Difference between typedef and #define in C

In C, both typedef and #define can be used to create aliases for existing types or values, but their behaviors and implementation differ significantly. typedef is a keyword used to create type aliases(nicknames) that are checked by the compiler, while #define is a preprocessor directive that performs simple textual substitution without any type checking.

The below table lists the primary differences between typedef and #define:

typedef in C

The typedef keyword is used to define a new type name for an existing type. It is used primarily to improve code readability by providing a more meaningful or simplified name for complex data types.

Example:

#include <stdio.h>


 // 'ulong' is now an alias for 'unsigned long'
typedef unsigned long ulong; 

int main() {
  
    // Using the new alias
    ulong num = 100;  
    printf("%lu\n", num);
    return 0;
}

100

Explanation: In this example, typedef creates an alias ulong for the existing type unsigned long. This makes the code easier to read, especially when dealing with long or complex type names.

When to Use typedef?

typedef should be used for:

• Creating aliases for complex types (e.g., function pointers or structures).
• Improving code readability and maintainability.
• Simplifying declarations involving pointers.

#define in C

The #define preprocessor directive is used to define constants or macros in C. It can be used to define values or even code that can be reused throughout the program. #define works by literally replacing the defined term with its value during the preprocessing step, before the compilation begins.

Example:

#include <stdio.h>

// Defining a constant
#define MAX_SIZE 100  

int main() {
  
    // Using the constant in the array size
    int arr[MAX_SIZE];  
    printf("%d", MAX_SIZE);
    return 0;
}

100

Explanation: In this example, #define creates a constant MAX_SIZE that is replaced with 100 during preprocessing. This helps make the code more flexible and easier to modify, as the size can be changed in one place.

When to Use #define?

#define should be used for:

• Defining constants or macros.
• Text replacement during preprocessing.
• Avoiding additional memory allocation.