Operator << in abstract classes, c++

Question

If I have an abstract class, let's call it "Vertebrate", it has a field std::string name; and it has a pure virtual method virtual void print(std::ostream&) const noexcept = 0; which will be overridden in child classes and called in operator<<.

I get how polymorphism works, and how to implement the operator<< in inherited classes.

What I don't get:

I don't get this: how to implement an operator<< in that abstract class, that uses the virtual print function. Why this code does not work? My abstract class needs to have operator<<.

virtual void print(std::ostream&) const noexcept = 0;
std::ostream & operator<<(std::ostream & str, Member &obj)
{
    return obj.print(str);
}

That is the abstract class code.

Answer 1

You are trying to return the result of print which is void, but operator<< should return std::ostream.

The following should work:

class Vertebrate
{
    // ...
    virtual void print(std::ostream&) const noexcept = 0;
};

std::ostream& operator<<(std::ostream& stream, Vertebrate& obj)
{
    obj.print(stream);
    return stream;
}