Rationale of enforcing some operators to be members

Question

There are 4 operators in C++ which can be overloaded but cannot be overloaded as freestanding (aka nonmember, standalone) functions. These operators are:

Answer 1

This thread on comp.std.c++ discusses the question.

Francis Glassborow, who was on the committee, stated:

The language designers did not want to support conversions and promotions on the left-hand operand of operator =, nor such on the operand of () and [].

Trying to avoid the situation where:

class A {};

class B { B(A& a) {} };

int operator()(B const& b) { return 0; }


int main(void)
{
    A a;

    // This works even though A doesn't have a () operator
    // It creates a temporary B and calls operator()(B& b)
    return a();                   
}