What is a callback function?

■ What is a callback function?

A function that is passed as an argument to another function.

The C language example on the following site is easy to understand. A callback function is assigned to the function pointer and passed as an argument.

http://shiroibanana.blogspot.jp/2012/09/callback.html

#include<stdio.h>
typedef void (* FUNC_POINTER)(char *); 

//Cobuck function 1
void callback1(char *s){
    printf("%s\n",s);
}

//Callback function 2
void callback2(char *s){
    printf("[");
    printf("%s",s);
    printf("].\n");
}

//A function that calls a callback function
void func(char *s,FUNC_POINTER p){
    //Call the callback function
    p(s);
}

int main(){
    FUNC_POINTER p;

    p = callback1;
    //Call callback function 1
    func("Cobuck function 1",p);

    p = callback2;
    //Call callback function 2
    func("Cobuck function 2",p);
    return 0;
}

Execution result.

Cobuck function 1
[Cobuck function 2].

■ Python example

An example of using a callback function in python. It seems that the function that calls the callback function is often described as handler ↓

def handler(func,*args):
    return func(*args)

http://www.phactory.jp/cms/blog/post/admin/2010/02/python_funcpointer/

As a flow,

1. Define a handler
2. Define a callback function
3. Assign a callback function to a function pointer
4. Pass a function pointer as a handler argument

# -*- coding: utf-8 -*-
def handler(func,*args):
    return func(*args)

def callback1(message1):
    print("%s" % message1)

def callback2(message1, message2):
    print("%s, %s" % ( message1, message2) )

# main
if __name__ == "__main__":

   #Callback function assignment
   call = callback1
   #Handler execution:Callback function call
   handler(call, "message1")

   #Callback function assignment
   call = callback2
   #Handler execution:Callback function call
   handler(call, "message1", "message2")

Execution result.

message1
message1, message2

■ Use class

The member "func" in the class has a variable that acts as a function pointer, but basically the same thing is done.

1. Define a class that acts as a handler
2. Define a member to be treated as a function pointer
3. Define a method that acts as a handler
4. Define a callback function
5. Assign a callback function to a function pointer
6. Pass a function pointer as a handler argument

# -*- coding: utf-8 -*-
class Callbacker():
    def __init__(self, func):
        self.func = func

    def handler(self, *args):
        self.func(*args)

def callback1(message1):
    print '%s' % message1

def callback2(message1, message2):
    print("%s, %s" % ( message1, message2) )

# main
if __name__ == "__main__":

    #Callback function assignment
    callback = Callbacker(callback1)
    print (type(callback.func))
    print ("callback.func "),
    print (callback.func)
    print ("callback1     "),
    print (callback1)
    #Handler execution:Callback function call
    callback.handler("message1")
    print ("")

    #Callback function assignment
    callback.func = callback2
    print (type(callback.func))
    print ("callback.func "),
    print (callback.func)
    print ("callback2     "),
    print (callback2)
    #Handler execution:Callback function call
    callback.handler("message1", "message2")

Execution result

<type 'function'>
callback.func  <function callback1 at 0x7fcff9c8a9b0>
callback1      <function callback1 at 0x7fcff9c8a9b0>
message1

<type 'function'>
callback.func  <function callback2 at 0x7fcff9c8ab18>
callback2      <function callback2 at 0x7fcff9c8ab18>
message1, message2

■ Summary

A callback function is a function that is passed as an argument to another function.

Basically

1. Define a handler
2. Define a callback function
3. Assign a callback function to a function pointer
4. Pass a function pointer as a handler argument