Function Pointers

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Typedef

The "typedef" keyword can be used to assign another name for a type declaration. This is often used in "C++" to make declarations easier to use.
File: type1.cpp

#include<stdio.h>

int main()
{
    int x1 ;
    int* ptr1 ;

    x1 = 100 ;
    printf("x1 value is %d\n" , x1 );
    ptr1 = &x1 ;
    printf("%d \n" , *ptr1 );

    return 0 ;
}
Output:
100
100

In the above example we are using an integer type and a pointer to an integer type. We can rewrite the above program as:
File: type2.cpp

#include<stdio.h>
int main()
{
  typedef int INTEGER ;
  typedef int* POINTER_TO_INTEGER ;

  INTEGER x1 ;
  POINTER_TO_INTEGER ptr1 ;
  x1 = 100 ;
  printf("x1 value is %d\n" , x1 );
  ptr1 = &x1 ;
  printf("%d \n" , *ptr1 );

  return 0 ;
}

The above example shows the declaration of "typedef" and it's usage. It's as if we had new types.

Function Pointers

We have seen examples of a pointer pointing to an integer but we can also have a pointer pointing to a function.
File: func1.cpp

#include<stdio.h>
void function1( int param1 )
{
    printf( "Am inside function1:  %d \n" , param1) ;
}

void function2( int param1 )
{
    printf( "Am inside function2:  %d \n" , param1) ;
}

int main()
{
    void (*POINTER_FUNCTION)(int a) ;

    POINTER_FUNCTION = function1 ;
    POINTER_FUNCTION(100) ;
    POINTER_FUNCTION = function2 ;
    POINTER_FUNCTION(200) ;
    return 0 ;
}
$ g++ func1.cpp ; ./a.exe
Am inside function1:  100
Am inside function2:  200

The declaration:

    void (*POINTER_FUNCTION)(int a) ;

states that the "POINTER_FUNCTION" is a pointer to a function.
However it is not just any function but a function with a
particular signature. The function must take an int as
an argument and return void.

The "POINTER_FUNCTION" is actually a variable and we can assign
the address using the form:

POINTER_FUNCTION = function1 ;

and then invoke the pointer using :

POINTER_FUNCTION(100) ;

This ends up calling the function with an argument
value of 100 .  We can change the value that the pointer
holds and assign the value of function 2 to it .

File: func2.cpp

#include<stdio.h>

void function1( int param1 )
{
    printf( "Am inside function1:  %d \n" , param1) ;
}
void function2( int param1 )
{
    printf( "Am inside function2:  %d \n" , param1) ;
}

int main()
{
    void (*POINTER_FUNCTION)(int a) ;
    POINTER_FUNCTION = function1 ;
    (*POINTER_FUNCTION)(100) ;
    POINTER_FUNCTION = function2 ;
    (*POINTER_FUNCTION)(200) ;
    return 0 ;

}
$ g++ func2.cpp ; ./a.exe
Am inside function1:  100
Am inside function2:  200

The above shows the syntax "(*POINTER_FUNCTION)(100)" that can be used to call the function also.
File: func3.cpp

#include<stdio.h>

void function1( int param1 )
{
    printf( "Am inside function1:  %d \n" , param1) ;
}
void function2( int param1 )
{
    printf( "Am inside function2:  %d \n" , param1) ;
}

int main()
{
    typedef void (*POINTER_FUNCTION_TYPE)(int a) ;
    POINTER_FUNCTION_TYPE ptr = function1 ;
    ptr(100) ;
    ptr = function2 ;
    ptr(200 ) ;
    return ( 0 ) ;
}

The notation
void (*POINTER_FUNCTION)(int a) ;
is kind of confusing . We are used to saying "int x1" where we have a type and then a variable of that type. We can use "typedef" to rewrite out progra
Using "typedef" above we define a type "POINTER_FUNCTION_TYPE" and then we can use this type to create a variable "ptr". This variable can hold the address of a function and is a function pointer.