第五十一课 C++对象模型分析（下）

1.继承对象模型

在C++编译器的内部类可以理解为结构体 子类是由父类成员叠加子类新成员得到的

51-1 继承对象模型初探

#include <iostream> #include <string> using namespace std; class Demo { protected: int mi; int mj; }; class Derived : public Demo { int mk; public: Derived(int i, int j, int k) { mi = i; mj = j; mk = k; } void print() { cout << "mi = " << mi << "," << "mj = " << mj << "," << "mk = " << mk << endl; } }; struct Test { int mi; int mj; int mk; }; int main() { cout << "sizeof(Demo) = " << sizeof(Demo) << endl; //8 cout << "sizeof(Derived) = " << sizeof(Derived) << endl; //12 Derived d(1,2,3); Test* p = reinterpret_cast<Test*>(&d); cout << "Before changing..." << endl; d.print(); p->mi = 10; p->mj = 20; p->mk = 30; cout &lt;&lt; "After changing..." &lt;&lt; endl; d.print(); return 0;

} 运行结果 sizeof(Demo) = 8 sizeof(Derived) = 12 Before changing… mi = 1,mj = 2,mk = 3 After changing… mi = 10,mj = 20,mk = 30

2.多态对象模型

C++多态的实现原理 当类中声明虚函数时，编译器会在类中生成一个虚函数表 虚函数表是一个存储成员函数地址的数据结构 虚函数表是由编译器自动生成与维护的 virtual成员函数会被编译器放入虚函数表中 存在虚函数时，每个对象中都有一个指向虚函数表的指针

51-2.c main.c 多态本质分析—C++是通过牺牲效率来实现虚函数的

C++中编写程序：51-2.cpp #include <iostream> #include <string> using namespace std; class Demo { protected: int mi; int mj; public: virtual void print() //声明为虚函数 { cout << "mi = " << mi << "," << "mj = " << mj << endl; } }; class Derived : public Demo { int mk; public: Derived(int i, int j, int k) { mi = i; mj = j; mk = k; } void print() { cout << "mi = " << mi << "," << "mj = " << mj << "," << "mk = " << mk << endl; } }; struct Test { void* p; int mi; int mj; int mk; }; int main() { cout << "sizeof(Demo) = " << sizeof(Demo) << endl; //8 cout << "sizeof(Derived) = " << sizeof(Derived) << endl; //12 Derived d(1,2,3); Test* p = reinterpret_cast<Test*>(&d); cout << "Before changing..." << endl; d.print(); p->mi = 10; p->mj = 20; p->mk = 30; cout << "After changing..." << endl; d.print(); return 0; } 运行结果 sizeof(Demo) = 16 sizeof(Derived) = 24 Before changing... mi = 1,mj = 2,mk = 3 After changing... mi = 10,mj = 20,mk = 30

用C语言实现面向对象的代码，来实现多态，也是C++内部的原理

51-2.h #ifndef _51_2_H_ #define _51_2_H_ typedef void Demo; typedef void Derived; Demo* Demo_Create(int i, int j); int Demo_GetI(Demo* pThis); int Demo_GetJ(Demo* pThis); int Demo_Add(Demo* pThis, int value); void Demo_Free(Demo* pThis); Derived* Derived_Create(int i, int j, int k); int Derived_GetK(Derived* pThis); int Derived_Add(Derived* pThis, int value); #endif 51-2.c #include "51-2.h" #include "malloc.h" static int Demo_Virtual_Add(Demo* pThis, int value); static int Derived_Virtual_Add(Demo* pThis, int value); struct VTable // 2. 定义虚函数表数据结构 { int (*pAdd)(void*, int); // 3. 虚函数表里面存储什么？？？ }; struct ClassDemo { struct VTable* vptr; // 1. 定义虚函数表指针==》虚函数表指针类型？？？ int mi; int mj; }; struct ClassDerived { struct ClassDemo d; int mk; }; static struct VTable g_Demo_vtbl = { Demo_Virtual_Add }; static struct VTable g_Derived_vtbl = { Derived_Virtual_Add }; Demo* Demo_Create(int i, int j) { struct ClassDemo* ret = (struct ClassDemo*)malloc(sizeof(struct ClassDemo)); if( ret != NULL ) { ret->vptr = &g_Demo_vtbl; // 4. 关联对象和虚函数表 ret->mi = i; ret->mj = j; } return ret; } int Demo_GetI(Demo* pThis) { struct ClassDemo* obj = (struct ClassDemo*)pThis; return obj->mi; } int Demo_GetJ(Demo* pThis) { struct ClassDemo* obj = (struct ClassDemo*)pThis; return obj->mj; } // 6. 定义虚函数表中指针所指向的具体函数 static int Demo_Virtual_Add(Demo* pThis, int value) { struct ClassDemo* obj = (struct ClassDemo*)pThis; return obj->mi + obj->mj + value; } // 5. 分析具体的虚函数！！！---对外的用户接口 int Demo_Add(Demo* pThis, int value) { struct ClassDemo* obj = (struct ClassDemo*)pThis; return obj->vptr->pAdd(pThis, value);//7.通过对象找到具体的虚函数表的指针vptr，通过指针找具体add函数，具体add函数的地址保存在pAdd成员里面，找到Demo_Virtual_Add } void Demo_Free(Demo* pThis) { free(pThis); } //8.开始完成子类的部分 Derived* Derived_Create(int i, int j, int k) { struct ClassDerived* ret = (struct ClassDerived*)malloc(sizeof(struct ClassDerived)); if( ret != NULL ) { ret->d.vptr = &g_Derived_vtbl; //10.将vptr关联到子类的虚函数中取 ret->d.mi = i; ret->d.mj = j; ret->mk = k; } return ret; } int Derived_GetK(Derived* pThis) { struct ClassDerived* obj = (struct ClassDerived*)pThis; return obj->mk; } //9.在实际的虚函数中进行相加操作 static int Derived_Virtual_Add(Demo* pThis, int value) { struct ClassDerived* obj = (struct ClassDerived*)pThis; return obj->mk + value; } int Derived_Add(Derived* pThis, int value) { struct ClassDerived* obj = (struct ClassDerived*)pThis; return obj->d.vptr->pAdd(pThis, value); } Main.c 程序 #include "stdio.h" #include "51-2.h" void run(Demo* p, int v) { int r = Demo_Add(p, v); printf("r = %d\n", r); } int main() { Demo* pb = Demo_Create(1, 2); Derived* pd = Derived_Create(1, 22, 333); printf("pb->add(3) = %d\n", Demo_Add(pb, 3)); printf("pd->add(3) = %d\n", Derived_Add(pd, 3)); run(pb, 3); run(pd, 3); Demo_Free(pb); Demo_Free(pd); return 0; } 运行结果 pb->add(3) = 6 pd->add(3) = 336 r = 6 //父类的调用版本 r = 336 //子类的调用版本

小结 继承的本质就是父子间成员变量的叠加 C++中的多态是通过虚函数表实现的 虚函数表是由编译器自动生成与维护的 虚函数的调用效率低于普通成员函数