Function Pointer

In C/C++, functions, like all data items, have an address. The name of a function is the starting address where the function resides in the memory, and therefore, can be treated as a pointer. We can pass a function pointer into function as well.

 The syntax for declaring a function pointer is:
// Function-pointer declaration
return-type (* function-ptr-name) (parameter-list)

// Examples
double (*fp)(int, int)  // fp points to a function that takes two ints and returns a double (function-pointer)
double *dp;             // dp points to a double (double-pointer)
double *fun(int, int)   // fun is a function that takes two ints and returns a double-pointer

 double f(int, int);      // f is a function that takes two ints and returns a double
fp = f;                 // Assign function f to fp function-pointer
Example
/* Test Function Pointers (TestFunctionPointer.cpp) */
#include
using namespace std;

int arithmetic(int, int, int (*)(int, int));
    // Take 3 arguments, 2 int's and a function pointer
    //   int (*)(int, int), which takes two int's and return an int
int add(int, int);
int sub(int, int);

int add(int n1, int n2) { return n1 + n2; }
int sub(int n1, int n2) { return n1 - n2; }

int arithmetic(int n1, int n2, int (*operation) (int, int)) {
   return (*operation)(n1, n2);
}

int main() {
   int number1 = 5, number2 = 6;

   // add
   cout << arithmetic(number1, number2, add) << endl;
   // subtract
   cout << arithmetic(number1, number2, sub) << endl;

}
Generic Pointer or void Pointer (void *)

A void pointer can hold address of any data type (except function pointer). We cannot operate on the object pointed to by void pointer, as the type is unknown. We can use a void pointer to compare with another address.

Constant Pointer vs. Constant Pointed-to Data

Non-constant pointer to constant data: Data pointed to CANNOT be changed; but pointer CAN be changed to point to another data. For example,
int i1 = 8, i2 = 9;
const int * iptr = &i1;  // non-constant pointer pointing to constant data
// *iptr = 9;   // error: assignment of read-only location
iptr = &i2;  // okay
Constant pointer to non-constant data: Data pointed to CAN be changed; but pointer CANNOT be changed to point to another data. For example,
int i1 = 8, i2 = 9;
int * const iptr = &i1;  // constant pointer pointing to non-constant data
 // constant pointer must be initialized during declaration
*iptr = 9;   // okay
// iptr = &i2;  // error: assignment of read-only variable
Constant pointer to constant data: Data pointed to CANNOT be changed; and pointer CANNOT be changed to point to another data. For example,
int i1 = 8, i2 = 9;
const int * const iptr = &i1;  // constant pointer pointing to constant data
// *iptr = 9;   // error: assignment of read-only variable
// iptr = &i2;  // error: assignment of read-only variable
Non-constant pointer to non-constant data: Data pointed to CAN be changed; and pointer CAN be changed to point to another data. For example,
int i1 = 8, i2 = 9;
int * iptr = &i1;  // non-constant pointer pointing to non-constant data
*iptr = 9;   // okay
iptr = &i2;  // okay

                    ...🔚

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