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:

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

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

Code:

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

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And the << operator is overloaded like so:

Code:

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

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My problem: I can do this....

Code:

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

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But if I do this...

Code:

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cout << s1 += s2;

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I get the following error:

Code:

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

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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?

Quote:

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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?

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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...