Difference between #define and const in C

In C, both #define and const define constant values, but these constants differ greatly in their behaviors and implementation. #define is a preprocessor directive used to define constants that are replaced by their value during preprocessing, before actual compilation begins. while const defines a typed constant with type safety.

The below table lists the primary differences between the constants defined using #define and using const keyword:

#define in C

#define in C is a preprocessor directive that replaces a name with a specific value before compilation. It works like a shortcut, where the preprocessor substitutes the defined name with the value, and it doesn't have a type like regular variables.

Example:

#include <stdio.h>

 // Define constant PI
#define PI 3.14 

int main() {
    printf("PI = %f\n", PI);
    return 0;
}

PI = 3.140000

Explanation: Here, #define is used to define a constant PI. The preprocessor replaces PI with 3.14 before compilation, and the program behaves as if the literal value 3.14 was written directly in place of PI.

When to Use #define?

The use of #define is prefferend in the following cases:

• Constants that do not require a specific type.
• Simple text replacement or macros.
• Avoiding additional memory allocation.

const in C

The const keyword is used to define constants with a specific type. Unlike #define, which is handled by the preprocessor, const is part of the C language and the compiler checks for type safety. A const variable behaves like any other variable, except its value cannot be modified after initialization.

#include <stdio.h>

// Define constant PI
const float PI = 3.14;  

int main() {
    printf("PI = %f\n", PI);
    return 0;
}

PI = 3.140000

Explanation: In this case, const is used to define a constant PI with type float. The compiler ensures type safety, and the value of PI cannot be changed after it is initialized.

When to Use const?

Use const for:

• Typed constants that benefit from type checking.
• Constants with a defined scope.
• Improved readability and debugging.