C language - pointer one touch ※

C Language pointer is a big difficulty for beginners to learn and understand , The pointer to C Language can be called C The soul of language , For beginners, the standard to judge whether it is a pointer is whether there is * , But is it true ？ Or what are the types of pointers , This article uses easy to understand language to help beginners learn pointer , Understand the pointer , Use pointer ！

Then let's start ！

Catalog

One 、 What is a pointer ？

1. Initial understanding of pointer  

 2. What is a pointer variable ？

Two 、 Initial order of pointer

1. Pointer and pointer type

2. Dereference of pointer *

3. Wild pointer

 3.1 The origin of wild pointer

 3.2 Avoid wild pointer

3、 ... and 、 Advanced pointer  

0. About array names （ About arr And &arr The difference between them, etc ）

 1. Character pointer    char*

 2. Pointer array  

3. Array pointer

 4. A function pointer

 5. Function pointer array

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One 、 What is a pointer ？

1. Initial understanding of pointer  

There are two points to understand pointer .

First , A pointer is a pointer in memory The number of the smallest unit , In a nutshell “ Address ”

secondly , Usually said pointer , Is usually Pointer to the variable , Variables used to store memory addresses

In short , It's memory , It's the address

 2. What is a pointer variable ？

Before understanding what pointer variables are , We need to understand every value , Every quantity needs space to be stored in the computer , The name of this space can be called address ,  adopt &（ Fetch address operator ） You can take out the address of this quantity . The following example ：

 

 

adopt VS The monitoring function of , Be able to find a The value of is 10,a The address of is 0x00effac, Namely &a yes  0x00effac.

Here we must Strictly distinguish between a The value of is equal to a The address of

             a The value of is int The type is 10         a The address is 0x00effac The type is int*（ Get to know ）

So for storage a The variable of the address of is the pointer variable ！！

So the value of the pointer variable here is the address of a variable , The address of the pointer variable is its own address ！！

Examples are as follows ：

 

  Create a Pointer to the variable p, take a The address of is assigned to p, Through monitoring ,p The value is &a ,&p Namely p The address of

Only this and nothing more , I have a certain understanding of pointers

To sum up ：

Pointer to the variable , The variable used to hold the address .（ The values stored in the pointer are treated as addresses ）.

Pointer to the variable , It's also a variable ！ It can be changed ！ Only the address is stored , Not a certain value ！
The pointer is used to Storage address Of , An address is a unique identifier of an address space .
The size of all pointers is 32 Bit platform is 4 Bytes , stay 64 Bit platform is 8 Bytes .

 

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Two 、 Initial order of pointer

1. Pointer and pointer type

The data types are int float double char short long etc.

Does the pointer have a corresponding pointer type ？            The answer is yes ！                Corresponding to the following ：

 

  The type of pointer variable , To correspond to the type of the corresponding stored variable , Storage int type Just apply int* To define the pointer

Here you can see , The way pointers are defined is ： type + *    So the meaning of the corresponding pointer below should be understood

  Secondly, the meaning of pointer type is not just to Type of storage , also Other special meanings , The demonstration is as follows ：

  summary ： The type of pointer determines how big the pointer moves forward or backward （ distance ）.

2. Dereference of pointer *

As mentioned earlier &（ Fetch address operator ） Then it's equivalent to finding Your room number, ！ Then you need to open the door The key , So that is Dereference operation *

We can understand first *&a The meaning of

 

  So contrast &（ Address operation ）* We can understand it as value taking operation ,* The object of is the address , Can be accessed ！

We put &a Assign to a pointer variable p1, therefore p1 Equivalent to &a, We can get *p1

From the above case, we can get   *p1 Namely   a  p1 yes a The address of , adopt * You can find a 了 ！

3. Wild pointer

         Many beginners have heard of wild pointer from his population , Have you heard of the power of wild pointer . If the pointer can be compared to an arrow , It refers to the address of a variable , Re pass * You can find this variable for assignment and other operations . As the name suggests , The wild pointer is just pointing *, Proceed again * Dereference will change the uncertain quantity , Will cause great harm ！

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

 3.1 The origin of wild pointer

Pointer not initialized                                                           Examples are as follows ：

  I want to pass the pointer p1 receive a The address of , Proceed again * Dereference operation pair a Assign values to , But if there is no assignment to the pointer , Then the object pointed to by the pointer is unknown , Extremely dangerous ！

If we want to define a pointer , But now I don't know the object , Then we can do it first NULL assignment

  The pointer is out of bounds                                                               Examples are as follows ：

#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;
}

 3.2 Avoid wild pointer

 1. Pointer initialization
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 // Avoid returning the address of a local variable when calling a function
5. Check the validity of the pointer before using it

