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This series will share some knowledge about pointer with you , Although the pointer may be difficult to learn compared with the previous knowledge , But don't be afraid to follow Xiaobian into the world of pointers ！

The pointer ：

What is a pointer ：

Every space in the memory will have a number , The number is the address , An address is a pointer .（ Number ---- Address ---- The pointer ）

The size of the pointer ：

Pointer in 32 On the bit platform is 4 Bytes , stay 64 On the bit platform is 8 Bytes .

The meaning of pointer type ：

Pointers are also typed , So what is the meaning of these types ？

1. The pointer type determines the permission of pointer dereference .

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int* pa;
	char* pc;
	float* pf;
	printf("%d\n", sizeof(pa));
	printf("%d\n", sizeof(pc));
	printf("%d\n", sizeof(pf));
	return 0;
}

The screenshot of the code running is as follows ：

2. The pointer type determines how far the pointer can go in one step （ step ）.

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int arr[10] = { 0 };
	int* p = arr;
	char* pc = arr;
	printf("%p\n", p);
	printf("%p\n", p + 1);
	printf("\n");
	printf("%p\n", pc);
	printf("%p\n", pc + 1);
	return 0;
}

The code screenshot is as follows ：

Wild pointer ：

Wild pointer means that the position pointed by the pointer is unknown .

The origin of wild pointer ：

1. Pointer not initialized

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
// The code is not runnable 
int main()
{
	int* p;
	*p = 20;// uninitialized 
	return 0;
}

2. The pointer is out of bounds

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int arr[10] = { 0 };
	int* p = arr;
	int i = 0;
	for (i = 0; i <= 10; i++)
	{
		*p = i;
		p++;
	}
	return 0;
}

The screenshot of the code running is as follows ：

An out of bounds occurrence is shown as follows ：

3. The space pointed to by the pointer frees up

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int* test()
{
	int a = 10;
	return &a;
}
int main()
{
	int* p = test();
	*p = 20;// When modifying the value a Space has been released 
	return 0;
}

How to avoid wild pointer ：

1. Pointer initialization

When you don't know why to initialize, initialize to NULL.

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int* p = NULL;
	return 0;
}

Clearly know the initialization value .

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int a = 20;
	int* p = &a;
	*p = 10;
	printf("%d\n", *p);
	return 0;
}

The code screenshot is as follows ：

2. Watch out for the pointer

stay C Array out of bounds behavior is not checked in the .

3. The pointer points to the space to release the immediate setting NULL

4. Check the validity of the pointer before using it

Pointer arithmetic :

1. The pointer +- Integers

The code is as follows ：

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

The code screenshot is as follows ：

2. The pointer - The pointer

The pointer - The pointer gets the number between two elements .

The code is as follows ：

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

The code screenshot is as follows ：

The premise of pointer subtraction is that two pointers point to the same space .

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
// Implement a function to calculate the length of characters 
int my_strlen(char* str)
{
	char* start = str;
	while (*str != '\0')
	{
		str++;
	}
	return str - start;
}
int main()
{
	int len = my_strlen("abc");
	printf("%d\n", len);
	return 0;
}

The screenshot of the code running is as follows ：

3. The relational operation of pointers

The standard stipulates ： Allows a pointer to an array element to be compared with a pointer to the memory location after the last element of the array , However, it is not allowed to compare with a pointer to the memory location before the first element .

Pointers and arrays ：

The array name is the address of the first element of the array .

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int arr[10] = { 0 };
	printf("%p\n", arr);// Array name 
	printf("%p\n", &arr[0]);// The first address of the array 
	return 0;
}

The code is shown in the following screenshot ：

The secondary pointer ：

See code for explanation , The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int a = 10;
	int* pa = &a;// Here pa Is the first level pointer 
	int** ppa = &pa;// Here ppa It's a secondary pointer 
	int*** pppa = &ppa;// Here pppa It's a three-level pointer 
	return 0;
}

Pointer array ：

Arrays also have pointers , Let's briefly introduce the format of pointer array .

The code is as follows ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
	int arr[10];
	char ch[5];
	int* paar[5];// Shaping array pointers 
	char* pch[5];// Character array pointer 
	return 0;
}

Conclusion :

That's all for you to share ！ If you want to see it C Language other series , Click on the link below ！ I hope that's helpful , Students who want to learn remember to pay attention to Xiaobian and study together ！ If there are any mistakes in the article, you are also welcome to point out the maze for Xiaobian in time （ I'd like to thank you guys first ！）

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