C binary operator overloading

Question

As per Deitel's 9ed C++ How to program, p. 439-440:

A binary operator can be overloaded as a non-static member function with one parameter or as a non-member function with two parameters (one of those parameters must be either a class object or a reference to a class object).

So, what other sort of object is there beside a class object or a reference to a class object? I cannot think of anything.

Answer 1

The language defines several fundamental types, such as int and double. 

Objects are instances of fundamental kinds.

Let's pretend you have:

struct Foo { ... };

You can use non-member functions to overload the operator+ function.

Foo operator+(Foo , int);
Foo operator+(Foo& , int);
Foo operator+(int, Foo);
Foo operator+(int, Foo&);

etc. 

One of the parameters in each of these circumstances is a Foo or a Foo&. 

Foo const& is another possibility. 

You cannot, however, overload it

int operator+(int, int);

because neither argument type is a class or a pointer to a class