C++类型转换

1、static_cast
可以将子类和基类互相转换，只验证两者类型是否相关。如：
子类到基类：
CBase * pBase = new CDerived();
CDerived * pDerived = static_cast<CDerived*>(pBase);//ok;

基类到子类：
CBase * pBase = new CBase();
CDerived * pDerived = static_cast<CDerived*>(pBase);//compiled success,but error on running.

不相关类型：
CBase * pBase = new CDerived();
CUnrelated * pUnrelated = static_cast<CUnrelated*>(pBase);//error. can't compiled success.


2、dynamic_cast
这种转换是在实际运行时执行检查，如果对象确实可以转换为该类型，他就能转换成功，而不管这两个类型之间有没有继承关系。如：
struct B1{
    virtual ~B1(){}
};
struct B2{
    virtual ~B2(){}
    void m(){}
};
struct D1 : B1, B2{};
int main()
{
    D1 d;
    B1* pb1 = &d;
    B2* pb2 = dynamic_cast<B2*>(pb1);//ok
    //if(pb2)
    // pb2->m();
    B2* pb22 = static_cast<B2*>(pb1);  //error,can't compile success.
    return 0;
}
上述定义中可以看到,B1和B2是不相关的类,从L1可以看到,dynamic_cast允许这种转换:只要B1存在多态方法.L2将编译失败,static_cast并不允许两个完全不相干的类互相转换.


3、reinterpret_cast
强制类型转换，而不关心两个类型是否相关。如：
CBase * pBase = new CBase();
CUnrelated * pUnrelated = reinterpret_cast<CUnrelated*>(pBase);//ok, can compile success. maybe will throw error on running.


4、const_cast
关闭对象的访问修饰符const。如：
CSomeClass
{
      public:
            void DisplayMembers();//should define as const.
}

void DisplayAllData(const CSomeClass & mData)
{
      mData.DisplayMembers();//compile failure, because we can't call a non const method on a const variable
      CSomeClass & refData = const_cast<CSomeClass&>(mData);
      refData.DisplayMembers();//compile success. close the const limitation.
}