单继承与多继承中的虚函数表和虚函数指针

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：

#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 Derive :public Base
{
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(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;
Derive d;

int* vptrBase = (int*)(*(int*)(&b));
int* vptrDeri = (int*)(*(int*)(&d));

PrintVfptr(vptrBase);
cout << endl;
PrintVfptr(vptrDeri);

}

int main()
{
Test();
system("pause");
return 0;
}

程序运行结果：
Base类(基类)的虚函数表：




Derive类(派生类)的虚函数表：



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：



Base2的虚函数表：



Derive的虚函数表：(有两个虚函数表)



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：


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。