Generic keyword in C ?

The _Generic keyword in C (introduced in C11) enables type-generic programming by allowing different expressions to be selected based on the type of an argument. This provides a solution to the type-safety limitations of traditional macros.

Syntax

_Generic(expression, type1: expression1, type2: expression2, ..., default: default_expression)

Problem with Traditional Macros

Traditional C macros lack type checking and perform the same operation regardless of data type −

#include <stdio.h>
#define INCREMENT(X) ++X

int main() {
    int x = 5; 
    float y = 2.56; 
    char z = 'A';
    printf("Integer Increment: %d<br>", INCREMENT(x));
    printf("Float Increment: %f<br>", INCREMENT(y));
    printf("Character Increment: %c<br>", INCREMENT(z));
    return 0;
}

Integer Increment: 6
Float Increment: 3.560000
Character Increment: B

Solution Using _Generic Keyword

The _Generic keyword allows us to define type-specific behavior for different data types −

#include <stdio.h>
#define INCREMENT(X) _Generic((X), char: X+10, int: X+1, float: X+2.5, default: 0)

int main() {
    int x = 5; 
    float y = 2.56; 
    char z = 'A';
    printf("Integer Increment: %d<br>", INCREMENT(x));
    printf("Float Increment: %f<br>", INCREMENT(y));
    printf("Character Increment: %c<br>", INCREMENT(z));
    return 0;
}

Integer Increment: 6
Float Increment: 5.060000
Character Increment: K

How It Works

The _Generic expression evaluates the type of the first argument and selects the corresponding expression:

• For int type: adds 1
• For float type: adds 2.5
• For char type: adds 10 (ASCII value increment)
• For unknown types: returns default value 0

Key Points

• _Generic is available in C11 and later versions
• Type selection happens at compile time, not runtime
• The default case is optional but recommended
• Only one expression is evaluated based on the type match

Conclusion

The _Generic keyword provides type-safe macro programming in C, allowing different behaviors for different data types while maintaining the efficiency of macros. It bridges the gap between C's type system and generic programming.