单继承与多继承中的虚函数表和虚函数指针

电玩女神 2022-05-28 08:52 232阅读 0赞

编译环境： VS 2013

首先，我们了解一下何为单继承，何为多继承？？

单继承：一个子类只有一个直接父类。

多继承：一个子类有两个或多个直接父类。

单继承中的虚函数表分析：

示例程序：

程序运行结果：

Base类(基类)的虚函数表：

[![wKioL1bcEMbyJxs-AAAQf1oQ-Ns235.png][]][wKioL1bcEMbyJxs-AAAQf1oQ-Ns235.png]

Derive类(派生类)的虚函数表：

[![wKiom1bcEHuxOB0QAAAcF7C983I617.png][]][wKiom1bcEHuxOB0QAAAcF7C983I617.png]

结论：

如果有虚函数表，那么只有一个虚函数表，并且按照虚函数声明的顺序顺序排列，派生类的虚函数紧接着基类的虚函数排列。

多继承中的虚函数表分析：

示例代码：

#include <iostream>  
    using namespace std;  
      
    typedef void(*FUNC)();  
    class Base  
    {  
    public:  
        virtual void func1() = 0;  
        virtual void func2() = 0;  
    };  
      
    class Base1:public Base  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Base1::func1()" << endl;  
        }  
        virtual void func2()  
        {  
            cout << "Base1::func2()" << endl;  
        }  
    private:  
        int _b1;  
    };  
      
    class Base2: public Base  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Base2::func1()" << endl;  
        }  
        virtual void func2()  
        {  
            cout << "Base2::func2()" << endl;  
        }  
    private:  
        int _b2;  
    };  
      
      
    class Derive : public Base1, public Base2  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Derive::func1()" << endl;  
        }  
        virtual void func3()  
        {  
            cout << "Derive::func3()" << endl;  
        }  
        virtual void func4()  
        {  
            cout << "Derive::func4()" << endl;  
        }  
    private:  
        int _d;  
    };  
      
    void PrintVfptr(Base* b)  
    {  
        int* vTable1 = (int*)(*(int*)b);  
        cout << "虚函数表指针:" << vTable1 << endl;  
        for (int i = 0; vTable1[i] != 0; ++i)  
        {  
            printf("第%d个虚函数指针: %p   >>", i, vTable1[i]);  
            FUNC f = (FUNC)vTable1[i];  
            f();  
        }  
        //int* vTable2 = (int*)(*((int*)b + sizeof(Base1) / 4));  
        int* vTable2 = (int*)(*((int*)((char*)b + sizeof(Base1))));  
        cout << "虚函数表指针:" << vTable2 << endl;  
        for (int i = 0; vTable2[i] != 0; ++i)  
        {  
            printf("第%d个虚函数指针: %p   >>", i, vTable2[i]);  
            FUNC f = (FUNC)vTable2[i];  
            f();  
        }  
    }  
      
    void Test()  
    {  
        Base1 b1;  
        Base2 b2;  
        Derive d;  
        PrintVfptr(&b1);  
        cout << endl;  
        PrintVfptr(&b2);  
        cout << endl;  
        PrintVfptr((Base1*)&d);  
    }  
      
    int main()  
    {  
        Test();  
        system("pause");  
        return 0;  
    }

程序运行结果：

Base1的虚函数表：

[![wKioL1bcFcSzAXRSAAAX-leGeyg336.png][]][wKioL1bcFcSzAXRSAAAX-leGeyg336.png]

Base2的虚函数表：

[![wKioL1bcFq2xTaMeAAAQJsD\_jQU490.png][wKioL1bcFq2xTaMeAAAQJsD_jQU490.png]][wKioL1bcFq2xTaMeAAAQJsD_jQU490.png]

Derive的虚函数表：(有两个虚函数表)

[![wKioL1bcFyCAR3u\_AAAq7ZbMpMs067.png][wKioL1bcFyCAR3u_AAAq7ZbMpMs067.png]][wKioL1bcFyCAR3u_AAAq7ZbMpMs067.png]

