Overloading friend operator method in template class

Question

I'm trying to overload the assignment ('=') operator of a template class, but I need the operator= method to be a friend function.

I though this would be simple, but I'm doing something wrong because the code below causes a compilation error:

error C2801: 'operator =' must be a non-static member

template <typename T>
class IDataStream
{
public:
    friend void operator=(const IDataStream& dataStream)
    {
        // set some private members, e.g.
        // this->{...} = dataStream.{...};
    };
}

Can someone show me the error of my ways- I've become pretty stuck on this :( Thanks.

Answer 1

Your error is using friend, which changes the function from a member-function to an inline-defined friend-function.

operator= can only be defined as a non-static member-function, and needs two arguments, the implicit this and the explicit right-hand-side.

Answer 2

As pointed out by 0x499602D2, operator= doesn't need to be a friend function. I'm blaming tiredness for managing to completely overlook that objects have access to private & protected members of objects of the same class...

Answer 3

Let's take a look at the C++ standard.

§ 9.3/1 Functions declared in the definition of a class, excluding those declared with a friend specifier (11.3), are called member functions of that class. A member function may be declared static in which case it is a static member function of its class (9.4); otherwise it is a non-static member function of its class (9.3.1, 9.3.2).

§ 13.5.3/1 An assignment operator shall be implemented by a non-static member function with exactly one parameter. [..]

So you cannot make operator= a friend.

In § 11 [class.access]:

1 A member of a class can be

— private; that is, its name can be used only by members and friends of the class in which it is declared.

Since a member function is a member of the class, it doesn't require the friend specifier to access private data members.

If you want to learn the proper way to overload an operator, please see c++-faq's Operator overloading, although the meat of overloading operator= is covered in What is the copy-and-swap idiom?.