【C++】 51_C++对象模分析 （下）

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class Dervied : public Demo{ int mk;};编程实验： 继承对象模型初探#include <iostream>using namespace std;class Demo{protected: int mi; int mj;};class Derived : public Demo{private: 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; cout << “sizeof(Derived) = " << sizeof(Derived) << endl; Derived d(1, 2, 3); Test* p = reinterpret_cast<Test*>(&d); // 注意这里！ cout << “Befor changeing …” << endl; d.print(); p->mi = 10; // 注意这里！ p->mj = 20; p->mk = 30; cout << “After changeing …” << endl; d.print(); return 0;}输出：sizeof(Demo) = 8sizeof(Derived) = 12Befor changeing …mi = 1, mj = 2, mk = 3After changeing …mi = 10, mj = 20, mk = 30结论：子类是由父类成员叠加子类新成员得到的多态对象模型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由编译器自动生成与维护virtual 成员函数被编译器放入虚函数表中存在虚函数时，每个对象都有一个指向虚函数表的指针多态：面向对象理论中的概念，与程序设计语言无关虚函数：C++ 中实现多态的唯一方式class Demo{private: int mi; int mj;public: virtual int add(int value) { return mi + mj + value; };};==>class Derived : public Demo{private: int mk;public: virtual int add(int value) { return mk + value; }};==>void run(Demo* p, int v){ p->add(v);}编译器确认 add() 是否为虚函数？Yes -> 编译器在对象 VPTR 所指向的虚函数表中查找 add() 的地址No -> 编译器可以直接确定被调成员函数的地址调用效率： 虚函数 < 普通成员函数编程实验：多态对象模型初探#include <iostream>using namespace std;class Demo{protected: int mi; int mj;public: virtual void print() { cout << “mi = " << mi << “, " << “mj = " << mj << endl; }};class Derived : public Demo{private: 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; // 注意这里！ int mi; int mj; int mk;};int main(){ cout << “sizeof(Demo) = " << sizeof(Demo) << endl; // 注意这里！ cout << “sizeof(Derived) = " << sizeof(Derived) << endl; Derived d(1, 2, 3); Test* p = reinterpret_cast<Test*>(&d); cout << “Befor changeing …” << endl; d.print(); p->mi = 10; p->mj = 20; p->mk = 30; cout << “After changeing …” << endl; d.print(); return 0;}输出：sizeof(Demo) = 12sizeof(Derived) = 16Befor changeing …mi = 1, mj = 2, mk = 3After changeing …mi = 10, mj = 20, mk = 30虚函数表指针位于对象最开始的 4 个字节处（32位平台）编程实验： 多态本质分析C 写面向对象、继承与多态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函数以上内容参考狄泰软件学院系列课程，请大家保护原创！