Question

In C++ please.

7. Define a callback. Name three kinds of callbacks that we

studied. Simplify the following code using a lambda-expression.

bool notZero(int e){return e!=0;}

...

auto it=find_if(vect.begin(), vect.end(), notZero);

Answer 1

Callback in C++

Callback or call-after function is any executable code that is passed as an argument to another

code,that other code is expected to execute(callback) the argument at a given time.

Example:

#include<iostream>

void A1() { //void is a return type like int

cout << "A1 function"; //print A1 function

}

void A2() {

cout <<"A2 function";

}

int main() {

void(*p) (); //function pointer

p= A1;

p() //output: A1 function

p=A2;

p(); //output: A2 function

}

3 kinds of Callback:

1) Function Pointer - See above example.

2) Function Objects/Functors

A functor is an object that acts like a function.

To create a functor ,we create a object that overloads the operator ().

Example code of Functor:

#include<iostream>

using namespace std;

class A1 {

int num; //variable declared

public:

A1() //constructor

{

num=0; //initialising num variable with 0

}

void print() {

cout<< num << endl; //output or print value of num variable on the output screen

}

void operator () (int val)

{

num+=val; //num=num+val

}

   };

int main() {

A1 a; //object declaration

a.print(); // 0

a(2); //Functor

a.print(); //2

a(8); //function object or Functor

a.print() //10

  return 0;

}

3) Lambda Expression

1- Anonymous notation representing something that performs calculation.

2- In Functional Programming ,it is used to produce first class and pure functions

3- This feature included in C++ 11.

3 Basic parts of Lambda Expression

1. Capturing List: [ ]

2. Parameter List: ( )

3. Body : { }

Order of above basic parts is - [ ] ( ) { }

1. bool notZero(int e){return e!=0;}

myLambda=[ ] (int e != 0) return true; //myLambda receives an integer and returns bool or boolean value(true or false)