Error when Overloading the operator <<

Question

_3DTocka operator<<(ostream& os, _3DTocka _3D){
    cout << "(" << _3D.x << "," << _3D.y << "," << _3D.z << ")" << endl;
}

The above is my code and it gives an error: it must take exactly one argument (1 line)

_3DTocka is the name of the class..

Answer 1

Three problems: First you're declaring the function wrong, it should return the ostream reference it receives as the first argument. Then it doesn't use that ostream argument, but is hardcoded to cout . Thirdly it doesn't return anything, which will lead to undefined behavior .

Regarding your actual compilation error, you're most likely defining the function as a member function in the class. An output operator defined as a class member is something completely different from an output operator defined as a stand-alone function: When declared as a member function it should take one argument, and it's a value to output to the object . If it's a stand-alone function (or defined as friend inside the class) then it's for outputting the object passed as the second argument "to" the object passed as the first argument.

---

Regarding the errors:

friend ostream& operator<<(ostream& os, _3DTocka _3D) {
    return os << '(' << _3D.x << ',' << _3D.y << ',' << _3D.z << ')';
}

A few notes: * I have made the string literals into character literals. Handling strings is more work than handling a single character * I now use the provided output stream, which means that you can use this for any kind of output stream (like files) * I have removed the endl , it's not needed and should be provided by the "caller" of this function.

Now you can do

_3DTocka o = ...;

cout << o << endl;