c ++ class length parsing

2019 Unicorn Enterprises heavily recruiting engineers Python standard >>>

　　Usually we define a class, how much space it occupies it?

First we look at the following class

class A{
public:
    void func1(void){
        printf("11111heihei\n");
    };
    void func2(void){
        printf("11111heihei\n");
    };
public:
    int number;
};

class B{
public:
    void func1(void){
        printf("heihei\n");
    };
    void func2(void){
        printf("heihei\n");
    };
public:
    char number;
    int number1;
};

 

We outputs of the two classes of length

 1 #include <stdio.h>
 2 class A{
 3 public:
 4     void func1(void){
 5         printf("11111heihei\n");
 6     };
 7     void func2(void){
 8         printf("11111heihei\n");
 9     };
10 public:
11     int number;
12 };
13 class B{
14 public:
15     void func1(void){
16         printf("heihei\n");
17     };
18     void func2(void){
19         printf("heihei\n");
20     };
21 public:
22     char number;
23     int number1;
24 };
25 int main(){
26     printf("A %d B %d\n",sizeof(A),sizeof(B));
27     getchar(); 
28      return 0;
29 }

View Code

 

got the answer:

 

　　As for why the second is 8, it is the memory alignment problem, refer to previous posts memory applications .

　　We found that the length of a class is its variable occupied space, that function is not to take up space. This is because when the object belongs to a class variable, and the function belongs to a class of this class.

　　So because of different objects and different virtual function what will happen?

　　We then look at the following two categories:

class A{
public:
    virtual void func1(void){
        printf("11111heihei\n");
    };
    void func2(void){
        printf("11111heihei\n");
    };
public:
    int number;
};
class B{
public:
    virtual void func1(void){
        printf("heihei\n");
    };
    virtual void func2(void){
        printf("heihei\n");
    };
public:
    int number1;
};

 

　　Output as a function of the following results were obtained:

 1 #include <stdio.h>
 2 class A{
 3 public:
 4     virtual void func1(void){
 5         printf("11111heihei\n");
 6     };
 7     void func2(void){
 8         printf("11111heihei\n");
 9     };
10 public:
11     int number;
12 };
13 class B{
14 public:
15     virtual void func1(void){
16         printf("heihei\n");
17     };
18     virtual void func2(void){
19         printf("heihei\n");
20     };
21 public:
22     int number1;
23 };
24 int main(){
25     printf("A %d B %d\n",sizeof(A),sizeof(B));
26     getchar(); 
27      return 0;
28 }

View Code

 

 

　　We found that more than 4 bytes. This is because the virtual function assigned to a virtual function table vptr object, virtual function is called by the pointer. Class virtual functions therefore, will be more space for a pointer. But only a pointer is assigned. Such as the first function with vptr [1] is represented by the second vptr [2] is represented, and 0 is the index information table.

　　If wrong place, please correct me.

 

Reproduced in: https: //my.oschina.net/u/204616/blog/545402