多继承中的菱形继承：

示例代码：

#include <iostream>  
    using namespace std;  
      
    typedef void(*FUNC)();  
    class Base  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Base::func1()" << endl;  
        }  
        virtual void func2()  
        {  
            cout << "Base::func2()" << endl;  
        }  
    private:  
        int _b;  
    };  
      
    class Parent1 :public Base  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Parent1::func1()" << endl;  
        }  
        virtual void func3()  
        {  
            cout << "Parent1::func3()" << endl;  
        }  
        virtual void func4()  
        {  
            cout << "Parent1::func4()" << endl;  
        }  
        virtual void func5()  
        {  
            cout << "Parent1::func5()" << endl;  
        }  
    private:  
        int _p1;  
    };  
      
    class Parent2 :public Base  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Parent2::func1()" << endl;  
        }  
        virtual void func3()  
        {  
            cout << "Parent2::func3()" << endl;  
        }  
        virtual void func6()  
        {  
            cout << "Parent2::func6()" << endl;  
        }     
        virtual void func7()  
        {  
            cout << "Parent2::func7()" << endl;  
        }  
    private:  
        int _p2;  
    };  
      
    class Child :public Parent1, public Parent2  
    {  
    public:  
        virtual void func1()  
        {  
            cout << "Child::func1()" << endl;  
        }  
        virtual void func3()  
        {  
            cout << "Child::func3()" << endl;  
        }  
        virtual void func4()  
        {  
            cout << "Child::func4()" << endl;  
        }  
        virtual void func6()  
        {  
            cout << "Child::func6()" << endl;  
        }     
        virtual void func8()  
        {  
            cout << "Child::func8()" << endl;  
        }  
    private:  
        int _c;  
    };  
      
    void PrintVfptr(int* vptr)  
        //打印虚函数表  
    {  
        cout << "虚函数表: " << vptr << endl;  
        for (int i = 0; vptr[i] != 0; ++i)  
        {  
            printf("第%d个虚函数：%p  >> ", i, vptr[i]);  
            FUNC f = (FUNC)(vptr[i]);  
            f();  
        }  
    }  
    void Test()  
    {  
        Base b;  
      
        Parent1 p1;   
        Parent2 p2;  
      
        Child c;  
      
        int* vptr_B = (int*)(*(((int*)(&b))));  
      
        int* vptr_P1 = (int*)(*(((int*)(&p1))));  
      
        int* vptr_P2= (int*)(*(((int*)(&p2))));  
      
        PrintVfptr(vptr_B);  
        cout << endl;  
      
        PrintVfptr(vptr_P1);  
        cout << endl;  
      
        PrintVfptr(vptr_P2);  
        cout << endl;  
      
        int* vptr_C_1 = (int*)(*(((int*)(&c))));  
        int* vptr_C_2 = (int*)(*(int*)((char*)(&c) + sizeof(Parent1)));  
      
        PrintVfptr(vptr_C_1);  
        cout << endl;  
        PrintVfptr(vptr_C_2);  
        cout << endl;  
    }  
    int main()  
    {  
        Test();  
        system("pause");  
        return 0;  
    }

运行结果：

[![wKioL1bcGALwhph7AABvr4tJVO0897.png][]][wKioL1bcGALwhph7AABvr4tJVO0897.png]其中，Child有两个虚函数表，因为它有两个直接父类。

总结：多继承中会有多个虚函数表，几重继承就会有几个虚函数表。这些虚函数表会按照派生的顺序依次排列。如果子类改写了父类的虚函数，那么就会用子类自己的虚函数覆盖相应的父类虚函数；如果子类有新的虚函数，那么就添加到第一个虚函数表的末尾。

**本文转自：**

**https://blog.csdn.net/fucangzxx/article/details/51534117**

[wKioL1bcEMbyJxs-AAAQf1oQ-Ns235.png]: /images/20220528/159ef28d5b4247809cb2404127ed224d.png
[wKiom1bcEHuxOB0QAAAcF7C983I617.png]: /images/20220528/157ca3f0a72b4ca8b926c4e81a869601.png
[wKioL1bcFcSzAXRSAAAX-leGeyg336.png]: /images/20220528/8185ef2fd5bc40939de54a2b813404e5.png
[wKioL1bcFq2xTaMeAAAQJsD_jQU490.png]: /images/20220528/6220018016724d0ab655b20c107df761.png
[wKioL1bcFyCAR3u_AAAq7ZbMpMs067.png]: /images/20220528/0d32bee7ca9843c6acb562f3532ec5b4.png
[wKioL1bcGALwhph7AABvr4tJVO0897.png]: /images/20220528/83b5cafcaf9c4fa8ac9a49a85a84a476.png