C++ 多态与虚函数表（二）—— 多继承

浅浅的花香味﹌ 2022-11-28 00:45 141阅读 0赞

## 一、前言 ##

个人笔记，不保证严谨和正确☺，如有错误还望指出！

[《C++ 多态与虚函数表（一）—— 单继承》][C_ _]

[《C++ 多态与虚函数表（二）—— 多继承》][C_ _ 1]

[《C++ 多态与虚函数表（三）—— 重复继承》][C_ _ 2]

[《C++ 多态与虚函数表（四）—— 虚继承》][C_ _ 3]

[《C++ 多态与虚函数表（五）拓展—— 对象大小》][C_ _ 4]

## 二、多继承 ##

### 2.1 类定义 ###

class Base1 
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (10) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void g() { cout << "Base1::g()" << endl; }
        virtual void h() { cout << "Base1::h()" << endl; }
    };
    
    class Base2 
    {
    public:
        long long ibase2;
        Base2 ():ibase2 (20) {}
        virtual void f() { cout << "Base2::f()" << endl; }
        virtual void g() { cout << "Base2::g()" << endl; }
        virtual void h() { cout << "Base2::h()" << endl; }
    };
    
    class Base3
    {
    public:
        long long ibase3;
        Base3 ():ibase3 (30) {}
        virtual void f() { cout << "Base3::f()" << endl; }
        virtual void g() { cout << "Base3::g()" << endl; }
        virtual void h() { cout << "Base3::h()" << endl; }
    };
    
    class Derive : public Base1, public Base2, public Base3 
    {
    public:
        long long iderive;
        Derive():iderive(100) {}
        virtual void f() { cout << "Derive::f()" << endl; }
        virtual void g_derive() { cout << "Derive::g_derive()" << endl; }
    };

### 2.2 类图 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70][]

### 2.3 内存布局 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 1][]

### 2.4 测试， 64位，G++ 4.6.4版本 ###

// 测试
    void test()
    {
        typedef void(*ptrType)(void);
        Derive b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << "sizeof(Derive): " << sizeof(Derive) << endl;
        
        int index = 0;  // 对象内存访问下标
        auto fun = [&](int basei, int maxj) 
        {
            cout << "[" << index << "] Base" << basei <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] Base" << basei <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun(1, 4);
        fun(2, 3);
        fun(3, 3);
        cout << "[" << index << "] Parent.iparent = " << (long long)*(&pVtab[index]) << endl; 
    }

### 2.5 执行结果 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 2][]

## 三、总结 ##

1.  每个父类都有自己的虚表。
2.  子类的成员函数被放到了第一个父类的表中。
3.  内存布局中，其父类布局依次按声明顺序排列。
4.  每个父类的虚表中的f()函数都被overwrite成了子类的f()。这样做就是为了解决不同的父类类型的指针指向同一个子类实例，而能够调用到实际的函数。
5.  当继承多个基类，每个有虚函数的基类，派生类中都会有一个指向基类虚函数表指针。

## 四、完整代码 ##

#include<iostream>
    using namespace std;
    
    class Base1 
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (10) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void g() { cout << "Base1::g()" << endl; }
        virtual void h() { cout << "Base1::h()" << endl; }
    };
    
    class Base2 
    {
    public:
        long long ibase2;
        Base2 ():ibase2 (20) {}
        virtual void f() { cout << "Base2::f()" << endl; }
        virtual void g() { cout << "Base2::g()" << endl; }
        virtual void h() { cout << "Base2::h()" << endl; }
    };
    
    class Base3
    {
    public:
        long long ibase3;
        Base3 ():ibase3 (30) {}
        virtual void f() { cout << "Base3::f()" << endl; }
        virtual void g() { cout << "Base3::g()" << endl; }
        virtual void h() { cout << "Base3::h()" << endl; }
    };
    
    class Derive : public Base1, public Base2, public Base3 
    {
    public:
        long long iderive;
        Derive():iderive(100) {}
        virtual void f() { cout << "Derive::f()" << endl; }
        virtual void g_derive() { cout << "Derive::g_derive()" << endl; }
    };
    
    // 测试
    void test()
    {
        typedef void(*ptrType)(void);
        Derive b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << "sizeof(Derive): " << sizeof(Derive) << endl;
        
        int index = 0;  // 对象内存访问下标
        auto fun = [&](int basei, int maxj) 
        {
            cout << "[" << index << "] Base" << basei <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] Base" << basei <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun(1, 4);
        fun(2, 3);
        fun(3, 3);
        cout << "[" << index << "] Parent.iparent = " << (long long)*(&pVtab[index]) << endl; 
    }
    
    int main()
    {
        test();
        return 0;
    }

[C_ _]: https://blog.csdn.net/nie2314550441/article/details/108037701
[C_ _ 1]: https://blog.csdn.net/nie2314550441/article/details/108037951
[C_ _ 2]: https://blog.csdn.net/nie2314550441/article/details/108038111
[C_ _ 3]: https://blog.csdn.net/nie2314550441/article/details/108038221
[C_ _ 4]: https://blog.csdn.net/nie2314550441/article/details/108038428
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