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在C++中 , 我們可以藉由呼叫 virtual function來dispatch到我們想要呼叫的那個class功能 , 

class Feature 

{

virtual void intersectWith(Feature* f) = 0;

virtual void intersectWith(Circle* f) = 0;

virtual void intersectWith(Square* f) = 0;

};
 

class Circle : public Feature

{

void intersectWith(Feature* f){ cout << "This is Feature in the Circle!!"; }

void intersectWith(Circle* f){ cout << "This is Circle!!"; }

void intersectWith(Square* f){ cout << "This is Square in the Circle!!"; }

};

class Square : public Feature

{

void intersectWith(Feature* f){ cout << "This is Feature in the Square!!"; }

void intersectWith(Circle* f){ cout << "This is Circle in the Square!!"; }

void intersectWith(Square* f){ cout << "This is Square!!"; }

}

以上面這個例子來說 , 若:

Feature* f1 = new Circle();

Feature* f2 = new Square();

Feature* f3;

此時我們若呼叫:

f1.intersectWith(f3);

f2.intersectWith(f3);

則分別會print出 This is Feature in the Circle!! 以及 This is Feature in the Square!!

這是由於virtual function會在run time時依照virtual table所記錄的內容自動幫我們動態決定應該要dispatch到哪一個class type.

但是若我們呼叫:

Feature* f3 = new Circle();

Feature* f4 = new Square();

f1.intersectWith(f4);

f2.intersectWith(f3);

照理說 , 我們想要印出的是 : This is Square in the Circle!! 以及 This is Circle in the Square!!

可是結果卻一樣是印出 This is Feature in the Circle!! 以及 This is Feature in the Square!!

這是由於argument的型別是在compile time時 , function overloading就已經幫我們決定好要使用哪個method function.

由於f1 以及 f2的type都是Feature* , 所以就會使用到argument為Feature*的那個method function.

如果我們要印出想要的內容 , 其時也很簡單 , 只要在動態決定一次就好了 , 而這就形成了double dispatching.

我們先要把intersectWith(Feature*) 這個virtual function的內容改為:

class Circle : public Feature

{

void intersectWith(Feature* f){ f.intersectWith(*this); }

void intersectWith(Circle* f){ cout << "This is Circle!!"; }

void intersectWith(Square* f){ cout << "This is Square in the Circle!!"; }

};

class Square : public Feature

{

void intersectWith(Feature* f){ f.intersectWith(*this); }

void intersectWith(Circle* f){ cout << "This is Circle in the Square!!"; }

void intersectWith(Square* f){ cout << "This is Square!!"; }

};

同樣呼叫:

Feature* f3 = new Circle();

Feature* f4 = new Square();

f1.intersectWith(f4);

f2.intersectWith(f3);

在第一次的dispatch中 , 我們會知道f2與f1是哪個class type , 此時我們會進到 void intersectWith(Feature* f){ f.intersectWith(*this); }

此時又進行一次的dispatch , 我們便會知道f4與f3是屬於哪個class type , 

因此最後便會印出:

This is Circle in the Square!! 以及 This is Square in the Circle!!

ref : http://en.wikipedia.org/wiki/Double_dispatch