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seeing the name of a thing one thinks of its function , The pointer can point to a certain position ; Pointer variable is the address used to store a certain number or a certain space ;

#include <stdio.h>
int main()
{
	int a = 10;// Open up a space in memory 
	int* pa = &a;// Here, for variables a Go to the address , Use & The operator 
				 /*a Variable occupancy 4 Bytes of space , Here is the general a Of 4 The address of the first byte of a byte is stored in p Variable 
	 in ,p Is a pointer variable to */
	printf("%d ", *pa);
	return 0;
}

Two 、 Pointers and pointer types

Among the data types we have learned , There are integers 、 Short 、 floating-point 、 Long integer and so on ; So does the pointer have a type ？ How pointers are defined ？
Pointer defined method ：
type + * + Variable name

	char* pc = NULL;// Character pointer  ---  The pointer variable is pc  Its type is char* 
	int* pi = NULL;// Integer pointer  ---  The pointer variable is pi  Its type is int* 
	short* ps = NULL;// Short integer pointer  ---  The pointer variable is ps  Its type is short* 
	long* pl = NULL;// Long pointer  ---  The pointer variable is pl  Its type is long* 
	float* pf = NULL;// Single precision floating point pointer  ---  The pointer variable is pf  Its type is float* 
	double* pd = NULL;// Double precision floating point pointer  ---  The pointer variable is pd  Its type is double*

2.1 Pointers add and subtract integers

Since the pointer stores the address of a certain number or a certain space , So what happens when the pointer adds or subtracts integers ？

#include <stdio.h>
int main()
{
	int n = 10;
	int* pi = &n;
    //%p ---  It prints the address in hexadecimal 
	printf("%p\n", &n);
	printf("%p\n", pi);
	printf("%p\n", pi + 1);
	return 0;
}

2.2 Dereference of pointer

The pointer stores the address of a certain number or a certain space , When we want to get this number or the content of this space , You need to dereference ; for instance ： You need to The key , Yes The key To open the door ; Login WeChat 、QQ、 Weibo needs Account and password , Yes Account and password To see the content ; These things we need can be understood as Dereference operation ;

#include <stdio.h>
int main()
{
	int n = 10;
	int* pi = &n;
	printf("%d\n", *pi);
	// Here it is pi Add one before '*' Sign means yes pi Dereference the stored address , Just to get the value corresponding to this address 
	return 0;
}

3、 ... and 、 Wild pointer

Concept ： The position of the pointer is unknown （ Random 、 incorrect 、 There is no definite limit to ）

3.1 The cause of the wild pointer

1. Pointer not initialized

We're creating local variable When , uninitialized , By default, this variable is a random value ; stay Local variable pointer in , It's the same ;

#include <stdio.h>
int main()
{
int *p;// Local variable pointer not initialized , The default value is random 
  *p = 20;
return 0;
}

2. Pointer cross boundary access

#include <stdio.h>
int main()
{
	int arr[10] = { 0 };
	int* p = arr;
	int i = 0;
	for (i = 0; i <= 11; i++)
	{
		// When the pointer points to an array arr The scope of time ,p It's a wild pointer 
		*(p++) = i;
	}
	return 0;
}

3. The space that the pointer points to is released

Here you can briefly understand , Let's not go into details , I will make a supplement for you later ;

3.2 How to avoid wild pointer

1. The pointer needs to be initialized
2. Watch out for the pointer
3. Pointer to space release even if set to NULL
4. Avoid returning the address of a local variable
5. Check the validity of the pointer before using it

Here is the fourth point ：

Look at the code below , Just touching the knowledge of pointer, I compiled such code , Thinking that the last print out is 10, The result is really 10 Do you ？
int* test()
{
	int a = 10;
	int* p1 = &a;
	return p1;
}
int main()
{
	int* p=test();
	printf("%d\n", *p);
	return 0;
}
Compile , The result is 10, I'm so happy ;
You think it's really 10 Do you ？
#include <stdio.h>

int* test()
{
	int a = 10;
	int* p1 = &a;
	return p1;
}
int main()
{
	int* p=test();
	printf("hehe\n");
	printf("%d\n", *p);
	return 0;
}
According to the code above , Just one more print in advance, hehe , In essence, it should not affect the final result ; But the results after debugging are as follows ：
The result is 5, Here it is explained to avoid returning the address of local variables
Simply, the space will be covered ;
The specific reason ：（ See the creation and destruction of function stack frames ）https://blog.csdn.net/sjsjnsjnn/article/details/122811828?spm=1001.2014.3001.5501

Four 、 The operation of the pointer

4.1 The pointer + - Integers

#include <stdio.h>
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int* p = arr;
	int i = 0;
	for (i = 0; i < 10; i++)
	{
		
		printf("%d ", *(p + i));// The pointer p+ Subscript （0~9）, The corresponding value is found through subscript access and dereference 
	}
	return 0;

4.2 The pointer - The pointer

#include <stdio.h>
int my_strlen(char* s)
{
	char* pc = s;
	while (*pc != '\0')
	{
		pc++;
	}// Here is to find '\0' The address of is subtracted from its first address （ The pointer  -  The pointer ） Get the number of elements 
	return pc - s;
}
int main()
{
	char c[]="abcdef";
	int ret = my_strlen(c);
	printf("%d\n", ret);
	return 0;
}

5、 ... and 、 Pointers and arrays

An array is a continuous space , If the address of the array is stored in the pointer variable , Combined with the above , What will change ？

#include <stdio.h>
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,0 };
	int* p = arr;
	printf("%p\n", arr);
	printf("%p\n", &arr[0]);
	printf("%p\n", p);
	return 0;
}

From the results of operation , The address of the array name 、 The address and of the first element of the array p The deposit address is the same ;
Conclusion ： When an array stores the address in a pointer variable, it stores the address of the first element of the array , It can also be said that p Deposit is 1 The address of ;

6、 ... and 、 summary

The above is the introduction of pointer , Help new contacts C The little friend of the language recognizes the initial face of the pointer , Update the advanced level of the pointer now , The content will be long , But it's rich .

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