Rules of operator overloading in C++

[abhishek gupta]

Hi,

I am having some trouble understanding operator overloading in c++.I have the following program-

class Abc{

    

int x=12;

    

double y=77;

    

public:

    

    

void operator+(){x+=10;}

    //-----d

void operator<<(){cout<<x<<endl;cout<<y<<endl;}  //ERROR - but it worked for '+'

   --- 'c'

};

int main() {

Abc ob;

ob.operator+();   //works fine ----- 'a'

ob+;              //ERROR ------    'b'

    

ob.operator<<();

return 0;

}

1)If statement 'a' works then why 'b' does not work?Aren't they both same?

2)Why do I get the compilation error for 'c'-

error: 'void Abc::operator<<()' must take exactly one argument

     void operator<<(){cout<<x<<endl;cout<<y<<endl;}

3)Is it necessary to overload << operator as friend function?

4)Similarly,is it a rule that overloading postfix operator requires a dummy argument & that to an int?

5)Is there a rule regarding which operator can be overloaded & out of them which can be member functions and which can be non-member functions?

I summary, I do not find any logical explanation regarding how an operator has to be overloaded.

[abhishek gupta]

Hi,

Can someone please resolve my doubts here?

[Andrew Tomazos]

As I previously told you, these sort of questions would be more appropriate for StackOverflow:  www.stackoverflow.com

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[Bo Persson]

No, they are not the same. A unary operator+ is used as +ob.

> 2)Why do I get the compilation error for 'c'-
>
> error: 'void Abc::operator<<()' must take exactly one argument
>      void operator<<(){cout<<x<<endl;cout<<y<<endl;}

The compiler tells you that there is no version of operator<< that takes
no parameters.

A member operator takes one parameter (plus *this). A non-member takes
two parameters (and has no *this).

>
> 3)Is it necessary to overload << operator as friend function?

No, but is often a convenient way to access private class members.

>
> 4)Similarly,is it a rule that overloading postfix operator requires
> a dummy argument & that to an int?

Yes, but only for the postfix operators ++ and --. For other operators
you cannot choose how they work.

>
> 5)Is there a rule regarding which operator can be overloaded & out
> of them which can be member functions and which can be non-member
> functions?

Yes. Most can be either, but a few operators have to be members, the
assignment operator in particular. See

https://en.cppreference.com/w/cpp/language/operators

>
> I summary, I do not find any logical explanation regarding how an
> operator has to be overloaded.
>

There is a logic, but it is kind of complex (for various historical
reasons). You cannot learn this by testing or asking about examples, but
have to read up on this (preferrably in a book).


Bo Persson