C++ 多态与虚函数表（一）—— 单继承

绝地灬酷狼 2022-11-28 00:44 174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、取地址 ##

class Base
    {
    public:
        long long value;
        virtual f();
    };
    typedef void(*ptrType)(void);
    Base b;
    cout << "虚函数表地址：" << (ptrType*)(&b) << endl;
    cout << "虚函数表 — 第一个函数地址：" << (ptrType*)*(ptrType*)(&b) << endl;
    
    // 方法一
    ptrType* pHead = (ptrType*)(&b);        // pHead: 指向的是类空间首地址（即，虚函数表地址）
    ptrType* pVptr = (ptrType*)(*pHead);    // pVptr: 虚函数表第一个函数地址
    
    ptrType F = (ptrType)*(pVptr+i);        // 取第i+1个虚函数指针， Base::f()
    F(); // i = 0, 执行第1个虚函数 Base::f()
    long long value = *(long long *)(pHead + 1);    // Base::value
    
    // 方法二
    ptrType** pVtab = (ptrType**)&b;           // pVtab[0] 内存布局首地址，如有虚指针为虚
    ptrType F = (ptrType)pVtab[0][0];          // 取第1个虚函数指针， Base::f()
    long long value = (long long)*(&pVtab[1];  // Base::value

## 三、含有虚函数的基类 ##

### 3.1 基类定义 ###

### 3.2 类图和内存布局 ###

**类图**

![20200816161213113.png][]

**内存布局图**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70][]

### 3.3 测试， 64位，G++ 4.6.4版本 ###

// 基类测试
    void test1()
    {
        typedef void(*ptrType)(void);
        Parent b;
        cout << "虚函数表地址：" << (ptrType*)(&b) << endl;
        cout << "虚函数表 — 第一个函数地址：" << (ptrType*)*(ptrType*)(&b) << endl;
    
        ptrType* pHead = (ptrType*)(&b);        // pHead: 指向的是对象空间首地址（即，虚函数表地址）
        ptrType* pVptr = (ptrType*)(*pHead);    // pVptr: 虚函数表第一个函数地址
    
    #define DEFFUN(i) {ptrType F = (ptrType)*((ptrType*)*(ptrType*)(&b)+i); cout << "   ["<<i<<"] "; F();}
        cout << "[0] Parent::_vptr->" << endl;
        DEFFUN(0);  // Parent::f()
        DEFFUN(1);  // Parent::g()
        DEFFUN(2);  // Parent::h()
        cout << "[1] Parent.iparent = " << *(long long *)(pHead + 1) << endl;
    }

### 3.4 执行结果： ###

![2020081616133778.png][]

## 四、子类继承父类测试1 ##

### 4.1 子类定义 ###

### 4.2 类图和内存布局 ###

**类图**

![20200816161505737.png][]

**内存布局图**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 1][]

### 4.3 测试， 64位，G++ 4.6.4版本 ###

// 子类继承父类测试1
    void test2()
    {
        typedef void(*ptrType)(void);
        Child b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << "[0] Child::_vptr->" << endl;
        for (int i = 0; (ptrType)pVtab[0][i] != NULL; ++i)
        {
            pFun = (ptrType)pVtab[0][i];
            cout << "   ["<<i<<"] ";
            pFun(); 
        }
    
        cout << "[1] Parent.iparent = " << (long long)*(&pVtab[1]) << endl;
        cout << "[2] Child.ichild = " << (long long)*(&pVtab[2]) << endl;
    }

### 4.4 执行结果 ###

![20200816161633278.png][]

## 五、子类继承父类测试2 ##

### 5.1 子类定义 ###

class Parent 
    {
    public:
        long long iparent;
        Parent ():iparent (10) {}
        virtual void f() { cout << "Parent::f()" << endl; }
        virtual void g() { cout << "Parent::g()" << endl; }
        virtual void h() { cout << "Parent::h()" << endl; }
    };
    
    class Child : public Parent 
    {
    public:
        long long ichild;
        Child():ichild(100) {}
        virtual void f() { cout << "Child::f()" << endl; }
        virtual void g_child() { cout << "Child::g_child()" << endl; }
        virtual void h_child() { cout << "Child::h_child()" << endl; }
    };
    
    class GrandChild : public Child
    {
    public:
        long long igrandchild;
        GrandChild():igrandchild(1000) {}
        virtual void f() { cout << "GrandChild::f()" << endl; }
        virtual void g_child() { cout << "GrandChild::g_child()" << endl; }
        virtual void h_grandchild() { cout << "GrandChild::h_grandchild()" << endl; }
    };

### 5.2 类图和内存布局 ###

**类图**

![20200816161748657.png][]

