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Function Pointers in C++ - With an Example

Updated on January 8, 2017

1. What is Function pointer?

You can pass a function as a parameter to some other function. The function's starting instruction can be represented as an address which comes from "Code Segment". So, C++ can admit storing that address in a pointer variable and thereby allow us to call the function using that stored pointer which is pointing to a function. Here we call this pointer as "Function Pointer".

In this example, we are going to write two functions. One will perform the square of given number and the other one will perform cubic of a given number. Then, we will pass these functions as arguments to some other function. The receiving function will make a call to those function by making use of the passed in function pointers.

2. Code Example for Function Pointer

The code for passing the function as a parameter is given below. First, go through this code and look at the next section for the explanation of the code flow and function pointer.

// TestIt.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "conio.h"

//Sample 01: Function that performs Square
long Square_it(long x)
{
	return (x * x);
}

//Sample 02: Function that perform Cube
long Cube_it(long x)
{
	return (x * x * x);
}

//Sample 03: Function that Take function pointer
long summation(long summationFn(long), int n)
{
	int sum = 0;
	for(int i=1; i<=n; i++)
	sum = sum + summationFn(i);
	return sum;
}

int _tmain(int argc, _TCHAR* argv[])
{
	int total = 0;

	//Sample 04: Calculate Square summation upto 4
	total = summation(Square_it, 4);
	printf("Square Summation upto 4= %ld\r\n", total);

	//Sample 05: Calculate Cubic Summation upto 4
	total = summation(Cube_it, 4);
	printf("Cube Summation upto 4 = %ld\r\n", total);
	getch();
	return 0;
}

3. Explanation

  • The functions Square_it, Cube_it will calculate the Square and cubic of any given numbers.
  • The function summation will take the first parameter as a function pointer. Here, the function pointer passed as a parameter is expected to be taking a long as a parameter and returning long to the caller. The second parameter for the summation function tells where we want to end the summation series. Have a look at the syntax of specifying the function pointer as a parameter.
  • The summation function will call the passed-in function for ā€˜nā€™ number of times by passing the number that increment from 1 to n.
  • In the program main, we called the summation function twice. For the first time, we passed the address of the square_it function. And in the second time, we passed the cube_it function as the first parameter. The result we get in the main function is the Square summation from 1 to 4 and cubic summation from 1 to 4.

Result of running the program is shown below:

Pic01

Fig.1 Code Output - Function Pointers
Fig.1 Code Output - Function Pointers | Source

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