Analysis of C language pointer function and function pointer

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Pointer function

  Definition ：

Pointer function , In a nutshell , It's a function that returns a pointer , Its essence is a function , The return value of this function is a pointer .
The declaration format is ：* Type identifier   Function name ( Parameters )

It doesn't seem hard to understand , Let's go further .
Take a look at the following function declaration ：

int fun(int x,int y);

This statement should be familiar , It's a simple one int Type of fun function , namely The return value is one int Type value .

int *fun(int x,int y);

The difference between this declaration and the above is that there is an additional pointer identifier *.

And this function is a pointer function . Its The return value is one int Pointer to type , It's an address .

This description should be easy to understand , So called pointer function is nothing special , Compared with ordinary functions, it's just Returned a pointer （ That is, the address value ） nothing more .

Pointer function writing

int *fun(int x,int y);
int * fun(int x,int y);
int* fun(int x,int y);

The above three ways of writing are all ok , This way of writing depends on personal habits , In fact, if * Close to the return value type, it's probably easier to understand its definition . That is, the first way of writing is very clear , At first glance, it is a pointer function .

Example  

#include <stdio.h>
int * fun();
int main()
{
	int *p;
	int i;
	p=(*fun)();
	for(i=0;i<2;i++)
		printf("arr[%d]=%d  ",i,*(p+i));
	return 0;
	
} 
 
int *fun()// Pointer function   It's a function that returns a pointer , Its essence is a function , The return value of this function is a pointer 
{
 	static arr[2]={10010,911};
 	int *p=arr;
 	return arr;
}

From the above code, you can clearly see the use of pointer functions , Define a arr Array , Store it in the static area , Then use the pointer *p Point to the first address of the array , Return the first address of the array , Call this pointer function in the main function , Print the array value returned at one time .

Running results ：arr[0]=10010  arr[1]=911

A function pointer  

Definition ：

A function pointer , Its essence is a pointer variable , This pointer points to this function . In conclusion , A function pointer is a pointer to a function .
Declaration format ： Type specifier (* Function name ) ( Parameters )
as follows ：

int (*fun)(int x,int y);

Function pointer is to assign the address of a function to it , There are two ways of writing ：

fun = &Function;
fun = Function;

Add or not add address characters & Fine . Because a function identifier represents its address , If it's a function call , It must also contain a parameter list enclosed in parentheses .

There are also two ways to call function pointers ：

x = (*fun)();
x = fun();

Either way , The second one looks no different from ordinary function calls , If you can , The first one is recommended , Because it can be clearly indicated that the function is called by pointer .

Example

#include <stdio.h>
/* Pointer function , In a nutshell , It's a function that returns a pointer , Its essence is a function , The return value of this function is a pointer */
// Define a solution int Pointer function of type number square value , Return the address with the square value 
int *sqare(int a)
{	
	a = a *a ;
	int *p = &a ;
	return p;// Return pointer , Square value address 
}
/* A function pointer , Its essence is a pointer variable , This pointer points to this function . In conclusion , A function pointer is a pointer to a function .*/
// Define function first 
int add(int x,int y)
{
    return x+y;
}
int sub(int x,int y)
{
    return x-y;
}
// Define a function pointer , Two functions int Type parameter , Function return value type is int
int (*fun)(int,int);

int main ()
{
	/* Pointer function */
	printf(" Example output of pointer function ：\n");
	int *m = sqare(10) ;//sqare(10) The value of is address , To assign a pointer variable 
    printf("10 The square of is %d\n",*m) ;// Print out 10 The square value of 
	printf("12 The square of is %d\n\n",*sqare(12)) ;// Print 12 The square value of 

	/* A function pointer */
	printf(" Example output of pointer function :\n");
	// Print add Address of function , It can be seen from the following print results add Add before or not & The printing results of symbols are the same 
	printf(" No addition & The symbol add The function address is %d, Add & Symbol add The address of the function is %d.\n",add,&add);
	printf(" As can be seen from the above print results add Add before or not & The printing results of symbols are the same \n");
	// A function pointer fun Point to add function 
	fun = add;
	printf("10 add 5 be equal to %d.\n",fun(10,5));
	// Function pointer to sub function 
	fun = &sub;
	printf("10 subtract 5 be equal to %d.\n\n",sub(10,5));
	// There are also two ways to call function pointers , Suggest the first , Because it can be clearly indicated that the function is called by pointer .
	int p = fun (8,6);// Call mode one 
	int q = (*fun)(8,6);// Call mode 2 
	printf(" Call mode one 8 subtract 6 As the result of the %d,\n Call mode 2 8 subtract 6 As the result of the %d\n",p,q);
	
	return 0;
}

  Running results ：

Difference between the two

Through the above introduction , We should be able to clearly understand the definition of both . So simply sum up the difference between the two ：

Different definitions

A pointer function is essentially a function , Its return value is a pointer .
A function pointer is essentially a pointer , It points to a function .

The writing is different
Pointer function ：int *fun(int x,int y);
A function pointer ：int (*fun)(int x,int y);
It can be simply understood as , Pointer function * It's data type , The asterisk of the function pointer belongs to the function name .
More simply , You can distinguish the two in this way ： The function name with parentheses is the function pointer , Otherwise, it's a pointer function .

