<< operator overloading as a class method in c++

Question

I want to overload the << operator for my class Complex.

The prototype is the following:

void Complex::operator << (ostream &out)
{
    out << "The number is: (" << re <<", " << im <<")."<<endl;
}

It works, but I have to call it like this: object << cout for the standart output. What can I do to make it work backwards, like cout << object?

I know that the 'this' pointer is by default the first parameter sent to the method so that's why the binary operator can work only obj << ostream. I overloaded it as a global function and there were no problems.

Is there a way to overload the << operator as a method and to call it ostream << obj?

You know there already is a [complex number](http://en.cppreference.com/w/cpp/numeric/complex) class in the standard library? — Some programmer dude, Jan 30 '13 at 10:38
Yes. Complex class was just a simple example and the point of the question is how can I overload the operator as a method. — Sorin, Jan 30 '13 at 10:40
You have already found the problem - as a member function the parameters are in the wrong order. There is no way to change this. The closest you get is making it a `friend` function. — Bo Persson, Jan 30 '13 at 10:46
http://stackoverflow.com/questions/4421706/operator-overloading, ctrl + f = stream — lucasmrod, Jan 30 '13 at 10:46

score 4 · Accepted Answer · answered Jan 30 '13 at 10:46

I would just use the usual C++ pattern of free function. You can make it friend to your Complex class if you want to make Complex class's private data members visible to it, but usually a complex number class would expose public getters for real part and imaginary coefficient.

class Complex
{
  ....

   friend std::ostream& operator<<(std::ostream &out, const Complex& c);
private:
   double re;
   double im;
};

inline std::ostream& operator<<(std::ostream &out, const Complex& c)
{
    out << "The number is: (" << c.re << ", " << c.im << ").\n";
    return out;
}

billz · Answer 2 · 2013-01-30T10:44:04.613

2

You could write a free stand operator<< function, try:

std::ostream& operator<< (std::ostream &out, const Complex& cm)
{
    out << "The number is: (" << cm.re <<", " << cm.im <<")." << std::endl;
    return out;
}

edited Jan 30 '13 at 10:44

answered Jan 30 '13 at 10:38

billz

44,644
9
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100

score 1 · Answer 3 · answered Jan 30 '13 at 10:42

1

You can define a global function:

void operator << (ostream& out, const Complex& complex)
{
     complex.operator<<(out);
}

answered Jan 30 '13 at 10:42

Zhi Wang

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<< operator overloading as a class method in c++

3 Answers3