Advanced level of C language pointer (Part 1)

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Column introduction ： Contains from C Explain the knowledge points from the beginning to the advanced level of the language , use C Language to write a variety of small programs , Generally around C Basic column ;

* Key points of this blog ️

1. Character pointer

2. Array pointer

3. Pointer array

4. Array parameter passing and pointer parameter passing

5. A function pointer

Foreword review ：

1. A pointer is a variable , It's used to store the address , The address uniquely identifies a piece of memory space .

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

3. Pointers are typed , The type of pointer determines the type of pointer +- Integer step size , The permission of pointer dereference operation . 4. The operation of the pointer .

 1. Character pointer

We should have seen it before char* Character pointer , Generally, the following use cases occur ：

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

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

The result of printing is hello, Is that pstr The string stored inside hello Well ？ Never think so , Pointer variables store addresses ,pstr It should be said that it is the address where the first element of the string is stored ,printf Print will be printed according to this address , encounter \0 Will stop printing ;

The above code is clear. Let's take a look at the following code ：

#include <stdio.h>
int main()
{
    char str1[] = "hello bit.";
    char str2[] = "hello bit.";
    const char *str3 = "hello bit.";
    const char *str4 = "hello bit.";
    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;
}

Ask what the print result is ？

Answer key ：

First understand str1==str2 and str3==str4 What does that mean? , This expression actually means to compare whether their addresses are the same , that str1 and str2 They are two different arrays , How could the address be the same ？ and str3 and str4 They are all character constants , There is no need to create new space in memory , Their first addresses all point to characters ‘h’ Of , So the address must be the same ;

2. Pointer array

A pointer array is an array of pointer types , The elements in it are all pointer types ;

int* arr1[10];           // An array of integer pointers

char *arr2[4];         // An array of first-order character pointers

char **arr3[5];        // An array of secondary character pointers

Priority understanding ：arr1 The first and [ 10 ] combination , It shows that this is an array , The type of array is before it int *, This is a pointer array , Used to store pointers ;

3. Array pointer

3.1 Definition of array pointer

What are array pointers ？

It's a pointer , Pointer to array , We already know there are int *pa  Integer pointer , float *pb Floating point pointer , They are all pointers to a certain type , So is the array pointer , The type it points to is an array ;

int * arr1[10] = { 0 };   

int (* arr2)[10] = { 0 };  //arr1 and arr2 What are the differences ？

arr1 We've already done that , It is an array of pointers ;

arr2 Pay attention to its （*arr2）, This means that in any case arr2 Has been and * Combined with the , This shows that arr2 It's the pointer, not the array name , Because it has no chance to [10] To combine , The type pointed to is int [10], So we call it array pointer ;

Be careful ： because [ ] The priority ratio * high ,arr2 I'll start with [10] combination , So if you want an array pointer, you must add （） Give Way * and arr2 First combine , In this way, we can guarantee arr2 It's an array pointer ;

3.2 & Array name VS Array name

   int arr[10] = { 0 };

&arr and arr Is there anything different ？ It seems to be exactly the same ,arr Is the address of the first element of the array &arr It is also the first element address , It's the same. Why bother to use it & Well ？

My answer is NO! Programming is rigorous , So its grammar is even more so , Just because the contents are the same doesn't mean they have the same meaning , Without proof, I will take you to study &arr and arr The difference between ;

We print their addresses exactly the same

#include <stdio.h>
int main()
{
    int arr[10] = {0};
    printf("%p\n", arr);
    printf("%p\n", &arr);
    return 0;
}

Running results ：

 

But let's look at the following code ：

#include <stdio.h>
int main()
{
	int arr[10] = { 0 };
	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;
}

Running results ：

What does this ？&arr and arr It's worth the same , But they certainly have different meanings , So some things we should not be confused by appearances , If there is any doubt about the matter, let's get to the bottom of it and kill it ; in fact &arr Is the address representing all elements of the array , But there must be an address to express them , But the space is limited, it is impossible to express all the element addresses , So we still use the first element of the array to represent them , But its meaning is different ;

reflection ：

&arr You can think about what it is ？ Is it an array pointer , Yes indeed ,&arr The point is arr Entire array , In this case, its type is int （*）[10];

Address of array +1, Skip the size of the entire array , therefore &arr+1 be relative to &arr The difference is 40.

3.3 The use of array pointers

Since there is an array pointer, how should we use it ？

chestnuts 1（ Not commonly used ）：

#include <stdio.h>
int main()
{
    int arr[10] = {1,2,3,4,5,6,7,8,9,0};
    int (*p)[10] = &arr;// Put the array arr To an array pointer variable p
    // But we seldom write code like this 
    return 0;
}

chestnuts 2（ Commonly used ）：

#include <stdio.h>
void print_arr1(int arr[3][5], int row, int col)
{
	int i = 0;
	for (i = 0; i < row; i++)
	{
		int j = 0;
		for (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++)
	{
		int j = 0;
		for (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;
}

After learning pointer array and array pointer, let's review and see what the following code means ：

int arr[5];                   // integer array

int *parr1[10];           // Pointer array

int (*parr2)[10];         // Array pointer

int (*parr3[10])[5];    // An array that holds array pointers

4. Array parameters , Pointer parameter

When writing code, it is inevitable to 【 Array 】 perhaps 【 The pointer 】 Pass to function , How to design the parameters of the function ？

4.1 One dimensional array parameters

#include <stdio.h>
void test(int arr[])//ok
{}
void test(int arr[10])//ok
{}
void test(int *arr)//ok
{}
void test2(int *arr[20])//ok
{}
void test2(int **arr)//ok
{}
int main()
{
 int arr[10] = {0};
 int *arr2[20] = {0};
 test(arr);
 test2(arr2);

return 0;
}

4.2 Two dimensional array parameters

void test(int arr[3][5])//ok？
{}
void test(int arr[][])//ok？
{}
void test(int arr[][5])//ok？
{}
// summary ： Two dimensional array parameters , Function parameter design can only omit the first [] The number of .
// Because for a two-dimensional array , I don't know how many lines there are , But you have to know how many elements in a row .
// So it's easy to calculate .
void test(int *arr)//ok？
{}
void test(int* arr[5])//ok？
{}
void test(int (*arr)[5])//ok？
{}
void test(int **arr)//ok？
{}
int main()
{
 int arr[3][5] = {0};
 test(arr);
}

4.3  First level pointer parameter transfer

#include <stdio.h>
void print(int *p, int sz)
{
 int i = 0;
 for(i=0; i<sz; i++)
 {
 printf("%d\n", *(p+i));
 }
}
int main()
{
 int arr[10] = {1,2,3,4,5,6,7,8,9};
 int *p = arr;
 int sz = sizeof(arr)/sizeof(arr[0]);
 // First level pointer p, Pass to function 
 print(p, sz);
 return 0;
}

When the parameter part of a function is a first-order pointer , What parameters can a function take ？

4.4  The secondary pointer transmits parameters

#include <stdio.h>
void test(int** ptr)
{
 printf("num = %d\n", **ptr); 
}
int main()
{
 int n = 10;
 int*p = &n;
 int **pp = &p;
 test(pp);
 test(&p);
 return 0;
}

When the parameter of the function is a secondary pointer , What parameters can be received ？