C + +: virtual function of class and space occupied by virtual inheritance

Char takes up one byte, but does not satisfy a multiple of 4, the data is aligned, using bytes (byte) in multiples of 4.

Gcc:

In GCC, either a virtual function or a virtual inheritance requires that the pointer be stored in a virtual function table, which occupies 4 bytes.

Inheritance inherits the base class's data, and the virtual function table, which is the space that inherits the base class.

Code:

* * * test.cpp * *  Created on:2014.04.20 * 
 author:spike/
      
/*eclipse CDT, gcc 4.8.1* /
      
#include <iostream>  
      
using namespace std;  
      
Class A {  
    char k[5];  
Public:  
    virtual void aa () {};  
      
Class B:virtual public A {  
    char j[6];  
Public:  
    virtual void bb () {};  
};  
      
Class C:virtual public B {  
    char i[7];  
Public:  
    virtual void cc () {};  
      
int main () {  
    std::cout << "sizeof (a) =" << sizeof (a) << Std::endl;  
    Std::cout << "sizeof (b) =" << sizeof (b) << Std::endl;  
    Std::cout << "sizeof (c) =" << sizeof (c) << Std::endl;  
      
    return 0;  
}

Output:

sizeof (A) =  
sizeof (B) =  
sizeof (C) = 36

Vs:

VS, in normal circumstances, all virtual functions are written in a virtual function table, that is, the derived class uses a virtual function table of the base class, and does not create a virtual function table of bytes;

If you use virtual inheritance, the derived class creates a new virtual function table and a virtual inheritance table, that is, an additional 8 bytes (two tables);

Output:

Author: csdn Blog spike_king

More Wonderful content: http://www.bianceng.cnhttp://www.bianceng.cn/Programming/cplus/