#include <stdio.h>
int main()
{
    int *p = NULL;
    //....
    int a = 10;
    p = &a;
    if(p != NULL)// Check pointer validity 
    {
        *p = 20;
    }
    return 0;
}

Sum up ： We have learned what pointer is , What is a pointer variable , The type of pointer variable , Two meanings of pointer variables ,& Fetch address operator ,* Dereference operator , What is the wild pointer , The origin of wild pointer , How to avoid wild pointer  . It's almost enough to learn about the initial application of so many pointers , Next is the higher-order learning of pointer , More difficult operation , More brainstorming ！

are you ready ！Are you ready？

  By * Anyone who points to it can learn ！

3、 ... and 、 Advanced pointer  

0. About array names （ About arr And &arr The difference between them, etc ）

Before further studying pointers , Review the little knowledge about array names , Sometimes the array name represents the address of the first element of the array , Sometimes it means the whole array , Often confused , Here is a little summary ！

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	printf("%p\n", arr);
	printf("%p\n", &arr);
	return 0;
}

  Ask yourself a question first ？    You can distinguish arr And &arr The difference between ？   

Through the above code, you will find that the results are consistent , But are they really the same ？ Let's demonstrate the following code

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	printf("arr = %p\n", arr);
	printf("arr+1 = %p\n", arr+1);
	printf("&arr = %p\n", &arr);
	printf("&arr+1 = %p\n", &arr+1);
	return 0;
}

  Through the above code, we can find that ：

arr+1 Skip is a int The length of and &arr+1 Skip the length of the entire array Namely int*10 The length of

arr The real meaning is arr[0] The address of , Is the whole array The first address of the first element

&arr The real meaning is The address of the entire array

Is there any exception ？ The answer is yes ！ But not much Just remember if sizeof（ Array name ） What you get is the length of the entire array

Summary ： About Array name

except sizeof( Array name ) Represents the length of the entire array , Other times The array name indicates Address of the first element of the array .

Array name +1 Skip the first element of the array

& Array name Represents the address of the entire array

& Array name +1 Skip the address of the entire array  

 1. Character pointer    char*

Among the types of pointers, we know that one pointer type is Character pointer char* ;

General usage ：

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {
	char ch = 'w';
	char* p = &ch;
	*p = 'm';
	return 0;
}

  besides ,char* There is another way to use character pointers

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {

	const char* pstr = "hello cainiao.";// Here is to put a string into pstr Is it in the pointer variable ？
	printf("%s\n", pstr);
		
	return 0;
}

  Many students will put const char* pstr = "hello cainiao."; This code is understood as hold hello cainiao. On the pstr in

It's actually the address of the first element h It's in storage The pointer pstr in ！

To help better understand const char* Demonstrate the following code ！ 

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {

	char str1[] = "hello cainiao.";
	char str2[] = "hello cainiao.";
	const char* str3 = "hello cainiao.";
	const char* str4 = "hello cainiao.";
	if (str1 == str2)
		printf("str1 and str2 are same\n");
	else
		printf("str1 and str2 are not same\n");
	if (str3 == str4)
		printf("str3 and str4 are same\n");
	else
		printf("str3 and str4 are not same\n");
		
	return 0;
}

  Did you send out soul torture ？？？what？

  About first if() It should be well understood

Even if the contents of two character arrays are consistent , But the content of comparing characters and numbers cannot be used == To compare （ Use strcmp）, here == The comparison is the address ,str1 And str2 It represents the address of the first element , Two character array , The address must be different , So it's definitely not equal

About the second if() 

Through the above code, you will find , The addresses of two constant character arrays are actually the same ？？？

0. here str3 and str4 Points to the same constant string .

1. actually C/C++ The constant string is stored in a separate memory area , When a few pointers . When pointing to the same string , They actually point to the same block of memory .

2. But use the same constant string to Initializing different arrays will open up different memory blocks . therefore str1 and str2 Different ,str3 and str4 Different .

  So if str3 And str4 Refers to different constant strings , Then it won't be the same , Another simple test .

  understand char*,const char* And the storage of constant strings in memory ！

 2. Pointer array  

  First, play a word game , whether Distinguished pointer array still Array pointer The central word of .

