Upcast and DownCast

这是我第一次去面试的时候面试官问我的一个很简单的问题，当时没经验，没回答出来，现在记录下来以防忘记，顺便再写博客的过程中学习学习

#include <iostream> using namespace std; class Base { public: void no_virtual() { cout<< "no_virtual in Base" <<endl; } virtual void has_virtual() { cout<< "has_virtual in Base"<<endl; } }; class Derived: public Base { public: void no_virtual() { cout<< "no_virtual in Derived"<<endl; } virtual void has_virtual() { cout<< "has_virtual in Derived"<<endl; } }; int main() { Base *base = new Derived();//base 是Base类型，但是它指向的类型编译器不知道 base->no_virtual(); // Base->no_virtual //函数调用是依据指针本身的类型来调用相应的函数，而非指针所指向的类型 base->has_virtual(); // Derived->has_virtual 这里has_virtual 是虚函数，进行晚绑定，调用的是Derived::has_virtual Base base2; Derived *derived = static_cast<Derived *>(&base2); //这是downcast derived->no_virtual(); // derived 本身是 Derived指针，所以调用的是 Derived::no_virtual derived->has_virtual(); // derived 指向的是Base ,虚函数进行晚绑定， 调用Base::has_virtual return 0; }

切割：覆盖方法和子类数据丢失的现象生成切割(slice)。

 

上面这个例子中 

 

Base *base = new Derived();//upcast, cast Derived to Base //upcasting： moving up an interitance hierarchy.When you upcast, you always derived from an ancestor class 

 

 

 

而所谓的downcasting 与upcasting 是相反的

c++ 提供了 dynamic_cast 这种安全的downcasting 机制，

/*C++ provides a special explicit cast called dynamic_cast that is a type-safe downcast operation. When you use dynamic_cast to try to cast down to a particular type, the return value will be a pointer to the desired type only if the cast is proper and successful, otherwise it will return zero to indicate that this was not the correct type. Here’s a minimal example: */ //: C15:DynamicCast.cpp #include <iostream> using namespace std; class Pet { public: virtual ~Pet(){}}; class Dog : public Pet {}; class Cat : public Pet {}; int main() { Pet* b = new Cat; // Upcast // Try to cast it to Dog*: Dog* d1 = dynamic_cast<Dog*>(b); // Try to cast it to Cat*: Cat* d2 = dynamic_cast<Cat*>(b); cout << "d1 = " << (long)d1 << endl; cout << "d2 = " << (long)d2 << endl; } ///:~

 

 

 

 

reference:

http://www.cppblog.com/mzty/archive/2008/02/19/42929.html

http://saturnman.blog.163.com/blog/static/557611201062910383394/

http://www.linuxtopia.org/online_books/programming_books/thinking_in_c++/Chapter15_028.html