show(std::ostream &os) const;
...
};

friend std::ostream& operator<<(std::ostream& os, const hand& obj)
{
    return obj.show(os);
}