Can C++ assignment operators be free functions?

c++ operator-overloading non-member-functions
Roddy picture Roddy · Jun 21, 2010 · Viewed 8k times · Source

I'm trying something like this:

Foo & operator=(Foo & to, const Bar &from);

But I'm getting this error: 

E2239 'operator =(Foo &, const Bar &)' must be a member function

Are there limitations on which operators can/cannot be defined as Free Functions, and if so, why?

Answer

James McNellis picture James McNellis · Jun 21, 2010

The assignment operator must be a non-static member function and must have exactly one parameter:

An assignment operator shall be implemented by a non-static member function with exactly one parameter (C++03 13.5.3/1).

operator(), operator[], and operator-> must also be implemented as non-static member functions.

Class-specific operator new and operator delete (and variants thereof) must be implemented as static member functions (note that these are implicitly static, even if they are not declared with the static keyword).

Related questions

Operator overloading : member function vs. non-member function?

I read that an overloaded operator declared as member function is asymmetric because it can have only one parameter and the other parameter passed automatically is the this pointer. So no standard exists to compare them. On the other hand, …

Read More
Multiplying an object with a constant from left side

I have a Matrix class and it has overloaded * operators for scalar and matrix multiplications. template <class T> class Matrix { public: // ... Matrix operator*(T scalar) const; // ... } // ... template <class T> Matrix<T> Matrix<T&…

Read More
What are the basic rules and idioms for operator overloading?

Note: The answers were given in a specific order, but since many users sort answers according to votes, rather than the time they were given, here's an index of the answers in the order in which they make the most …

Read More