PASSING FUNCTIONS AS ARGUMENT
DISCLAIMER: Though program(s) mentioned here is a
working program and also, I have tried my best to make this content error free,
but still I do not claim that every concept mentioned here is correct.
Therefore, content available here is not meant for primary source of learning.
Introduction
We have passed variable, constants, structure variables to a
function as argument. But have you ever tried or thought to pass a function as an argument to another function.
If not, then let us do it together. At first sight it may appear a bit
confusing to you but trust me it is as simple as writing “int main()” .
Story behind this
story(blog):
Well, I want to share with you that how I thought to learn
this concept. Once I was programming a graphical Tic Tac Toe game in C++ using SDL library,
in that I created 3 buttons to perform different actions like “New Game”,
”Exit”, ”High Score”. Therefore, it is obvious that these buttons will perform
different tasks, but for this I have to rewrite the same code 3 times to create
and display button that will perform its defined task. But, I was feeling too
lazy for that, so I thought that my work would get easier if I could somehow
pass function as an argument to a function, By doing this, I can simply write common
code for button just once and can pass different functions that perform their
defined tasks like start a new game, show high score etc. So, I searched on
Internet, got some hint, and completed my program.
Now let’s begin
Learning………….
I believe that you have some knowledge of pointers in C/C++.
REMEMBER:
FUNCTION NAME ACTUALLY REFERS TO THE ADDRESS OF THE FUNCTION.
If you had read and understood above line then you must have
guessed that we will need pointers to deal with memory addresses.
(We will see a complete working program later in this blog.)
A.
HOW
TO DECLARE POINTERS TO FUNCTION:
return_type (*pointer_name)(parameter_list);
Here, * tells that we declared a
pointer to function.
NOTE: If the function to be pointed does not take any arguments,
then you have to place () or (void) in place of parameter list.
 |
| Declaration of Function |
Above image clearly explains what these data types and names
means.
Please Note:
Parenthesis of (*compute) i.e. ‘(‘ and ‘)’ are compulsory(as per my knowledge) to remove ambiguity for
compiler, I mean If you do not enclose *compute
in parenthesis then compiler will get confused that * is used with void and your declaration will be treated as void * compute(int,int), i.e. a function named compute taking two arguments
of int type and returning pointer of void
type.
Q. Create a
pointer for char sayHello(int,char) function.
Ans. char (*fp)(int,char);
B.
INITIALIZING
FUNCTION POINTERS WITH A FUNCTION
function_pointer_name=function_name;
EXAMPLE Let a function sum:
void
sum(int a, int b)
{
cout<<”Sum
=”<<a+b;
}
To initialize
compute(created earlier) we will write
compute=sum;
NOTE: do not use
parenthesis with sum( i.e do not write sum()).
C.
CALLING
FUNCTIONS USING FUNCTION POINTERS
function_pointer_name(arguments);
EXAMPLE to call sum function using compute function pointer write
compute(5,2);
or,
compute(x,y); //where x,y may be a variable or a constant
See this(screen scrap of code):
 |
| Calling Function |
D.
WRITING
FUNCTIONS THAT TAKE FUNCTION AS ARGUMENT
Functions taking functions as
argument are nothing but normal functions which contain one or more
pointers to function.
 |
| Function that takes function as argument |
EXAMPLE
See that the function takeFunction
is written in usual way except that there is a function pointer in its
parameter list (see void (*fp) (int,int)
above).
Please note that I have used two
more parameters a, b of int type that are used as arguments
while calling the function (see fp(a,b);), Its
was not mandatory to include int a, int
b in parameter list but still I included it for convenience. I might have
used the following definition which is also true, but you can see that each
time the function is called with same data (i.e. 5, 2) which is probably of no
use (see below).
void
takeFunction(void (*fp) (int,int))// is not much reliable
{
fp(5,2);
}
E.
CALLING
FUNCTIONS THAT TAKE FUNCTION AS ARGUMENT
Now, it’s time to call the function (ex. takeFunction defined above).
See example below to understand
takeFunction (sum, 5, 2);
What I have done is just passed the sum function (defined in section B) to
this takeFunction function as
argument for its fp function pointer
, other arguments 5, 2 are passed in usual way as you do normally.
NOTE: do not use
parenthesis with sum( i.e sum())
when passing it as argument.
F.
COMPLETE
C++ PROGRAM THAT USES ABOVE CONCEPTS
#include<iostream.h>
#include<conio.h>
void sum(int
a, int b)
{
cout<<"\nSum ="<<a+b;
}
void diff(int
a, int b)
{
cout<<"\nDifference
="<<a-b;
}
void
takeFunction(void (*fp) (int,int),int a, int b)
{
fp(a,b);
}
int main()
{
clrscr();
void (*compute)(int,int);
compute=sum;
compute(5,2);
compute=diff;
compute(5,2);
takeFunction(sum,2,3);
takeFunction(diff,2,3);
getch();
return 0;
}
NO SEE
HOW DECLARATION OF VARIABLE POINTER IS DIFFERENT FROM FUCNTION POINTER
THANKS!!!!!!!!!!!!!!!!!
