[C/C++] The difference between Struct and Class in C and C++

1. Structure

1.1 C structure and C++ structure

The structure in C++ is an extension of the C structure, adding some new usages and differences when using attributes

:

struct Human
{
    
    
	char name[15];
	int age;
};
typedef struct Human human_; C语言中取别名
struct Human tom;			 C语言中使用
Human tom; 					 C语言别名使用/C++中使用

Fundamental: The
structure in the C language is a collection of different types of elements, and does not involve the algorithm for processing data
. The structure in C++ not only contains data but also encapsulates different methods of processing data.

1.2 C++ structure and C++ class

1. The default permissions of internal members are different

struct student1		成员默认public属性
{
    
    
	int age;
};
class student2		成员默认private属性
{
    
    
	int age;
};

2. Different default permissions under inheritance

struct human
{
    
    
	int age;
}
struct student : human		默认public继承属性
{
    
    
	int Stu_ID;
}
student tom;
tom.age = 19;	在class下会报错，class默认private属性

3. Use of templates

定义一个模板：
template <class T> 或者 template <typename T>，但不能是：struct

1.3 Structure memory alignment

Purpose: In order to improve the efficiency of program operation, the CPU needs to access unaligned memory twice, while aligned memory access only needs one time
. The address of the CPU access must meet an integer multiple of 4. Memory alignment is a sacrifice of space in exchange for efficiency Plan

Alignment rules:

1. The first element does not need to be aligned
2. Other members are aligned to integer multiples of their own type
3. If the default alignment number is specified, it will be aligned to the address with the smaller value of its own type and the default alignment number
4. Total size: an integer multiple of the maximum number of alignments

struct A	// 24
{
    
    
	char a;		
	double b;
	char c;	
};
struct B	// 12
{
    
    
	char a;
	int b;
	char c;
};
struct C	// 8 
{
    
    
	char a;
	char b;
	int c;
};
struct D	// 16 
{
    
    
	double a;
	char b;
	int c;
};
struct E	// 32
{
    
    
	char a;
	struct D b;
	double c;
};

Second, position

1.1 What is a position?

1. Bit segment is a kind of data compression, without considering efficiency, paying attention to space usage, so there is no need for memory alignment
2. Bit segments are used in TCP/IP headers

struct A
{
    
    
	char a : 3;
	char b : 4;
	char c : 5;
	char d : 4;
};
int main()
{
    
    
	A a = {
    
     0 };	// 初始化，防止里面是随机值
	a.a = 10;
	a.b = 12;
	a.c = 3;
	a.d = 4;
	cout << sizeof(struct A) << endl;
	return 0;
}

1.2 The use of bit segment in TCP/IP protocol header

struct 
{
    
    
	unsigned int vison : 4;
	unsigned int handler lend : 4
	unsigned int TOS : 8
	unsigned int total length : 16
};

Three, joint

The members of the union share the same space, so the union is also called a union

union Un 
{
    
     
 char c; 
 int i; 
}; 
//联合变量的定义
union Un un; 
//计算连个变量的大小
printf("%d\n", sizeof(un));
un.i = 1
if (un.c)
	printf("小端\n");
else
	printf("大端\n");