**内存布局图**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 2][]

### 5.3 测试， 64位，G++ 4.6.4版本 ###

// 子类继承父类测试2
    void test3()
    {
        typedef void(*ptrType)(void);
        GrandChild b;
        ptrType pFun = NULL;
        long long** pVtab = (long long**)&b;
        cout << "[0] GrandChild::_vptr->" << endl;
        // 在本机测试这种情况通过判断指针是否为空可以，但并不是所有情况都可以
        for (int i = 0; (ptrType)pVtab[0][i]!=NULL; ++i)
        {
            pFun = (ptrType)pVtab[0][i];
            cout << "   ["<<i<<"] ";
            pFun(); 
        }
    
        //cout << "1值：" << *(long long*)((long long*)&b + 1) << endl;
        cout << "[1] Parent.iparent = " << (long long)*(&pVtab[1]) << endl;
        cout << "[2] Child.ichild = " << (long long)*(&pVtab[2]) << endl;
        cout << "[3] GrandChild.igrandchild = " << (long long)*(&pVtab[3]) << endl;
    }

### 5.4 执行结果 ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L25pZTIzMTQ1NTA0NDE_size_16_color_FFFFFF_t_70 3][]

## 六、总结 ##

1.  虚函数表在最前面的位置。
2.  成员变量根据其继承和声明顺序依次放在后面。
3.  在单一的继承中，被overwrite的虚函数在虚函数表中得到了更新。

## 七、完整代码 ##

#include<iostream>
    using namespace std;
    
    class Parent 
    {
    public:
        long long iparent;
        Parent ():iparent (10) {}
        virtual void f() { cout << "Parent::f()" << endl; }
        virtual void g() { cout << "Parent::g()" << endl; }
        virtual void h() { cout << "Parent::h()" << endl; }
    };
    
    class Child : public Parent 
    {
    public:
        long long ichild;
        Child():ichild(100) {}
        virtual void f() { cout << "Child::f()" << endl; }
        virtual void g_child() { cout << "Child::g_child()" << endl; }
        virtual void h_child() { cout << "Child::h_child()" << endl; }
    };
    
    class GrandChild : public Child
    {
    public:
        long long igrandchild;
        GrandChild():igrandchild(1000) {}
        virtual void f() { cout << "GrandChild::f()" << endl; }
        virtual void g_child() { cout << "GrandChild::g_child()" << endl; }
        virtual void h_grandchild() { cout << "GrandChild::h_grandchild()" << endl; }
    };
    
    // 基类测试
    void test1()
    {
        typedef void(*ptrType)(void);
        Parent b;
        cout << "虚函数表地址：" << (ptrType*)(&b) << endl;
        cout << "虚函数表 — 第一个函数地址：" << (ptrType*)*(ptrType*)(&b) << endl;
    
        ptrType* pHead = (ptrType*)(&b);        // pHead: 指向的是对象空间首地址（即，虚函数表地址）
        ptrType* pVptr = (ptrType*)(*pHead);    // pVptr: 虚函数表第一个函数地址
    
    #define DEFFUN(i) {ptrType F = (ptrType)*((ptrType*)*(ptrType*)(&b)+i); cout << "   ["<<i<<"] "; F();}
        cout << "[0] Parent::_vptr->" << endl;
        DEFFUN(0);  // Parent::f()
        DEFFUN(1);  // Parent::g()
        DEFFUN(2);  // Parent::h()
        cout << "[1] Parent.iparent = " << *(long long *)(pHead + 1) << endl;
    }
    
    // 子类继承父类测试1
    void test2()
    {
        typedef void(*ptrType)(void);
        Child b;
        ptrType pFun = NULL;
        ptrType** pVtab = (ptrType**)&b;
        cout << "[0] Child::_vptr->" << endl;
        // 在本机测试这种情况通过判断指针是否为空可以，但并不是所有情况都可以
        for (int i = 0; (ptrType)pVtab[0][i] != NULL; ++i)
        {
            pFun = (ptrType)pVtab[0][i];
            cout << "   ["<<i<<"] ";
            pFun(); 
        }
    
        cout << "[1] Parent.iparent = " << (long long)*(&pVtab[1]) << endl;
        cout << "[2] Child.ichild = " << (long long)*(&pVtab[2]) << endl;
    }
    
    // 子类继承父类测试2
    void test3()
    {
        typedef void(*ptrType)(void);
        GrandChild b;
        ptrType pFun = NULL;
        long long** pVtab = (long long**)&b;
        cout << "[0] GrandChild::_vptr->" << endl;
        for (int i = 0; (ptrType)pVtab[0][i]!=NULL; ++i)
        {
            pFun = (ptrType)pVtab[0][i];
            cout << "   ["<<i<<"] ";
            pFun(); 
        }
    
        //cout << "1值：" << *(long long*)((long long*)&b + 1) << endl;
        cout << "[1] Parent.iparent = " << (long long)*(&pVtab[1]) << endl;
        cout << "[2] Child.ichild = " << (long long)*(&pVtab[2]) << endl;
        cout << "[3] GrandChild.igrandchild = " << (long long)*(&pVtab[3]) << endl;
    }
    
    int main()
    {
        test1();
        test2();
        test3();
        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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[2020081616133778.png]: /images/20221124/1295ab2de4e146dcb37500fce90781b2.png
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