Method/function operators

The following operators can be redefined with a method or a function: binary $+$, unary $+$, binary $-$, unary $-$, $*$, $/$, $\%$, $==$, $!=$, $\&\&$, $\vert\vert$, $!$, $<$, $>$, $<=$, $>=$, $+=$, $-=$, $*=$, $/=$, $\%=$, $\&$, $\vert$, $\wedge$, $\sim$, $«$, $»$, $\&=$, $\vert=$, $++$, $-$. The advantage of a function operator over a method operator is the possibility to define it independently from the class. For example, to use a method:

struct   Number
{
  Number  operator+(const Number &n) const { /* ... */ }
  Number  operator-() const { /* ... */ }
  bool    operator==(const Number &n) const { /* ... */ }
  Number  &operator+=(const Number &n) { /* ... */ }
  Number  &operator++() { /* ... */ }   // postfixed
  Number  operator++(int) { /* ... */ } // prefixed
  double  &operator*() { /* ... */ }    // prefixed
  double  &operator->() { /* ... */ }   // prefixed
};

struct   Stream
{
  Stream   &operator<<(const Number &n) { /* ... */ }
};

They can also be redefined with a function (possibly friend). For example:

Number  operator+(const Number &n0, const Number &n1) { /* ... */ }
Number  operator-(const Number &n) { /* ... */ }
bool    operator==(const Number &n0, const Number &n1) { /* ... */ }
Stream  &operator<<(Stream &is, const Number &n) { /* ... */ }
Number  &operator+=(Number &n0, const Number &n1) { /* ... */ }
Number  &operator++(const Number &n) { /* ... */ }     // postfixed
Number  operator++(const Number &n, int) { /* ... */ } // prefixed
double  &operator*(const Number &n) { /* ... */ }      // prefixed
double  &operator->(const Number &n) { /* ... */ }     // prefixed