C language: callback function

What is the callback function ？

A callback function is a function called through a function pointer . Put the function pointer （ Address ） Pass as argument to another function , When this function is used to call the function it points to , It's called a callback function . The callback function is not called directly by the function's implementer , It's called by another party when a particular event or condition occurs , Used to respond to the event or condition .

This concept is more abstract , Next , Through two implementation of the computer code comparison , To learn more about callback functions .

Purpose ： The simulation realizes that the computer makes the operation of addition, subtraction, multiplication and division .

Before using the callback function to simulate the implementation of the computer , First use another method to help understand , Encapsulate functions that implement different functions , It is called in an array , This method is called using an array of function pointers to store different kinds of calculations . The first thing to know is , A function pointer is a pointer to a function , The array of function pointers is to store these pointers to functions in an array , Then call the required function by subscript in this array , This code is relatively simple , Don't explain in detail , Go straight to the code ：

#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 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 Div(int x, int y)
{
	return x / y;
}

int main()
{
	int input = 0;
	int x = 0;
	int y = 0;
	int ret = 0;
	//pfArr Is an array of function pointers , Also called transfer table 
	int (*pfArr[5])(int, int) = { 0, Add, Sub, Mul, Div };
	do
	{
		menu();
		printf(" Please select :>");
		scanf("%d", &input);
		if (input == 0)
		{
			printf(" Exit the computer \n");
			break;
		}
		else if (input >= 1 && input <= 4)
		{
			printf(" Please enter 2 Operands :>");
			scanf("%d %d", &x, &y);
			ret = (*pfArr[input])(x, y);
			printf(" The result of the calculation is :%d\n", ret);
		}
		else
		{
			printf(" Input error , Re input \n");
		}
	} while (input);
	return 0;
}

Understand this code to achieve the computer , After you can roughly know how to implement , Then there is the second method ： Use the callback function to call the required function for calculation .

This is a flexible method , First use in the main function switch Statement to determine the user's choice , Then pass the addresses of different functions to another function through different choices , Call the required function again through this address in this function , This is not called directly by the implementer of the function , It's called by another party when a particular event or condition occurs , The function used to respond to this event or condition is called a callback function , See below for the specific code ：

#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 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 Div(int x, int y)
{
	return x / y;
}

void calc(int (*pf)(int, int))
{
	int x = 0;
	int y = 0;
	int ret = 0;
	printf(" Please enter 2 Operands :>");
	scanf("%d %d", &x, &y);
	ret = pf(x, y);
	printf(" The result of the calculation is :%d\n", ret);
}

int main()
{
	int input = 0;
	do
	{
		menu();
		printf(" Please enter :>");
		scanf("%d", &input);
		switch (input)
		{
		case 1:
			calc(Add);
			break;
		case 2:
			calc(Sub);
			break;
		case 3:
			calc(Mul);
			break;
		case 4:
			calc(Div);
			break;
		case 0:
			printf(" Exit the computer \n");
			break;
		default:
			printf(" Input error , Re input \n");
			break;
		}
	} while (input);
	return 0;
}

This article introduces the concept of callback function through a simple computer code , Next, there will be an article on sorting algorithm , It will be used again 、 And get a little deeper into the callback function .

The use of callback function in actual code is flexible , This article is just a summary of its preliminary understanding , For reference only .