Question

Consider the following code:

struct A {
    A& add( int i ) { return *this; }
};

A& operator+=( A& a, int i ) { return a; }

void f() {
    A a1 = A().add( 1 ); // expr 1
    A a2 = A() += 1;     // expr 2
}

Both expressions 1 and 2 are accepted by VC. Expression 2 is rejected by GCC with “No viable overloaded ‘+='”. However such an expression is accepted for operators taking a reference to constant object:

const A& operator+=( const A& a, int i ) { return a; }

Why does GCC force this temporary to be constant in the operator context when not in the method call context, and is this correct behavior?

Answer 1

A();

Creates temporary.

A& operator+=( A& a, int i ) { return a; }

temporary cannot be binded to reference. MSVC accept this, since it’s non-standard extension. Look at rvalue to lvalue conversion Visual Studio