Kill the general and seize the "pointer" (Part 2)

The same beautiful appearance , One in a million interesting souls

Pointer is C The soul of language

There should be many friends who are still confused about the pointer

In this issue Violet Then I will take you through the pass and kill the generals , Catch “ The pointer ”

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Catalog

1. Character pointer

2. Pointer array

3. Array pointer  

3.1 Definition of array pointer  

3.2 & Array name vs Array name

 3.3 The use of array pointers

4. A function pointer  

5. Function pointer array  

6.  A pointer to an array of function pointers  

7. Callback function  

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1. Character pointer

Character pointer ： Point to character Of The pointer , The pointer type is char*

	char ch = 'a';
	char* pc = &ch;

General usage ：

#include<stdio.h>
int main()
{
	char ch = 'a';
	char* pc = &ch;
	*pc = 'b';
	printf("%c\n", ch);

	return 0;
}

There's another way to use it ：

#include<stdio.h>
int main()
{
	const char* pc = "abcdef";
	printf("%s\n", pc);

	return 0;
}

reflection 1： Why? const char* pc = "abcdef" Add in front.  const？

answer ： Because character pointers pc Pointing to Constant string , Constants cannot be changed , So add const Avoid modifying constant strings . 

reflection 2：const char* pc = "abcdef", It's a abcdef String into character pointer variable pc Medium ？

answer ： No , Is to put the string First character Of Address Put in pc in , That is the a My address is in pc in . 

  Interview questions ：

#include<stdio.h>
int main()
{
	char arr[] = { "abcdef" };
	char brr[] = { "abcdef" };
	const char* crr = "abcdef";
	const char* drr = "abcdef";
	if (arr == brr)
	{
		printf("arr and brr are same\n");
	}
	else
	{
		printf("arr and brr not are same\n");
	}
	if (crr == drr)
	{
		printf("crr and drr are same\n");
	}
	else
	{
		printf("crr and drr not are same\n");
	}

	return 0;
}

  Running results ：

 arr and brr Is a character array , It's a abcdef Stored in a character array , Their memory is not the same , So their addresses are different . and crr and drr It's a character pointer , It's a abcdef The address of the first element of the constant string is stored in the character pointer , So they point to the same memory space . 

 2. Pointer array

  Pointer array ： It's a Array , Array Each element is a type pointer

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

  First arr Follow me first [10] It is an array with ten elements , Every element is int* Type of The pointer .

3. Array pointer  

3.1 Definition of array pointer  

Array pointer ： It's a The pointer , Point to a Array  

	int arr[6] = { 1, 2, 3, 4, 5, 6 };
	int(*pi)[6] = &arr;

pi The first and * The combination shows that it is a Pointer to the variable ,[6] That means it points to a Array , Array has 6 Elements ,int Explain that each element is int type .

• notes ：[] Priority is higher than * The no. , So we have to add （） To guarantee p The first and * combination . 

3.2 & Array name vs Array name

int arr[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

•   Array name arr ： Express First element Of Address
• & Array name ： Express Entire array Of Address  

arr and &arr They print out the same address  , But they mean different things .arr Represents the address of the first element of the array , and &arr Represents the address of the entire array ,arr + 1 Skipped an element , and &arr + 1 Skipped the entire array .

sizeof（arr[0]） It's calculation arr[0] The amount of space taken up , and sizeof（arr） It's calculation Entire array The amount of space taken up  .

Conclusion ：& Array name and sizeof（ Array name ）  Express Entire array , The rest Array name All means Address of the first element of the array .

 3.3 The use of array pointers

  since Array pointer Pointing to Array , that Array pointer Which is stored in Address of array .

#include<stdio.h>
int main()
{
	int arr[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
	int(*pi)[10] = &arr;

	return 0;
}

hold Array arr The address of Assign values to array pointer variables p , But we seldom write code like this  , Because there is no point in writing code like this .

Generally, we use array pointers for Two dimensional array

#include<stdio.h>
void print_arr(int(*arr)[3], int row, int col)
{
	for (int i = 0; i < row; i++)
	{
		for (int j = 0; j < col; j++)
		{
			printf("%d ", arr[i][j]);
		}
		printf("\n");
	}
}
int main()
{
	int arr[3][3] = { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } };
	print_arr(arr, 3, 3);

	return 0;
}

Array name arr, Represents the address of the first element   . But the first element of a two-dimensional array is a two-dimensional array first line   , So the message here is arr, In fact, it's equivalent to The address on the first line , yes One dimensional array address .

