Multiple dispatch in C++

Question

I am trying to understand what multiple dispatch is. I read a lot of various texts but I still have no idea what multiple dispatch is and what it is good for. Maybe the thin

Answer 1

In single dispatch the function executed depends on just the object type. In double dispatch the function executed depends on the object type and a parameter.

In the following example, the function Area() is invoked using single dispatch, and Intersect() relies on double dispatch because it takes a Shape parameter.

class Circle;
class Rectangle;
class Shape
{
    virtual double Area() = 0; // Single dispatch

    // ...
    virtual double Intersect(const Shape& s) = 0; // double dispatch, take a Shape argument
    virtual double Intersect(const Circle& s) = 0; 
    virtual double Intersect(const Rectangle& s) = 0; 
};

struct Circle : public Shape
{
    virtual double Area() { return /* pi*r*r */; }

    virtual double Intersect(const Shape& s); 
    { return s.Intersect(*this)  ; }
    virtual double Intersect(const Circle& s); 
    { /*circle-circle*/ }
    virtual double Intersect(const Rectangle& s); 
    { /*circle-rectangle*/ }
};

The example is based on this article.