Virtual overloaded operators >> and <<

Question

I need an interface that would require its subclasses to overload << and >>, but I'm not quite sure how since these operators aren't overloaded as member functions:

std::istream& operator>> (std::istream& in, Student& student) {
    in >> student.name >> student.group;
    for (int& i : student.marks) { in >> i; }
    return in;
}

Maybe there's a way to make it a member function?

Declare a member virtual function for example like virtual std::ostream & out( std::ostream &os = std::cout ) const; And call it inside the body of the overloaded operator <<. — Vlad from Moscow, Jun 01 '20 at 21:11

score 10 · Accepted Answer · edited Jun 01 '20 at 21:25

10

You could do something like this:

class StudentInterface
{
public:
    virtual void readSelfFrom(std::istream& in) = 0;
};

std::istream& operator>> (std::istream& in, StudentInteface& student) 
{
    student.readSelfFrom(in);
    return in;
}

And then let users derive from StudentInterface, eg:

class Student: public StudentInterface
{
public:
    void readSelfFrom(std::istream& in) override
    {
        in >> name >> group;
        for (int& i : marks) { in >> i; }
    }
};

edited Jun 01 '20 at 21:25

Remy Lebeau

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answered Jun 01 '20 at 21:12

Jeffrey

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The `Student` is actually the one that should be derived, so I suppose that I leave `virtual void readSelfFrom(std::istream& in) = 0`. – Martian Jun 01 '20 at 21:14
1

@Ayxan nice! Thank you. – Martian Jun 01 '20 at 21:22

score 5 · Answer 2 · answered Jun 01 '20 at 21:13

A general approach in such a case is to declare in the base class a virtual member function like

virtual std::ostream & out( std::ostream &os = std::cout ) const;

In derived classes the function will be overriden.

Then the operator << can look like

std::ostream & operator <<( std::ostream &os, const Base &obj )
{
    return obj.out( os );
}

A similar way can be defined the operator >> only in this case the virtual member function will not be constant..

Virtual overloaded operators >> and <<

2 Answers2