存在虚基类时,类对象的大小

最新推荐文章于 2022-09-07 21:53:59 发布

wyjvip333

最新推荐文章于 2022-09-07 21:53:59 发布

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分类专栏：语法函数文章标签： C C++ C# J#

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本文通过几个实例展示了C++中虚继承与多重虚继承的内存布局特性，包括虚基类指针的作用及其如何影响派生类的大小。

1，
实例代码:


#include <iostream>
using namespace std;

class Top
{
protected:
    int x;
public:
    Top(int n)
    {
        x = n;
    }
    virtual ~Top() {}
};

class Left : virtual public Top
{
protected:
    int y;
public:
    Left(int m, int n) : Top(m)
    {
        y = n;
    }
};

class Right : virtual public Top
{
protected:
    int z;
public:
    Right(int m, int n) : Top(m)
    {
        z = n;
    }
};

class Bottom : public Left, public Right
{
    int w;
public:
    Bottom(int i, int j, int k, int m) : Top(i), Left(0, j), Right(0, k)
    {
        w = m;
    }
};

int main()
{
  Bottom b(1, 2, 3, 4);

  cout << sizeof(Top) << endl;
  //Top: int+虚表指针  8
  cout << sizeof(Left) << endl;
  //Left: 大小为Top+int+指向虚基类的指针  16
  cout << sizeof(Bottom) << endl;
  //Bottom: 大小为Top+Left(int + 指向虚基类的指针)+Right(int + 指向虚基类的指针)+int 28
}

(2)一个比较奇怪的地方


#include <iostream>
using namespace std;

class Top
{
public:
    Top()
    {
    }
    virtual ~Top() {}
private:
    int i;
};

class Left : virtual public Top
{
public:
    Left()
    {
    }
};

class Right : virtual public Top
{
public:
    Right()
    {
    }
};

class Bottom : public Left, Right
{
};


class Top2
{
public:
    Top2()
    {
    }
    virtual ~Top2() {}
};

class Left2 : virtual public Top2
{
public:
    Left2()
    {
    }
};

class Right2 : virtual public Top2
{
public:
    Right2()
    {
    }
};

class Bottom2 : public Left2, Right2
{
};

int main()
{
    cout << sizeof(Top) << endl;
    cout << sizeof(Left) << endl;
    //Left: Top+虚表指针  12
    cout << sizeof(Bottom) << endl;
    //Left: Top+虚表指针+虚表指针  16

    cout << sizeof(Top2) << endl;
    cout << sizeof(Left2) << endl;
    //Left2: 为什么是4？
    cout << sizeof(Bottom2) << endl;
    //为什么 是 8?
}

(3) 多重虚继承的情况


#include <iostream>
using namespace std;

class A
{
public:
     virtual void aa()
     {
     }
private:
     char k[3];
};

class B : virtual public A
{
public:
     virtual void bb()
     {
     }
private:
     char j[3];
};

class C : virtual public B
{
public:
     virtual void cc()
     {
     }
private:
     char i[3];
};

class D : public B
{
public:
     virtual void cc()
     {
     }
private:
     char i[3];
};

int main()
{
     A a;
     B b;
     C c;
     cout<<"sizeof(A):"<<sizeof(A)<<endl;
     cout<<"sizeof(B):"<<sizeof(B)<<endl;
     //A+4+虚表指针 16
     cout<<"sizeof(C):"<<sizeof(C)<<endl;
     //B+4+续表指针 24
     cout<<"sizeof(D):"<<sizeof(D)<<endl;
     //B+4 20
     return 0;
}