我的4个字节在哪里？

Question

#include <iostream>
#include <cstdlib>

using std::cout;

class A
{

public :
    A() { cout << "A()" << this << "\n";}
    ~A() { cout << "~A()" << this << "\n";}
    //void func()  { }
    virtual void debug(int a)  { cout << "A::debug";}
private :
    int a;
};

class A1 : public A
{
public :
    A1() { cout << "A1()"<< this << "\n";}
    ~A1() { cout << "~A1()"<< this << "\n";}
private :
    int a1;
};
class A2 : public A
{
public :
    A2() { cout << "A2()"<< this << "\n";}
    ~A2() { cout << "~A2()"<< this << "\n";}
private :
    int a2;
};

class B : public A1, public A2
{
public :
    B() { cout << "B()"<< this << "\n";}
    ~B() { cout << "~B()"<< this << "\n";}
    void debug() { cout << "B::debug()";  }
private :
    int a3;
};
int main()
{
    cout << "sizeof(int)" << sizeof(int) << "\n";
    cout << "sizeof(void*)" << sizeof(void*) << "\n";
    cout << "sizeof(A): " << sizeof(A) << "\n";
    cout << "sizeof(A1): " << sizeof(A1) << "\n";
    cout << "sizeof(A2): " << sizeof(A2) << "\n";
    cout << "sizeof(B): " << sizeof(B) << "\n";
    B b;
    b.debug();

}

输出:

sizeof(int)4
sizeof(void*)4
sizeof(A): 8
sizeof(A1): 12
sizeof(A2): 12
**sizeof(B): 28**
A()0x28fef4
A1()0x28fef4
**A()0x28ff00**
A2()0x28ff00
B()0x28fef4
B::debug()~B()0x28fef4
~A2()0x28ff00
~A()0x28ff00
~A1()0x28fef4
~A()0x28fef4

A1和A2都是4(vtbl)+ 4(A'sint)+4(相应的int)= 12个字节但是B是28个字节我知道它不能保证但是可能使用那些4个字节......我没有看到任何填充问题？任何人都可以指出我错过了什么？

Answer 1

sizeof(A): 8

该类型A具有类型的成员int,在您的平台中是4个字节.它还有一个虚函数,这意味着vptr为你的类的每个对象分配一个(虚拟表指针),它的大小是另外4个字节.

**sizeof(B): 28**

B包含一个类型的对象A1(12个字节)和一个类型的对象A2(另外12个字节),它int为总共12+12+4 = 28字节添加另一个.这很简单.