Void Pointers Recap

Part of the Object Oriented Programming section of Coddy's C journey. Lesson 49 of 61.

A void* is C's way of saying "a pointer to something, but I'm not telling you what." It can hold the address of any data type: an int, a float, a struct, or anything else. This makes it the foundation for writing generic code in C.

The catch is that you cannot dereference a void* directly. The compiler doesn't know how many bytes to read or how to interpret them. To use the data, you must cast it back to the correct type:

void print_int(void* data) {
    int* p = (int*)data;
    printf("%d\n", *p);
}

int main() {
    int x = 42;
    print_int(&x);
    return 0;
}

This flexibility comes with responsibility. If you cast to the wrong type, the compiler won't stop you, but your program will produce garbage or crash. That's why generic functions often pair a void* with a type indicator (like an enum or flag) so the function knows how to interpret the data correctly.

In the upcoming lessons, you'll use void* to build containers that can store any type of data, enabling truly reusable data structures.

Challenge

Easy

Write a function called print_value that accepts two parameters: a void* pointer to data and an int type flag. The function should interpret and print the data based on the type flag:

If the type flag is 0, cast the pointer to int* and print the integer value
If the type flag is 1, cast the pointer to float* and print the float value with 2 decimal places

Your program will receive two inputs:

A type flag (0 for integer, 1 for float)
A numeric value

Based on the type flag, store the value in the appropriate variable type, then call print_value with a pointer to that variable and the type flag.

Example output when the inputs are 0 and 42:

Example output when the inputs are 1 and 3.14:

3.14

Example output when the inputs are 0 and -17:

-17

Example output when the inputs are 1 and 99.5:

99.50

Try it yourself

#include <stdio.h>

// TODO: Write your print_value function here


int main() {
    int type_flag;
    scanf("%d", &type_flag);
    
    if (type_flag == 0) {
        int int_value;
        scanf("%d", &int_value);
        
        // TODO: Call print_value with a pointer to int_value and the type flag
        
    } else if (type_flag == 1) {
        float float_value;
        scanf("%f", &float_value);
        
        // TODO: Call print_value with a pointer to float_value and the type flag
        
    }
    
    return 0;
}