C++的四种类型转换例子

　　C++有四种类型转换方法，分别是 reinterpret_cast、static_cast、dynamic_cast、const_cast。下面是四种类型转换使用的具体代码，在调试代码的时候可以尝试着把某些注释去掉后再编译运行，查看结果。经过自己动手和思考后，才会有更深刻的记忆。

　　代码：

#include <iostream>
using namespace std;

unsigned short 
Hash(void* p)
{
    unsigned int val = reinterpret_cast<unsigned int>(p);
    cout << "Hash unsigned int " << val << " ## ";
    cout << "Hash unsigned short " << (unsigned short) (val ^ (val >> 16)) << endl;
    return (unsigned short) (val ^ (val >> 16));
}

void show_in_short(void* a, int len)
{
    if (NULL == a)
        return;

    unsigned short* p = reinterpret_cast<unsigned short*>(a);
    cout << "######### show in short ##################" << endl;
    for (int i=0; i<len/sizeof(short); i++)
    {
        cout << "[" << i << "] = " << *p++ << endl;
        
    }
    
}

class A
{
public:
    virtual void xxx(){ cout << "this is A" << endl; };
};

class B : public A
{
public:
    virtual void xxx(){ cout << "this is B" << endl; };
};

class C
{
};

int
main(int argc, char* argv[])
{
    int array[2];
    for (int i=0; i<20; i++)
    {
        array[i] = i + 10;
        Hash(array + i);
    }

    show_in_short(array, sizeof(array));

    A a;
    B b;
    C c;
    A* pa = NULL;
    B* pb = NULL;
    C* pc = NULL;

    pa = reinterpret_cast<A*>(&a);
    pa = reinterpret_cast<A*>(&b);
    pa = reinterpret_cast<A*>(&c);

    pa = static_cast<A*>(&a);
    pa = static_cast<A*>(&b);
    //static_cast是不安全的，static_cast只适用于
    //1.基类和子类之间的转换
    //2.基础数据类型转换
    //3.空指针转换成目标类型指针EG： CHAR* P = NULL
    //4.任何类型的表达式转换成void类型 
    //static_cast的表达式不能去掉类型的const, volitale
    //so 下面这行代码会在编译时报错
    //pa = static_cast<A*>(&c); 

    pa = dynamic_cast<A*>(&a);
    pa = dynamic_cast<A*>(&b); 
    //dynamic_cast是安全的，它和static_cast相似，但是比static_cast更严格
    //dynamic_cast仅仅对指针和引用有效,一般用于基类和子类之间的转换。
    //如果是父类转为子类，会返回NULL。类中需要有虚函数，否则会在编译时报错
    //so 下面这行代码会在编译时报错
    //pa = dynamic_cast<A*>(&c);
    
    pb = dynamic_cast<B*>(&a); // pb = NULL;
    cout << "excute pb = dynamic_cast<B*>(&a) " << endl;
    cout << "pb = " << pb << endl;

    const A *const_pa = &a;
    //const_pa->xxx(); //fail because xxx() is not const function
    pa = const_cast<A*>(const_pa);
    pa->xxx();
}

　　执行结果

xiaol-luo@CentOS:~/link/net_codes/c++/cast$ g++ -g -o cast cast.cpp && ./cast
cast.cpp: In function ‘int main(int, char**)’:
cast.cpp:86: 警告：从‘A a’到‘class B*’的动态转换永远不会成功
Hash unsigned int 3216571380 ## Hash unsigned short 17484
Hash unsigned int 3216571384 ## Hash unsigned short 17472
Hash unsigned int 3216571388 ## Hash unsigned short 17476
Hash unsigned int 3216571392 ## Hash unsigned short 17336
Hash unsigned int 3216571396 ## Hash unsigned short 17340
Hash unsigned int 3216571400 ## Hash unsigned short 17328
Hash unsigned int 3216571444 ## Hash unsigned short 17292
Hash unsigned int 3216571448 ## Hash unsigned short 17280
Hash unsigned int 3216571452 ## Hash unsigned short 17284
Hash unsigned int 3216571456 ## Hash unsigned short 17400
######### show in short ##################
[0] = 10
[1] = 0
[2] = 11
[3] = 0
excute pb = dynamic_cast<B*>(&a) 
pb = 0
this is A