Question

Consider the following code:

class A
{
public:
    A& operator=( const A& );
    const A& operator+( const A& );
    const A& operator+( int m );
};

int main()
{
    A a;
    a = ( a + a ) + 5;   // error: binary '+' : no operator found which takes a left-hand operand of type 'const A'
}

Can anyone explain why the above is returned as an error?

“( a + a )” calls “const A& operator+( const A& )” and returns a constant reference which is then passed to “const A& operator+( int m )” if I’m not mistaken.

How can one fix the above error (without creating a global binary operator+ or a constructor that accepts an int) such that the statement inside main() is allowed?

Answer 1

which is then passed to “const A& operator+( int m )” if I’m not mistaken

No. Since the LHS is a const A& and RHS is an int, it will call*

[anyType] operator+ (int rhs) const
//                            ^^^^^ note the const here.

as you’ve only provided the non-const version const A& operator+( int m ), the compiler will complain.

_{*: Or operator+(const int& rhs) const or operator+(float rhs) const… The crucial point is that it must be a const method.}