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copied!<p>First: the two different ways are really "overload as a member" and "overload as a non-member", and the latter has two different ways to write it (as-friend-inside class definition and outside class definition). Calling them "inside class" and "outside class" is going to confuse you.</p> <hr> <p>Overloads for +=, +, -=, -, etc. have a special pattern:</p> <pre><code>struct Vector2 { float x, y; Vector2& operator+=(Vector2 const& other) { x += other.x; y += other.y; return *this; } Vector2& operator-=(Vector2 const& other) { x -= other.x; y -= other.y; return *this; } }; Vector2 operator+(Vector2 a, Vector2 const& b) { // note 'a' is passed by value and thus copied a += b; return a; } Vector2 operator-(Vector2 a, Vector2 const& b) { return a -= b; } // compact </code></pre> <p>This pattern allows the conversions mentioned in the other answers for the LHS argument while simplifying the implementation considerably. (Either member or non-member allows conversions for the RHS when it's passed either as a <code>const&</code> or by value, as it should be.) Of course, this only applies when you do actually want to overload both += and +, -= and -, etc., but that is still common.</p> <hr> <p>Additionally, you sometimes want to declare your non-member op+, etc. as <a href="http://codepad.org/ACNeZcCf" rel="noreferrer">friends</a> within the class definition using the <a href="http://en.wikipedia.org/wiki/Barton-Nackman_trick" rel="noreferrer">Barton-Nackman trick</a>, because due to quirks of templates and overloading, it may not be found <a href="http://codepad.org/3jbvxRN9" rel="noreferrer">otherwise</a>.</p>

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