KwCheng's blog

在C++中  , 若有兩個derived class 繼承同一個abstract base class時 , 

我們若以base class pointer (or reference )來接derived class的instance , 並使用base class的pointer

做為virtual function的argument傳入時 , 對於這個argument我們通常沒辦法知道他的type .

for example :

Shape* s1 = new Circle();

Shape* s2 = new Square();

bool b = s1.intersectWith(s2); // we can't know the type of s2.

而要解決這個問題 , 就我們目前所知道的 , 可以使用double dispatching , 或typeid兩種方法 .

而除了這兩種方法 , 我們還可以使用dynamic_cast在run time時做強制轉型確認 (用來將一個基底類別的指標強制轉型到derived class的pointer).

( dynamic_cast can be used only with pointers and references to objects. 

  Its purpose is to ensure that the result of the type conversion is a valid complete object of the requested class) -- from ref1

以上面的example來說:

bool judge = dynamic_cast<Square* > (s2);

其中dynamic_cast<>會先判斷s2是否符合Square class的type , 

若true則轉型並return 1, 

否則就不轉型並return 0.

for example :

#include <iostream> 

#include <exception> 

using namespace std; 

class CBase { virtual void dummy() {} }; 

class CDerived: public CBase { int a; }; 

int main () 

{ 

try 

{ 

CBase * pba = new CDerived; 

CBase * pbb = new CBase; 

CDerived * pd;

pd = dynamic_cast<CDerived*>(pba); 

if (pd==0) cout << "Null pointer on first type-cast" << endl; 

pd = dynamic_cast<CDerived*>(pbb); 

if (pd==0) cout << "Null pointer on second type-cast" << endl; 

} 

catch (exception& e) 

{

cout << "Exception: " << e.what();

} 

return 0; 

}

Even though both are pointers of type CBase*, pba points to an object of type CDerived, 

while pbb points to an object of type CBase. 

(如果使用參考的話，dynamic_cast在轉換失敗之後會丟出bad_cast例外，所以您必須使用try...catch來處理例外)

ref : http://www.cplusplus.com/doc/tutorial/typecasting/

ref : http://caterpillar.onlyfun.net/Gossip/CppGossip/dynamicCast.html