RTTI和类型转换

通过运行时类型识别（RTTI），程序能够使用基类的指针或引用来检索这些指针或引用所指对象的实际派生类型。通过下面两个操作符提供 RTTI：

dynamic_cast

typeid 操作符

实例

#include <iostream>
#include <typeinfo>

using namespace std;

class Base
{
     public:
          virtual void func(){cout << "I, Base." << endl;}
};

class Derived:public Base
{
     public:
          void dfunc(){cout << "I, Derived." << endl;}
};

class BadDerived
{
     public:
          virtual void func(){cout << "I, BadDerived." << endl;}
};
class T
{
     public:
          void dfunc(){cout << "I, T." << endl;}
};
int main()
{
     Base *baseptr = new Derived;
     BadDerived *badptr = new BadDerived;

     cout << endl << "*********Test 0************" << endl;
     if(T *deriveptr = dynamic_cast<T*>(baseptr))
     {
          cout << "success---->";
          deriveptr->dfunc();
     }
     else
     {
          cout << "failed----->";
          baseptr->func();
          cout << "DEBUG" << endl;
          cout << "typeid(T).name = " << typeid(T).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(*baseptr).name() << endl;
     }
     cout << endl << "*********Test 1************" << endl;
     if(Derived *deriveptr = dynamic_cast<Derived*>(baseptr))
     {
          cout << "success---->";
          deriveptr->dfunc();
     }
     else
     {
          cout << "failed----->";
          baseptr->func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(*baseptr).name() << endl;
     }
     cout << endl << "*********Test 2************" << endl;
     if(Derived *deriveptr = dynamic_cast<Derived*>(badptr))
     {
          cout << "success---->";
          deriveptr->dfunc();
     }
     else                            
     {                               
          cout << "failed----->";
          badptr->func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(*badptr).name() << endl;
     }

     Derived deriveobj;
     Base &baseref = deriveobj;
     BadDerived badobj;
     BadDerived &badref = badobj;

     cout << endl << "*********Test 3************" << endl;
     try
     {
          Derived &deriveref = dynamic_cast<Derived&>(baseref);
          cout << "success---->";
          deriveref.dfunc();
     }
     catch(bad_cast)
     {
          cout << "failed----->";
          baseref.func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(baseref).name() << endl;
     }
     cout << endl << "*********Test 4************" << endl;
     try
     {
          Derived &deriveref = dynamic_cast<Derived&>(badref);
          cout << "success---->";
          deriveref.dfunc();
     }
     catch(bad_cast)
     {
          cout << "failed----->";
          baseref.func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(badref).name() << endl;
     }
     cout << endl << "*********Test 5************" << endl;
     if(typeid(Derived) == typeid(baseref))
     {
          Derived &deriveref = dynamic_cast<Derived&>(baseref);
          cout << "success---->";
          deriveref.dfunc();
     }
     else
     {
          cout << "failed----->";
          baseref.func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(baseref).name() << endl;
     }
     cout << endl << "*********Test 6************" << endl;
     if(typeid(Derived) == typeid(badref))
     {
          Derived &deriveref = dynamic_cast<Derived&>(badref);
          cout << "success---->";
          deriveref.dfunc();
     }
     else
     {
          cout << "failed----->";
          badref.func();
          cout << "DEBUG" << endl;
          cout << "typeid(Derived).name = " << typeid(Derived).name() << endl;
          cout << "typeid(Baseref).name = " << typeid(badref).name() << endl;
     }
}

执行结果

*********Test 0************
failed----->I, Base.
DEBUG
typeid(T).name = 1T
typeid(Baseref).name = 7Derived

*********Test 1************
success---->I, Derived.

*********Test 2************
failed----->I, BadDerived.
DEBUG
typeid(Derived).name = 7Derived
typeid(Baseref).name = 10BadDerived

*********Test 3************
success---->I, Derived.

*********Test 4************
failed----->I, Base.
DEBUG
typeid(Derived).name = 7Derived
typeid(Baseref).name = 10BadDerived

*********Test 5************
success---->I, Derived.

*********Test 6************
failed----->I, BadDerived.
DEBUG
typeid(Derived).name = 7Derived
typeid(Baseref).name = 10BadDerived

类型转换

static_cast <new_type> (expression) 静态转换

dynamic_cast <new_type> (expression) 动态转换

动态转换确保类指针的转换是合适完整的，它有两个重要的约束条件，其一是要求new_type为指针或引用，其二是下行转换时要求基类是多态的（基类中包含至少一个虚函数），如果发现下行转换不安全，dynamic_cast返回一个null指针。当待转换指针是void*或者转换目标指针是void*时，dynamic_cast总是认为是安全的。

reinterpret_cast <new_type> (expression) 重解释转换

这个转换是最“不安全”的，两个没有任何关系的类指针之间转换都可以用这个转换实现。reinterpret_cast可以把整型数转换成地址（指针），这种转换在系统底层的操作，有极强的平台依赖性，移植性不好;它同样要求new_type是指针或引用。

const_cast <new_type> (expression) 常量向非常量转换

const_cast可以将常量转成非常量，但不会破坏原常量的const属性，只是返回一个去掉const的变量。从char *cc = const_cast<char *>(c)可以看出了这个转换的作用了，但切记，这个转换并不转换原常量本身，即c还是常量，只是它返回的结果cc是非常量了。