Pointers to functions in C++

In C++, just as we can have pointers to variables, we can have pointers to functions as well.

The general syntax of a pointers to a functions is:

return-type (*pointer-name) (list of parameter);

For example:

1	void (*p) (float, int);

In the above declaration, a pointer to a function returns void and takes the two formal arguments of float and int types respectively.

After declaring a pointer to a function, the address of the function must be assigned to a pointer as follows:

p = &function-name;

where p is a pointer variable.

Through pointer to function, the function is called as follows:

1	A = (*p) (x, y);

where, x and y are actual parameters, and
A is a variable in which the function’s return value is stored,
p is a pointer name which points to the function

Let us illustrate pointer to function with a programming example as follows:

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#include<iostream.h> void main() {    float add(float, float);   //function prototype    float a, b, s;    float (*p) (float, float); //declaration of pointer to function    p = &add;                  //assigning function add to p      a = 20.5;    b = 10.3;    s = (*p) (a,b);            //calling function add through pointer                               //to function p    cout << "s= " << s << endl; }   float add(float a1, float a2) {                              //add function defined    int sum;    sum = a1 + a2;    return(sum); }

The output of the above program would be:

s = 30.8

In the above program:

• The statement:
  
  1	float (*p) (float, float);

  
  declares a pointer p which is a pointer to function
• The statement:
  
  1

  p = &add;

  
  assigns the address of function add to p, i.e., now pointer p points to function add
• The statement:
  
  1	s = (*p) (a, b);

  
  calls the function which is pointed by p (i.e., add) and then assigns the returned value of function to s
• It is worthwhile to note here that if in pointer to function declaration we skip the bracket in which pointer name is is written the C++ compiler interprets it as a function which returns a pointer type value. For example, if in above program we write:
  
  1	float *p (float, float);

  
  then this statement means p is a function which returns pointer and the pointer points to float type data. i.e., function returns a pointer to float