Operator overloading << error: must take exactly one argument

Hi all!

I can't get my head around the compile error I get when overloading the << operator:
error: ‘QDataStream& Coordinate::operator<<(QDataStream&, const Coordinate&)’ must take exactly one argument

Here is the code:

coordinate.h:

Code:

---

class Coordinate
{
public:
    Coordinate();
 
    double x, y, z;
 
    // output
    QDataStream &operator <<(QDataStream& out, const Coordinate& c);
};

---

coordinate.cpp:

Code:

---

#include "coordinate.h"
 
Coordinate::Coordinate()
{
    x = y = z = 0.;
}
 
QDataStream& operator <<(QDataStream& out, const Coordinate& c)
{
    out <<  "(" <<  c.x << ", " << c.y << ", " << c.z << ")";
    return out;
}

---

What is wrong / am I not getting?

Thanks!

The operator should be a standalone function and not a method of your class.

If you want to make the data members private you can add friend operator inside your class:

Code:

---

class Coordinate
{
public:
    Coordinate();
    // output
    friend QDataStream &operator <<(QDataStream& out, const Coordinate& c);
private:
        double x, y, z;
};

---

Quote:

---

Originally Posted by wysota

The operator should be a standalone function and not a method of your class.

---

Worked! Thank you!

Why does operator== not give any troubles?

Quote:

---

Originally Posted by Zlatomir

If you want to make the data members private you can add friend operator inside your class:

---

Ok, thanks for your info!

Quote:

---

Originally Posted by nomiz

Worked! Thank you!

Why does operator== not give any troubles?

---

If you put the operator inside the class definition, then the operator takes the current object ("this") implicitly as its first parameter.

So operator== comparing A against B can be placed in class A but not in B (or it can be a standalone function taking two parameters -- A and B). If you have a standalone operator << that takes QDataStream and MyStruct as its parameters, you could put it in QDataStream class but not in MyStruct class. Since in C++ you can't modify a class that has already been defined, you can't put it in QDataStream class and you're left with the standalone option.

Aha! Thanks alot. I'll buy and read a proper C++ book ;)

That's usually a good idea :)