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];// Array pointer 
int (*parr2)[10];// Pointer array 
int (*parr3[10])[5];//？？？

 int (*parr3[10])[5]： First parr3 Follow [10] The combination shows that it is an array , Each element is a pointer array type .

4. A function pointer  

A function pointer ： First, it is a pointer , It points to a function  

  Function name and & Function name ： Are all expressions of the function Address

  If you want to save the address of the function , You have to use function pointers

p Follow me first * combination That means it's The pointer , The one at the back () Indicates that it points to a Nonparametric functions , Ahead void Represents the return value void.

《 C Pitfalls and pitfalls 》 There are two codes in this book ：

// Code 1
(*(void (*)())0)();

take 0  Cast type to void(*)() Function pointer type , that 0 It is regarded as the address of a function , Then dereference the call , This is actually a function call , It's called 0 As a function at the address .

// Code 2
void (*signal(int , void(*)(int)))(int);

signal Follow first Brackets combination , explain signal yes Function name , The first argument to the function argument is int, The second parameter is A function pointer （ The function parameter pointed to by the function pointer is int, The return type is void）, Return value Also a A function pointer , The function parameter pointed to by the function pointer is int, The return type is void, So this is a Function declaration . 

5. Function pointer array  

Function pointer array ： First of all, it's a Array , Every element of the array is The pointer Pointing to function . 

int(*arr[10])();

  First arr Follow me first [10] The combination shows that it is a Array , Every element in the array is int（*）（） Type of A function pointer .

  Purpose of function pointer array ： Transfer table

#include<stdio.h>
void menu()
{
	printf("*********************\n");
	printf("*** 1.add   2.sub ***\n");
	printf("*** 3.mul   4.div ***\n");
	printf("****** 0.exit *******\n");
	printf("*********************\n");
}
int add(int a, int b)
{
	return a + b;
}
int sub(int a, int b)
{
	return a - b;
}
int mul(int a, int b)
{
	return a * b;
}
int div(int a, int b)
{
	return a / b;
}

int main()
{
	int a, b;
	int input = 1;
	int ret = 0;
	int(*p[5])(int a, int b) = { 0, add, sub, mul, div }; // Transfer table 
	while (input)
	{
		menu();
		printf(" Please select ：");
		scanf("%d", &input);
		if ((input <= 4 && input >= 1))
		{
			printf(" Enter the operands ：");
			scanf("%d %d", &a, &b);
			ret = (*p[input])(a, b);
		}
		else
			printf(" Incorrect input \n");
		printf("ret = %d\n", ret);
	}
	return 0;
}

6.  A pointer to an array of function pointers  

A pointer to an array of function pointers  ： First of all, it's a The pointer , It points to a Array , Every element in the array is A function pointer .

	void(*p)();// A function pointer 
	void(*p1[1])();// Function pointer array 
	void(*(*p2)[2])();// Pointer to function pointer array pointer 

7. Callback function  

A callback function is one that passes through The function called by the function pointer . If you take the function of The pointer （ Address ） Pass as a parameter to another function , When this pointer is used Call the function it points to when , Let's just say this is Callback function . The callback function is not controlled by this function The implementer of directly calls , But by... When a specific event or condition occurs Called by the other party , Used to enter the event or condition Row response . 

qosrt  Function USES ：

#include <stdio.h>
//qosrt The user of the function has to implement a comparison function 
int int_cmp(const void * p1, const void * p2)
{
  return (*( int *)p1 - *(int *) p2);
}
int main()
{
    int arr[] = { 1, 3, 5, 7, 9, 2, 4, 6, 8, 0 };
    int i = 0;
    
    qsort(arr, sizeof(arr) / sizeof(arr[0]), sizeof (int), int_cmp);// Pass the address of one function to another function 
    for (i = 0; i< sizeof(arr) / sizeof(arr[0]); i++)
   {
       printf( "%d ", arr[i]);
   }
    printf("\n");
    return 0;
}