Returning a function pointer from a function in C/C++
In C/ C++, like normal data pointers(int *, char *, etc), there can be pointers to functions. Every function created in a program gets an address in memory since pointers can be used in C/C++, so a pointer to a function can also be created.
Syntax:
return type (*function_pointer_name) (argument_type_1, argument_type_2, ……, argument_type_n) = &function_name;
OR
return type (*function_pointer_name) (argument_type_1, argument_type_2, ……, argument_type_n) = function_name;
NOTE: Arguments type and return type of function pointer should match with the actual function present in the program.
Program 1:
- C
- C++
C
// C program for the above approach#include <stdio.h>// Function to add the value 10 to// the variable avoid demo(int* a) { *a += 10; }// Driver Codeint main(){ int num = 20; // ptr contains address of demo // function or void void (*ptr)(int*) = &demo; // or (*ptr)(&num); ptr(&num); printf("%d", num); return 0;} |
C++
30
Return Function Pointer From Function: To return a function pointer from a function, the return type of function should be a pointer to another function. But the compiler doesn’t accept such a return type for a function, so we need to define a type that represents that particular function pointer.
Syntax :
typedef return type (*function_pointer_name) (argument_type_1, argument_type_2, ……, argument_type_n);
This creates a type which represents a pointer for a particular function.
Program 2:
- C
- C++
C
// C program for the above approach#include <stdio.h>typedef int (*ptr)(int*);typedef ptr (*pm)();int fun1(int* y){ printf("geeks!!\n"); return *y + 10;}// Function that return type ptrptr fun(){ printf("Hello "); // or return fun1; /* or int(*pt)(int*)=fun1; return pt */ return &fun1;}// Driver Codeint main(){ int a = 10; pm u = fun; printf("%d", (*u())(&a)); return 0;} |
C++
Hello geeks!! 20
Declaring an array that has two function pointers as to its elements and these function pointers, in turn, return other function pointers which point to other functions. The logic of the driver code main() function can be changed in the above program as:
Program 3:
- C
- C++
C
// C program for the above approach#include <stdio.h>// This defines a type for// function prototypestypedef int (*ptr)(int*);typedef ptr (*pm)();int fun1(int* y){ printf("geeks!!\n"); return *y + 10;}// fun() is a function with// return type ptrptr fun(){ printf("Hello "); // or return fun1; /* or int(*pt)(int*)=fun1; return pt */ return &fun1;}// Driver codeint main(){ int a = 10; pm u = fun; /* Above line assigns 'u' which is of type 'pm' to an array of size 1 which has function pointers as its elements and these function pointers in turn return other function pointer which points to other functions. Now this 'p' array contains a function pointer 'u' which points to fun() and this fun() returns another function pointer which points to fun1(). */ int (*(*p[1])())(int*) = { u }; printf("%d", (*p[0]())(&a));} |
C++
Hello geeks!! 20
In both C and C++, you can return a function pointer from a function by declaring the return type of the function as a pointer to a function.
Here’s an example in C:
- C
C
#include <stdio.h>int add(int a, int b) { return a + b;}int subtract(int a, int b) { return a - b;}int (*operation(char op))(int, int) { if (op == '+') { return &add; } else if (op == '-') { return &subtract; } else { return NULL; }}int main() { int (*func_ptr)(int, int) = operation('+'); printf("%d\n", func_ptr(4, 5)); return 0;} |
9
In this example, we have two functions called add() and subtract() that take two integer arguments and return an integer value. We also have a function called operation() that takes a character argument op and returns a function pointer to add() or subtract() depending on the value of op. Finally, in main(), we call operation() with ‘+’ as the argument and store the result in a function pointer called func_ptr. We then call func_ptr() with 4 and 5 as arguments and print the result to the console.
Here’s the same example in C++:
- C++
C++
#include <iostream>using namespace std;int add(int a, int b) { return a + b;}int subtract(int a, int b) { return a - b;}int (*get_operation(char op))(int, int) { if (op == '+') { return &add; } else if (op == '-') { return &subtract; } else { return NULL; }}int main() { int (*op)(int, int) = get_operation('+'); int result = op(3, 4); cout << "Result: " << result << endl; return 0;} |
Result: 7