Pointer array The understanding of should be compared integer array , Floating point array , A character array ......, So the central word is Array

Compare and understand , It can be clearly concluded that An array of pointers is an array , The data type stored in the array is pointer ！

  Compare and define pointer arrays

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {

	char str1[] = "hello cainiao.";// A character array 
	int a[10] = { 1,2,3,4,5,6,7,8,9,10 };// integer array 
	float b[10] = { 1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0 };// Floating point array 
	//......

	char* pstr[10]; // Character pointer array 
	int* pint[10];  // Array of integer pointers 
	float* pf[10];  // Floating point pointer array 
    //......
	return 0;
}

  The application of pointer array

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {

	int a = 1;
	int b = 1;
	int c = 0;
	int d = 1;
	int e = 1;
	int f = 9;
	
	int* pint[6] = {&a,&b,&c,&d,&e,&f};  // Array of integer pointers 
	for (int i = 0; i < 6; i++) {
		printf("%d", *pint[i]);
	}
		
	return 0;
}

3. Array pointer

Array pointer The understanding of is to compare Integer pointer , Floating point pointer , Character pointer ...... The central word is The pointer

therefore Array pointer It's a pointer to an array , Namely A pointer stores the address of the array ！

int *p1[10];
int (*p2)[10];
//p1, p2 What are the differences ？

 int *p1[10];      Pointer array

* The priority of is relatively low, so p1 With the first [] Combine with * combination , therefore int* Is the type of contents stored in the array It's the pointer
int (*p2)[10];      Array pointer

stay () With the first * combination , therefore p2 It's a pointer , The type pointed to is int (*)[10]

  Compare and define array pointers

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

int main() {

	int* a ;// Integer pointer 
	float* b ;// Floating point pointer 
	double* c ;
	char* d ;// Character pointer 
	//......
	
	int arr1[6] = { 1,2,3,4,5,6 };
	float arr2[6] = { 1.0,2.0,3.0,4.0,5.0,6.0 };
	char arr3[6] = { 'n','i','h','a','o','\0' };
	int(*pint)[6] = &arr1;// Integer array pointer     The type is int（*）[6]
	float(*pf)[6] = &arr2;// Floating point array pointer   The type is float（*）[6]
	char(*pch)[6] = &arr3;// Character array pointer     The type is char（*）[6]
	//......
	return 0;
}

  Array pointer , Is the address used to receive the array , therefore We must add & Fetch address operator , If not , Is to get the address of the first element of the array , There will be big mistakes , want Note note ！

  Application of array pointer

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>

void print_arr1(int arr[3][5], int row, int col)
{
	int i = 0;
	for (i = 0; i < row; i++)
	{
		for (int j = 0; j < col; j++)
		{
			printf("%d ", arr[i][j]);
		}
		printf("\n");
	}
}
void print_arr2(int(*arr)[5], int row, int col)
{
	int i = 0;
	for (i = 0; i < row; i++)
	{
		for (int j = 0; j < col; j++)
		{
			printf("%d ", arr[i][j]);
		}
		printf("\n");
	}
}
int main()
{
	int arr[3][5] = { 1,2,3,4,5,6,7,8,9,10 };
	print_arr1(arr, 3, 5);
	// Array name arr, Represents the address of the first element 
	// But the first element of a two-dimensional array is the first row of a two-dimensional array 
	// So the message here is arr, It's actually equivalent to the address on the first line , Is the address of a one-dimensional array 
	// You can use an array pointer to receive 
	print_arr2(arr, 3, 5);
	return 0;
}

 4. A function pointer

  Pointers are used to store addresses , Then you can see that every function also has an address with a function pointer ！

Let's start with a piece of code

#include <stdio.h>
void test()
{
	printf("hehe\n");
}
int main()
{
	printf("%p\n", test);
	printf("%p\n", &test);
	return 0;
}

 

So let's say Function name and & Function name All means Address of function  

The definition of function pointer applies

#include <stdio.h>
int  add(int x,int y)
{
	return x + y;
}
int main()
{
	int (*p)(int, int) = add;
	printf("%d", p(1, 2));
	return 0;
}

 int (*p)(int, int) = add;

A function pointer p The type is int(*)(int,int)    first int Is the return type ,（int,int） It's a parameter list

 5. Function pointer array

int Add(int x,int y) {
	return x + y;
}
int Sub(int x, int y) {
	return x - y;
}
int Mul(int x, int y) {
	return x * y;
}
int Diy(int x, int y) {
	return x / y;
}

int main() {
	int (*pfArr[])(int, int) = { 0, Add, Sub, Mul,  Diy };// Function pointer array 
	printf("%d\n", (pfArr[1])(2, 4));
	printf("%d\n", (pfArr[2])(2, 4));
	printf("%d\n", (pfArr[3])(2, 4));
	printf("%d\n", (pfArr[4])(2, 4));

	return 0;
}

 

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summary

That's what we're going to talk about today , This article only briefly introduces the use of pointers , There is an initial order of pointer , Higher order of pointer , I hope students can get what they need , The study of pointer cannot stop at reading articles , It is more important to practice , Code friends, come on ！