C++虚继承中的对象内存布局

钻石型虚拟继承

虚继承是为了解决多继承中的数据冗余而出现的。

eg：

#define _CRT_SECURE_NO_WARNINGS 1

#include <iostream>
using namespace std;

typedef void(*FUNC)();

class Base
{
public:
virtual void fun1()
{
cout << "Base::fun1()" << endl;
}
public:
int _b;
};

class Parent1:virtual public Base
{
public:
virtual void fun1()
{
cout << "Parent1::fun1()" << endl;
}
virtual void fun2()
{
cout << "Parent1::fun2()" << endl;
}
public:
int _p1;
};

class Parent2 :virtual public Base
{
public:
virtual void fun1()
{
cout << "Parent2::fun1()" << endl;
}
virtual void fun3()
{
cout << "Parent2::fun3()" << endl;
}
public:
int _p2;
};

class Child : public Parent1,public Parent2
{
public:
virtual void fun1()
{
cout << "Child::fun1()" << endl;
}
virtual void fun2()
{
cout << "Child::fun2()" << endl;
}
virtual void fun3()
{
cout << "Child::fun3()" << endl;
}
virtual void fun4()
{
cout << "Child::fun4()" << endl;
}
public:
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 PrintfMove(int* vbptr)
//打印偏移量
{
cout << "偏移量 >: " << endl;
for (int i = 0; vbptr[i] != 0; ++i)
{
printf("第%d个虚函数：%d\n", i, vbptr[i]);
}
cout << endl;
}
void Test()
{
Child c;
c._b = 1;
c._p1 = 2;
c._p2 = 3;
c._c = 4;

cout << "sizeof(Base)::" << sizeof(Base) << endl;
cout << "sizeof(Parent1)::" << sizeof(Parent1) << endl;
cout << "sizeof(Parent2)::" << sizeof(Parent2) << endl;
cout << "sizeof(Child)::" << sizeof(Child) << endl;

int* cAddress = (int*)&c;
cout << "Parent1::";
int* tmpP1 = (int*)(*cAddress);
PrintVfptr(tmpP1);
cout << "Parent1::";
int* moveP1 = (int*)(*(cAddress + 1));
PrintfMove(moveP1);
cout << endl;

cout << "Parent2::";
int* tmpP2 = (int*)(*(cAddress + 3));
PrintVfptr(tmpP2);
cout << endl;
cout << "Parent1::";
int* moveP2 = (int*)(*(cAddress + 4));
PrintfMove(moveP2);
cout << endl;

int* tmpBase = (int*)(*(cAddress + 7));
PrintVfptr(tmpBase);
cout << endl;
}

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

程序运行结果：



对象在内存中的布局：


大家可以发现：
0x0014F8F8 + 0xFFFFFFFC(-4) = 0x0014F8F4, 0x0014F8F8 + 0x00000018 = 0x0014F910;
0x0014F904 + 0xFFFFFFFC(-4) = 0x0014F900, 0x0014F904 + 0x0000000c = 0x0014F910.
所以，有以下结论：
在虚继承时，类中会自动加一个指针(VBPTR),该变量指向一个全类共享的偏移量表，如果本类有虚函数，那么第一项记录着当前子对象相对与虚基类表指针（VBPTR指针）的偏移，是FF FF FF FC(也就是-4)，如果没有则是零；第二项起是被继承的基类（上述例子为Base类）子对象相对于VBPTR指针的偏移量。