Operator Overloading in struct

Question

Suppose I define this structure:

struct Point {
   double x, y;
};

How can I overload the + operator so that, declared,

Point a, b, c;
double k;

the expression

c = a + b;

yields

c.x = a.x + b.x;
c.y = a.y + b.y;

and the expression

c = a + k;

yields

c.x = a.x + k;
c.y = a.y + k; // ?

Will the commutative property hold for the latter case? That is, do c = a + k; and c = k + a; have to be dealt with separately?

Answer 1

Just do it:

Point operator+( Point const& lhs, Point const& rhs );
Point operator+( Point const& lhs, double rhs );
Point operator+( double lhs, Point const& rhs );

With regards to your last question, the compiler makes no assumptions concerning what your operator does. (Remember, the + operator on std::string is not commutative.) So you have to provide both overloads.

Alternatively, you can provide an implicit conversion of double to Point (by having a converting constructor in Point). In that case, the first overload above will handle all three cases.

Answer 2

Here is how I would do it.

struct Point {
   double x, y;
   struct Point& operator+=(const Point& rhs) { x += rhs.x; y += rhs.y; return *this; }
   struct Point& operator+=(const double& k) { x += k; y += k; return *this; }
};

Point operator+(Point lhs, const Point& rhs) { return lhs += rhs; }
Point operator+(Point lhs, const double k) { return lhs += k; }
Point operator+(const double k, Point rhs) { return rhs += k; }