Homework 1 

#include <stdio.h>
/*
 Write a program , Make a statement 3*5 And initialize , Specific values are optional , Program print out 
 The number , Then double the value , Then print out the new value again , To write - A function to display the contents of the array ,
 Writing another function to perform the doubling function , The array name and the number of array rows are passed to the function by the program as parameters 
*/
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int fuzhi(int arr[3][5],int x,int y);
int doublt(int arr[3][5],int x,int y);

int main()
{
	int arr[3][5]={0};
	fuzhi(arr,3,5);
	printf("\n");
	doublt(arr,3,5);
 	return 0;
}

int fuzhi(int arr[3][5],int x,int y)
{
	int i,j;
	srand(time(0));
	for(i=0;i<x;i++)
	{
		for(j=0;j<y;j++)
			arr[i][j]=rand()%20+1;
	}
	for(i=0;i<x;i++)
	{
		for(j=0;j<y;j++)
			printf("%d\t",arr[i][j]);
		printf("\n");
	}
}

int doublt(int arr[3][5],int x,int y)
{
	int i,j;
	for(i=0;i<x;i++)
	{
		for(j=0;j<y;j++)
			arr[i][j] *= 2;
	}
	for(i=0;i<x;i++)
	{
		for(j=0;j<y;j++)
			printf("%d\t",arr[i][j]);
		printf("\n");
	}
}

  Running results ：

Homework 2

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int fuzhi(int arr[],int n);
//fun(int *arr,int n,int *odd,int *even);
//int *sum_oushu(int arr[],int n);
//int *sum_jishu(int arr[],int n);
int fun(int *arr,int n,int *odd,int *even); 
int main()
{
	int i;
	int sum01=0, sum02=0;
 	int arr[10]={0};
 	int *odd,*even;
 	fuzhi(arr,10);
 	for(i=0;i<10;i++)
 	{
 		printf("%d ",arr[i]);	
	}
	printf("\n");
	odd = &sum01;
	even = &sum02;
	fun(arr,10,odd,even);
	printf(" The sum of even numbers is ：%d\n",*odd);
	printf(" The sum of odd numbers is ：%d",*even);
	
 	return 0;
}

int fuzhi(int arr[],int n)
{
	int i;
	srand(time(0));
	for(i=0;i<n;i++)
	{
		arr[i]=rand()%10+1; 
	}
}

//int *sum_oushu(int arr[],int n)
//{
//	int i,sum=0;
//	for(i=0;i<n;i++)
//	{
//		if(arr[i]%2==0)
//			sum += arr[i];
//	}
//	int *even=&sum; 
//	printf("%d",*even);
//	return even;
//}
//
//int *sum_jishu(int arr[],int n)
//{
//	int i,sum=0;
//	for(i=0;i<n;i++)
//	{
//		if(arr[i]%2!=0)
//			sum += arr[i];
//	}
//	int *odd=&sum;
//	return odd;
//}
int fun(int *arr,int n,int *odd,int *even)
{
	int i;
	int j;
	for(i=0;i<n;i++)
	{
		if(arr[i]%2==0)
			*odd += arr[i];
	}

	for(j=0;j<n;j++)
	{
		if(arr[j]%2!=0)
			*even += arr[j];
	}
}

Running results ： 

Homework 3 

#include <stdio.h>

int add(int x,int y)
{
    return x+y;
}
int sub(int x,int y)
{
    return x-y;
}
int cheng(int x,int y)
{
    return x*y;
}
int chu(int x,int y)
{
    return x/y;
}

int (*fun[4])(int,int)={add,sub,cheng,chu};// A function pointer , Encapsulate function modules , By an array of function pointers (*fun[]) Unified management  

int main ()
{
	int x,y;
	char ch; 
	printf(" Please enter operator ：\n");
	ch=getchar();
	switch(ch)
	{
		case '+':
			printf(" Please enter two integers ：\n");
			scanf("%d %d",&x,&y);
			printf("%d+%d=%d",x,y,fun[0](x,y));
			break;
		case '-':
			printf(" Please enter two integers ：\n");
			scanf("%d %d",&x,&y);
			printf("%d-%d=%d",x,y,fun[1](x,y));
			break;
		case '*':
			printf(" Please enter two integers ：\n");
			scanf("%d %d",&x,&y);
			printf("%d*%d=%d",x,y,fun[2](x,y));
			break;
		case '/':
			printf(" Please enter two integers ：\n");
			scanf("%d %d",&x,&y);
			printf("%d/%d=%d",x,y,fun[3](x,y));
			break;
		default:
			break;
	}
//	fun = &add;
//	printf("10 add 5 be equal to %d\n",fun(10,5));
//	fun = &sub;
//	printf("10 subtract 5 be equal to %d\n",fun(10,5));
//	fun = &cheng;
//	printf("10 multiply 5 be equal to %d\n",fun(10,5));
//	fun = &chu;
//	printf("10 except 5 be equal to %d",fun(10,5));
	
	return 0;
}

Running results ：