Define operators primarily to mimic conventional usage

Reason

Minimize surprises.

Example

class X {
public:
    // ...
    X& operator=(const X&); // member function defining assignment
    friend bool operator==(const X&, const X&); // == needs access to representation
                                                // after a = b we have a == b
    // ...
};

Here, the conventional semantics is maintained: Copies compare equal.

Example, bad

X operator+(X a, X b) { return a.v - b.v; }   // bad: makes + subtract

Note

Non-member operators should be either friends or defined in the same namespace as their operands. Binary operators should treat their operands equivalently.

Enforcement

Possibly impossible.