[C language] comprehensively analyze the pointer and sort out the pointer knowledge points

Catalog

Preface ：

1. The concept of pointer

2. The type of pointer

3. Wild pointer

3.1 The origin of wild pointer ：

 3.2 How to avoid wild pointer ？

4. The operation of the pointer

5. Pointers and arrays

6. The secondary pointer

7. Pointer array

summary ： 

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Preface ：

Yes C language , Pointer is a difficult point , If you use C Language to write data structures , It is necessary to master the usage of pointers , If the pointer doesn't learn well , It's hard to learn data structure . So I hope you must master the pointer ！！！

1. The concept of pointer

1. The pointer is a Variable , be used for Storage address , The address uniquely represents a piece of memory space .

ps:（ Memory number = Address = The pointer ）

2. What is the size of the pointer fixed 4/8 Bytes （32 Bit platform /64 Bit platform ）

2. The type of pointer

Pointers are typed , The type of pointer determines the pointer +- Integer step size , Pointer dereference permission .

Now let me explain the meaning of the red part above , for instance , Take a look at the following code and running results ：

#include<stdio.h>
int main()
{
	int a = 4;
	int* p1 = &a;
	char* p2 = &a;
	printf("%p\n", p1);
	printf("%p\n", p2);
	printf("%p\n", p1+1);
	printf("%p\n", p2+1);
	return 0;
}

  At first p1 and p2 The address is the same , But back let p1 and p2 separately +1, The later result is different ,p1 Plus 1 yes int Type of 1, and p2+1 Plus char Type of 1.

As we mentioned above, the size of the pointer is fixed 4/8 Bytes , The assumption is 32 Bit platform , Then a pointer will occupy 4 Bytes . If I define an integer pointer and a character pointer , So this Integer pointer When dereferencing, you can access 4 Bytes , and Character pointer You can only visit 1 Bytes .

3. Wild pointer

The concept of wild pointer is ： The position of the pointer is unknown

3.1 The origin of wild pointer ：

It's a wild pointer origin There are two ：1. Pointer not initialized    2. Pointer out of bounds access

Explain to you ：

Pointer not initialized

This should be well understood , That is, when we create a pointer variable, we don't let it point to any object

for example ： int* p;    such p It's a local variable ,p It's just one. Random value  

Pointer out of bounds access

Take a look at the following code ：

#include<stdio.h>
int main()
{
	int arr[5] = { 1,2,3,4,5 };
	int* p = arr;
	int i = 0;
	for (i = 0; i < 6; i++)
	{
		printf("%d ", *p);
		p++;
	}
	return 0;
}

First, explain the principle of this code ,int* p = arr; here arr It's an array name , The array name is the address of the first element

So now p Is the address of the first element   Yes p Dereference is *p ,*p The value is 1

p++;  This line of code is to make p The address of ++; What is the size of the pointer fixed 4/8 Bytes , int Type data in C Language is also 4/8 Bytes , All the pointers we get are data The address of the first byte , And the array in memory is continuity Of ,p++ Just move one data back .

But now arr The array has only 5 Elements , But the cycle 6 This will inevitably lead to the out of bounds of the array , Let's take a look at the running results

front 5 The number is arr The number in the array , The first 6 A value is a random value . Because when you cycle to the 6 When the time ,p There is no object to point to , here p It's a wild pointer .

 3.2 How to avoid wild pointer ？

1. Good at using NULL, Initialize the pointer in time

If you are defining pointer variables , I have thought of the object pointed to by the pointer variable , Then initialize directly .

If you are defining , It's not clear what the pointer points to , It's not clear whether to use a pointer later , Then assign the pointer variable NULL   

NULL It means empty , If int *p=NULL; So at this time p Is a null pointer , You can re assign values later , It does not affect the later use . If a pointer is a null pointer , Don't use it before you initialize it .

2. Avoid pointer out of bounds

3. Avoid returning the address of a local variable

4. The operation of the pointer

Look at the following code ：

#include<stdio.h>
int main()
{
	// Pointer address plus or minus integer 
	int arr[5] = { 1,2,3,4,5 };
	int* p1 = arr;
	int i = 0;
	for (i = 0; i < 5; i++)
	{
		printf("%d ", *p1);
		p1++;
	}
	printf("\n");
	// Dereferenced pointer plus or minus integers 
	int b = 10;
	int* p2 = &b;
	(*p2)++;
	printf("%d", *p2);
	return 0;
}

p1++ Is an integer that adds or subtracts an address , We've already talked about that , Now I won't introduce more

and (*p2)++, I defined a b Variable , And then assigned to 10, And then put b The address of p2,*p2 By dereferencing, we get 10,（*p）++  amount to 10++ , What you get is 11.

So let's see what happens ：

 ps: Pointers can compare sizes

The pointer can also be subtracted

for instance ：

#include<stdio.h>
int main()
{
	int arr[5] = { 1,2,3,4,5 };
	printf("%d", &arr[4] - &arr[0]);
	return 0;
}

There is no pointer variable subtraction , It's the same thing . After all, the pointer is the address .

When two pointers point to the same space , The absolute value of pointer minus pointer is the number of data between these two pointers .

Note that this is not Number * The size of the data type  C According to the language

5. Pointers and arrays

1. Arrays can be accessed through pointers , You can refer to the code I wrote above .

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

But there are exceptions to everything ：

        1.sizeof( Array name )  What you get is the size of the entire array  

        2.&+ Array name Here is the address of the entire array .

3. When transferring function parameters , If the parameter is an array , Formal parameters can be designed as pointers , Of course, if the formal parameter is an array , You can also pass a pointer as an argument .

6. The secondary pointer

The secondary pointer is used to store the primary pointer （ Pointer to the variable ） The address of .

#include<stdio.h>
int main()
{
	int a = 5;
	int* pa = &a;
	int** ppa = &pa;
	return 0;
}

here pa Is the first level pointer ,ppa It's a secondary pointer ,ppa It's a pa Take out the address of and put it in ppa Inside

**ppa Namely *pa find pa, In the face of pa Dereference to find a

7. Pointer array

The definition of pointer array ：int* Array name [ size ]  

Pointer array usage ：

#include<stdio.h>
int main()
{
	int arr1[3] = { 1,2,3 };
	int arr2[3] = { 4,5,6, };
	int* arr3[2] = {arr1,arr2};
	int i = 0;
	int j = 0;
	for (i = 0; i < 2; i++)
	{
		for (j = 0; j < 3; j++)
		{
			printf("%d ", *(arr3[i] + j));
		}
		printf("\n");
	}
}

The pointer array can be simulated as a two-dimensional array ,arr[i] What's in it is arr1 and arr2 The address of .+j Is to get the address of each array element , Coming * Dereference to get the value inside . Export that place also Can be replaced by   printf("%d ", arr3[i][j]); The effect is the same .

summary ： 

Pointers are really important ！ Pointers are really important ！ Pointers are really important ！（ Important things are to be repeated for 3 times ） Be sure to master

I hope this article can help you （ Level co., LTD. , If there are questions , Welcome the boss to correct ！ thank !）