Question

The warning is being generated by the ostream & operator<<(ostream &os, A &A0) function.

Here’s how the class’s defined:

class A
{
    public:
        friend ostream & operator<<(ostream &os, A &A0);
        A& operator=(string strSlot_);
        A& operator+(string strSlot_);
        A& operator+(const A &B);
        A& operator=(const A &B);
        string slotReturn();
        A(string strSlot_);
        A(const A &object);
        void slotChange();
        void sCout();
        ~A();
        A();
    private:
        string strSlot;
        int n;
};


ostream & operator<<(ostream &os, A &A0)
{
    os << "strSlot = \"" << A0.slotReturn() << "\"" << endl;
}

string A::slotReturn()
{
    return strSlot;
}

---

The question is, what is it supposed to return? *this doesn’t seem to work (because it’s a friend?). (It’s merely a warning, but still, I just want to know.)

Also, why can’t I pass A &A0 as a const (in which case the error is: “passing ‘const A’ as ‘this’ argument of ‘std::string A::slotReturn()’ discards qualifiers”)?

Answer 1

You need to return the std::ostream& itself:

ostream & operator<<(ostream &os, A &A0)
{
   return  os << "strSlot = \"" << A0.slotReturn() << "\"" << endl;
}

As an aside, the terminology is slightly off. There is no “friend class”. The std::ostream& operator<< is the friend here. But it doesn’t even need to be a friend, because it just calls a public member function of A. So you can remove the friend declaration.

You should probably also make slotReturn() a const method:

string slotReturn() const;
        //            ^ const method

and modify the ostream& operator<< to take a const reference:

ostream & operator<<(ostream& os, const A& A0) { .... }

This will allow you to print out temporaries:

std::cout << A("I am a temporary!") << "\n";