C++ 多态与虚函数表（四）—— 虚继承

妖狐艹你老母 2022-11-28 00:45 203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、单虚继承Base1 ##

只有一层继承关系且为虚继承，只是用于测试，实际工作中不会这么样。

### 2.1 类定义 ###

### 2.2 类图 ###

![20200816163745601.png][]

### 2.3 内存布局 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70][]

### 2.4 测试， 64位，G++ 4.6.4版本 ###

// 测试 Base1
    void test1()
    {
        typedef void(*ptrType)(void);
        Base1 b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << ">>>>>>sizeof(Base1): " << sizeof(Base1) << endl;
    
        int index = 0;  // 对象内存访问下标
        auto fun = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base1", 4);
        fun("Base", 3);
    }

### 2.6 小结 ###

通过结果我们发现在单虚继承的时候，对象内存布局为：指向子类的虚表指针，子类成员对象，指向父类的虚表指针，父类成员对象。

## 三、单虚继承Base2 ##

同理单虚继承Base1

### 3.1 类定义 ###

class Base2 : virtual 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 b2_1() { cout << "Base2::b2_1()" << endl; }
        virtual void b2_2() { cout << "Base2::b2_2()" << endl; }
        virtual void b2_3() { cout << "Base2::b2_3()" << endl; }
        virtual void b2_4() { cout << "Base2::b2_4()" << endl; }
    };

### 3.2 测试， 64位，G++ 4.6.4版本 ###

// 测试 Base2
    void test2()
    {
        typedef void(*ptrType)(void);
        Base2 b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << ">>>>>>sizeof(Base2): " << sizeof(Base2) << endl;
    
        int index = 0;  // 对象内存访问下标
        auto fun = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base2", 6);
        fun("Base", 3);
    }

### 3.3 执行结果 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 1][]

## 四、菱形继承 ##

### 4.1 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 : virtual public Base
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (100) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void b1_1() { cout << "Base1::b1_1()" << endl; }
        virtual void b1_2() { cout << "Base1::b1_2()" << endl; }
        virtual void b1_3() { cout << "Base1::b1_3()" << endl; }
    };
    
    class Base2 : virtual 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 b2_1() { cout << "Base2::b2_1()" << endl; }
        virtual void b2_2() { cout << "Base2::b2_2()" << endl; }
        virtual void b2_3() { cout << "Base2::b2_3()" << endl; }
        virtual void b2_4() { cout << "Base2::b2_4()" << 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; }
    };

### 4.2 类图 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 2][]

### 4.3 内存布局 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 3][]

### 4.4 测试， 64位，G++ 4.6.4版本 ###

// 测试 Derive
    void test3()
    {
        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 = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base1", 5);
        fun("Base2", 6);
    
        // Derive::iderive
        cout << "[" << index << "] Derive.iderive = " << (long long)*(&pVtab[index]) << endl; 
        ++index;
    
        // 虚继承的基类 Base
        fun("Base", 3);
    }

### 4.5 执行结果 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 4][]

## 五、总结 ##

### 5.1 何时使用虚继承 ###

1.  当一个派生类从多个基类派生，而这些基类又有一个共同的基类，则在对该基类中声明的名字进行访问时，可能产生二义性。
2.  如果在多条继承路径上有一个公共的基类，那么在继承路径的某处汇合点，这个公共基类就会在派生类的对象中产生多个基类子对象。
3.  要使这个公共基类在派生类中只产生一个子对象，必须对这个基类声明为虚继承，使这个基类成为虚基类

### 5.2 虚继承中内存布局 ###

1.  单虚继承时，对象内存布局为：指向子类的虚表指针，子类成员对象，指向父类的虚表指针，父类成员对象。例如：单虚继承Base1
2.  当一个派生类有多个基类，这些基类都虚继承一个公共的基类，内存布局是在普通多继承基础上添加了一个指向公共基类的虚指针和公共基类成员对象。例如：四、菱形继承

## 六、完整代码 ##

#include<iostream>
    #include<string>
    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 : virtual public Base
    {
    public:
        long long ibase1;
        Base1 ():ibase1 (100) {}
        virtual void f() { cout << "Base1::f()" << endl; }
        virtual void b1_1() { cout << "Base1::b1_1()" << endl; }
        virtual void b1_2() { cout << "Base1::b1_2()" << endl; }
        virtual void b1_3() { cout << "Base1::b1_3()" << endl; }
    };
    
    class Base2 : virtual 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 b2_1() { cout << "Base2::b2_1()" << endl; }
        virtual void b2_2() { cout << "Base2::b2_2()" << endl; }
        virtual void b2_3() { cout << "Base2::b2_3()" << endl; }
        virtual void b2_4() { cout << "Base2::b2_4()" << 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 Base1
    void test1()
    {
        typedef void(*ptrType)(void);
        Base1 b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << ">>>>>>sizeof(Base1): " << sizeof(Base1) << endl;
    
        int index = 0;  // 对象内存访问下标
        auto fun = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base1", 4);
        fun("Base", 3);
    }
    
    // 测试 Base2
    void test2()
    {
        typedef void(*ptrType)(void);
        Base2 b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << ">>>>>>sizeof(Base2): " << sizeof(Base2) << endl;
    
        int index = 0;  // 对象内存访问下标
        auto fun = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base2", 6);
        fun("Base", 3);
    }
    
    // 测试 Derive
    void test3()
    {
        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 = [&](string strName, int maxj) 
        {
            cout << "[" << index << "] " << strName <<"::vptr->" << endl;
            for (int j = 0; j < maxj; ++j)
            {
                pFun = (ptrType)pVtab[index][j];
                cout << "   [" << j <<"] ";
                pFun(); 
            }
            ++index;
            cout << "[" << index << "] " << strName <<".ibase = " << (long long)*(&pVtab[index]) << endl; 
            ++index;
        };
    
        fun("Base1", 5);
        fun("Base2", 6);
    
        // Derive::iderive
        cout << "[" << index << "] Derive.iderive = " << (long long)*(&pVtab[index]) << endl; 
        ++index;
    
        // 虚继承的基类 Base
        fun("Base", 3);
    }
    
    int main()
    {
        test1();    // Base1
        test2();    // Base2
        test3();    // Derive
        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
[20200816163745601.png]: /images/20221124/0c1943fd6df14b1f886798aec66ee3d8.png
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