Calling Function Pointers

Part of the Object Oriented Programming section of Coddy's C journey — lesson 34 of 61.

Now that you know how to declare a function pointer and assign a function to it, the next step is actually calling the function through that pointer.

The syntax is straightforward — you use the function pointer just like you would call a regular function:

int add(int a, int b) {
    return a + b;
}

int main() {
    int (*operation)(int, int) = add;
    
    int result = operation(5, 3);  // Calls add(5, 3)
    printf("%d\n", result);        // Output: 8
    return 0;
}

Simply write the pointer name followed by parentheses containing the arguments. C automatically dereferences the pointer and executes the function it points to.

You can also use explicit dereference syntax with (*operation)(5, 3), but it's optional and less common. Both forms produce identical results.

The real power emerges when you reassign the pointer to different functions:

int subtract(int a, int b) {
    return a - b;
}

operation = subtract;
result = operation(5, 3);  // Now calls subtract(5, 3)
printf("%d\n", result);    // Output: 2

The same variable operation now executes completely different logic. This ability to swap behavior at runtime is the foundation of polymorphism in C.

Challenge

Easy

Let's practice calling functions through pointers and swapping which function gets executed. You'll build a simple calculator that can switch between addition and subtraction at runtime.

Write a program that:

Defines two functions:
- add — takes two int parameters and returns their sum
- subtract — takes two int parameters and returns the first minus the second
In main, declares a function pointer called calc that can point to functions taking two int parameters and returning an int
Reads four inputs: two integers and two operation codes
For each operation code:
- Assign the appropriate function to calc (1 for add, 2 for subtract)
- Call the function through the pointer with the two numbers
- Print the result

You will receive four inputs: the first number, the second number, the first operation code (1 or 2), and the second operation code (1 or 2).

Your output should show two results on separate lines—one for each operation performed using the same two numbers:

15
5

In this example, with inputs 10, 5, 1, 2: the first operation (code 1) adds 10 + 5 = 15, then the pointer is reassigned, and the second operation (code 2) subtracts 10 - 5 = 5.

This demonstrates how the same pointer variable can execute completely different logic by reassigning it between calls.

Cheat sheet

To call a function through a pointer, use the pointer name followed by parentheses with arguments:

int add(int a, int b) {
    return a + b;
}

int main() {
    int (*operation)(int, int) = add;
    
    int result = operation(5, 3);  // Calls add(5, 3)
    printf("%d\n", result);        // Output: 8
    return 0;
}

You can also use explicit dereference syntax (*operation)(5, 3), but it's optional.

Function pointers can be reassigned to point to different functions at runtime:

int subtract(int a, int b) {
    return a - b;
}

operation = subtract;
result = operation(5, 3);  // Now calls subtract(5, 3)
printf("%d\n", result);    // Output: 2

This allows the same pointer variable to execute different logic by reassigning it between calls.

Try it yourself

#include <stdio.h>

// TODO: Define the add function here


// TODO: Define the subtract function here


int main() {
    // Read inputs
    int num1, num2, op1, op2;
    scanf("%d", &num1);
    scanf("%d", &num2);
    scanf("%d", &op1);
    scanf("%d", &op2);
    
    // TODO: Declare a function pointer called 'calc' that can point to
    // functions taking two int parameters and returning an int
    
    
    // TODO: For the first operation code (op1):
    // - Assign the appropriate function to calc (1 for add, 2 for subtract)
    // - Call the function through the pointer and print the result
    
    
    // TODO: For the second operation code (op2):
    // - Reassign calc to the appropriate function
    // - Call the function through the pointer and print the result
    
    
    return 0;
}