Operator overloading and precedence

I've been experimenting with some simple operator overloading. I have a simple class which contains an int. I've overloaded the + operator to add two objects and it returns a new object:

Code:

Simple operator+(const Simple& rhs) {return Simple(n + rhs.n);}

I've also overloaded the += operator and it returns a reference to the modified object:

Code:

Simple& operator +=(const Simple& rhs)
{
    n += rhs.n;
    return *this;
}

And the << operator is overloaded like so:

Code:

 friend ostream&
        operator<<(ostream& os, const Simple& cs)
{
    return os << "Object: n = " << cs.n << endl;
}

My problem: I can do this....

Code:

Simple s1(15), s2(20);
cout << s1 + s2;

But if I do this...

Code:

cout << s1 += s2;

I get the following error:

Code:

binary '+=' : 'std::ostream' does not define this operator 
or a conversion to a type acceptable to the predefined operator

However, if I use parenthesis, as in cout << (s1 += s2); it works fine, but I can't quite see why. Is it something to do with operator precedence?

Originally Posted by Sharke

However, if I use parenthesis, as in cout << (s1 += s2); it works fine, but I can't quite see why. Is it something to do with operator precedence?

Yes, the precedence of assignment operators is lower than that of shift operators, though the precedence of shift operators is lower than that of additive operators.

Aha, I see! And I guess that's why I should just throw parenthesis around things just to make sure...