Class size calculation

First of all, let's make a summary, and then we will give an actual example

The size of the empty class is 1 byte
In a class, the virtual function itself, member functions (including static and non-static) and static data members do not occupy the storage space of class objects
For classes containing virtual functions, no matter how many virtual functions there are, there is only one virtual pointer, the size of VPTR
Ordinary inheritance, derived class inherits all the functions and members of the base class, and the size is calculated according to byte alignment
Virtual function inheritance, whether single inheritance or multi inheritance, inherits the VPTR of the base class(32 bit operating system 4 bytes, 64 bit operating system 8 bytes)！
Virtual inheritance inherits the VPTR of the base class

1.Rule 1

c++

/**
 * @file blackclass.cpp
 * @brief The size of the empty class is 1 byte
 * @author 光城
 * @version v1
 * @date 2019-07-21
 */
#include<iostream>
using namespace std;
class A{};
int main()
{
    cout<<sizeof(A)<<endl;
    return 0;
}

2.Rule 2

c++

/**
 * @file static.cpp
 * @brief
 * 
 * @author 光城
 * @version v1
 * @date 2019-07-21
 */
#include<iostream>
using namespace std;
class A
{
    public:
        char b;
        virtual void fun() {};
        static int c;
        static int d;
        static int f;
};

int main()
{
    /**
     * @brief 16 
     */
    cout<<sizeof(A)<<endl; 
    return 0;
}

3.Rule 3

c++

/**
 * @file morevir.cpp
 * @brief 
 * @author 
 * @version v1
 * @date 2019-07-21
 */
#include<iostream>
using namespace std;
class A{
    virtual void fun();
    virtual void fun1();
    virtual void fun2();
    virtual void fun3();
};
int main()
{
    cout<<sizeof(A)<<endl; // 8
    return 0;
}

4.Rule 4 and 5

c++

/**
 * @file geninhe.cpp
 * @brief 
 * 
 * 
 * @author 光城
 * @version v1
 * @date 2019-07-21
 */

#include<iostream>

using namespace std;

class A
{
    public:
        char a;
        int b;
};

/**
 * @brief 
 * char a
 * int b
 * short a
 * long b
 * 根据字节对齐4+4=8+8+8=24
 */
class B:A
{
    public:
        short a;
        long b;
};
class C
{
    A a;
    char c;
};
class A1
{
    virtual void fun(){}
};
class C1:public A1
{
};

int main()
{
    cout<<sizeof(A)<<endl; // 8
    cout<<sizeof(B)<<endl; // 24
    cout<<sizeof(C)<<endl; // 12
    /**
     * @brief 
     */
    cout<<sizeof(C1)<<endl; // 8 
    return 0;
}

5.Rule 6

c++

/**
 * @file virnhe.cpp
 * @brief 
 * @author 光城
 * @version v1
 * @date 2019-07-21
 */

#include<iostream>
using namespace std;
class A
{
    virtual void fun() {}
};
class B
{
    virtual void fun2() {}
};
class C : virtual public  A, virtual public B
{
    public:
        virtual void fun3() {}
};

int main()
{
    /**
     * @brief 8 8 16  The derived class inherits multiple virtual functions and inherits the VPTR of all virtual functions
     */
    cout<<sizeof(A)<<" "<<sizeof(B)<<" "<<sizeof(C);

    return 0;
}