dynamic_cast, RTTI， 整理

主要是参考下图，了解内存布局，然后写个实例程序就差不多明白了，但是需要熟悉指针转换。

1) 只有多态类才有RTTI信息，dynamic_cast正是运用RTTI进行转换，属于运行时类型检查。

2) dynamic_cast判断两个指针是否能转换时，用RTTI可以知道当前实际对象，然后遍历自己所有的父类，看是否有与目标类型一致的，如果有就可以进行转换。

3) dynamic_cast是安全的，可以通过检查返回值或异常捕捉来判断是否转成功。其中检查返回值用于指针转换，异常捕捉用于引用转换。

4) 补充一点，与static_cast不同，即使两个类没有直接继承关系，但是只要在一个类层次结构中，就有可能指向同一个对象，也就可以进行dynamic_cast。以下图为例，C继承自A，B。A, B指针就可以进行dynamic_cast，并可能成功。

    C * pc = new C;
    A* pa = pc;
    B* pb = pc;

    pb = dynamic_cast<B*>(pa); // 可以成功进行转换，因为二者指向的都是C对象。

 下面的例子主要是关于RTTI的，打印出一个对象和她所有父类的运行时信息，这里的运行时信息主要是类的名字。

#include "iostream"
#include "string" 
#include <typeinfo>
using namespace std;


class Zero
{
public:
    virtual void f111() { }
};

class Base : public Zero
{
public:
    virtual void f() { }
};

class Deri1234567890ve : public Base
{
};

typedef unsigned long DWORD;

struct PMD
{
    int mdisp;  //member displacement
    int pdisp;  //vbtable displacement
    int vdisp;  //displacement inside vbtable
};

struct RTTIBaseClassDescriptor
{
    struct TypeDescriptor* pTypeDescriptor; //type descriptor of the class
    DWORD numContainedBases; //number of nested classes following in the Base Class Array
    struct PMD where;        //pointer-to-member displacement info
    DWORD attributes;        //flags, usually 0
};


struct TypeDescriptor
{
    DWORD ptrToVTable;
    DWORD spare;
    char name[ ];
};

struct RTTIClassHierarchyDescriptor
{
    DWORD signature;      //always zero?
    DWORD attributes;     //bit 0 set = multiple inheritance, bit 1 set = virtual inheritance
    DWORD numBaseClasses; //number of classes in pBaseClassArray
    struct RTTIBaseClassArray* pBaseClassArray;
};

struct RTTICompleteObjectLocator

{

    DWORD signature; //always zero ?

    DWORD offset;    //offset of this vtable in the complete class

    DWORD cdOffset;  //constructor displacement offset

    struct TypeDescriptor* pTypeDescriptor; //TypeDescriptor of the complete class

    //int * ptr;
    struct RTTIClassHierarchyDescriptor* pClassDescriptor; //describes inheritance hierarchy

};




int main()
{
    /*Base *pderive = new Deri1234567890ve();
    int **ptr = (int **)(&pderive);
    int *ptable = (int *)(*(int *)(*ptr)); 
    int * rtti = ptable -1;

    RTTICompleteObjectLocator * RIIT_locator =   (RTTICompleteObjectLocator *)( *(int*)rtti);
    cout<<RIIT_locator->pTypeDescriptor->name<<endl;*/


    Base *pderive = new Deri1234567890ve();
    int *ptable = (int*)*(int*)pderive;
    int * rtti = ptable -1;

    // 显示当前类的名字“。。Deri1234567890ve。。”
    RTTICompleteObjectLocator * RIIT_locator =   (RTTICompleteObjectLocator *)( *(int*)rtti);
    cout<<RIIT_locator->pTypeDescriptor->name<<endl;

    // 显示自己和所有父类的名字
    int * p1 = (int*)(RIIT_locator->pClassDescriptor->pBaseClassArray);
    int * p2 = (int*)*(p1+2);
    TypeDescriptor* pDesc = (TypeDescriptor*)(*p2);
    cout<<"One of Base Classes: "<<pDesc->name<<endl;

    p1 = (int*)(RIIT_locator->pClassDescriptor->pBaseClassArray);
    p2 = (int*)*(p1 + 1);
    pDesc = (TypeDescriptor*)(*p2);
    cout<<"One of Base Classes: "<<pDesc->name<<endl;

    p1 = (int*)(RIIT_locator->pClassDescriptor->pBaseClassArray);
    p2 = (int*)*(p1);
    pDesc = (TypeDescriptor*)(*p2);
    cout<<"One of Base Classes: "<<pDesc->name<<endl;
}

参考：

浅议 Dynamic_cast 和 RTTI   http://www.cnblogs.com/zhyg6516/archive/2011/03/07/1971898.html

http://www.openrce.org/articles/full_view/23 

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