C++ 多态与虚函数表（三）—— 重复继承

曾经终败给现在 2022-11-28 00:45 113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、重复继承 ##

### 2.1 类定义 ###

class Base
    {
    public:
        long long ibase;
        Base ():ibase (10) {}
        virtual void f() { cout << "Base::f()" << endl; }
        virtual void g() { cout << "Base::g()" << endl; }
        virtual void h() { cout << "Base::h()" << endl; }
    };
    
    class Base1 : public Base
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (100) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void g_base1() { cout << "Base1::g_base1()" << endl; }
        virtual void h_base1() { cout << "Base1::h_base1()" << endl; }
    };
    
    class Base2 : public Base
    {
    public:
        long long ibase2;
        Base2 ():ibase2 (200) {}
        virtual void f() { cout << "Base2::f()" << endl; }
        virtual void g() { cout << "Base2::g()" << endl; }
        virtual void h_base2() { cout << "Base2::h_base2()" << endl; }
    };
    
    class Derive : public Base1, public Base2
    {
    public:
        long long iderive;
        Derive():iderive(1000) {}
        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;
            cout << "[" << index << "] Base" << basei <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun(1, 6);
        fun(2, 4);
        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 ##

最顶端的父类Base其成员变量存在于Base1和Base2中，并被Derive给继承下去了。而在Derive中，其有Base1和Base2的实例，于是B的成员在D的实例中存在两份，一份是Base1继承而来的，另一份是B2继承而来的。所以，如果我们使用以下语句，则会产生二义性编译错误：

Derive d;
    d.iderive = 0; //二义性错误
    d.Base1::ibase = 1; //正确
    d.Base2::ibase = 2; //正确

注意，上面例程中的最后两条语句存取的是两个变量。虽然我们消除了二义性的编译错误，但Base类在Derive中还是有两个实例，这种继承造成了数据的重复，我们叫这种继承为重复继承。重复的基类数据成员可能并不是我们想要的。所以，C++引入了虚基类的概念。

## 四、完整代码 ##

#include<iostream>
    using namespace std;
    
    class Base
    {
    public:
        long long ibase;
        Base ():ibase (10) {}
        virtual void f() { cout << "Base::f()" << endl; }
        virtual void g() { cout << "Base::g()" << endl; }
        virtual void h() { cout << "Base::h()" << endl; }
    };
    
    class Base1 : public Base
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (100) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void g_base1() { cout << "Base1::g_base1()" << endl; }
        virtual void h_base1() { cout << "Base1::h_base1()" << endl; }
    };
    
    class Base2 : public Base
    {
    public:
        long long ibase2;
        Base2 ():ibase2 (200) {}
        virtual void f() { cout << "Base2::f()" << endl; }
        virtual void g() { cout << "Base2::g()" << endl; }
        virtual void h_base2() { cout << "Base2::h_base2()" << endl; }
    };
    
    class Derive : public Base1, public Base2
    {
    public:
        long long iderive;
        Derive():iderive(1000) {}
        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;
            cout << "[" << index << "] Base" << basei <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun(1, 6);
        fun(2, 4);
        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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