# and ## Operators in C ?

In C, the "#" and "##" are preprocessor operators that are used to manipulate macro parameters during compilation. The # operator converts a macro parameter to a string literal, while the ## operator concatenates two tokens into a single token.

The macro parameters are the parameters of the macro definition, which are declared using the #define directive.

The "#" operator is known as the Stringizing operator, whereas the "##" operator is known as the Token Pasting operator.

Syntax

/* Stringizing operator */
#define MACRO_NAME(param) #param

/* Token pasting operator */
#define MACRO_NAME(param1, param2) param1##param2

The Stringizing Operator ('#')

The "#" operator is a preprocessor Stringizing operator in C that converts a macro argument into a string literal. It can only be used within macro definitions −

Example

The following example demonstrates how the Stringize (#) operator converts macro parameters into string literals −

#include <stdio.h>

#define STRINGIFY(x) #x 
#define PRINT_VAR(var) printf(#var " = %d
", var)

int main() {
    int number = 42;
    
    /* Convert macro argument to string */
    printf("String: %s
", STRINGIFY(Hello World));
    
    /* Print variable name and value */
    PRINT_VAR(number);
    
    return 0;
}

String: Hello World
number = 42

The Token Pasting ('##') Operator

The "##" operator is a preprocessor Token Pasting operator that concatenates two macro tokens into a single token. This is useful for creating identifiers dynamically −

Example

The following example shows how the Token pasting (##) operator concatenates tokens −

#include <stdio.h>

#define CONCAT(x, y) x##y
#define DECLARE_VAR(type, name, num) type name##num

int main() {
    /* Concatenate numbers */
    int result = CONCAT(12, 34);
    printf("Concatenated number: %d
", result);
    
    /* Create variable names dynamically */
    DECLARE_VAR(int, var, 1) = 100;
    DECLARE_VAR(int, var, 2) = 200;
    
    printf("var1 = %d
", var1);
    printf("var2 = %d
", var2);
    
    return 0;
}

Concatenated number: 1234
var1 = 100
var2 = 200

Key Points

• The # operator works only with macro parameters, not with regular variables.
• The ## operator creates valid C identifiers by concatenating tokens.
• Both operators are processed during preprocessing, before actual compilation begins.
• These operators cannot be used outside of macro definitions.

Conclusion

The preprocessor operators # (stringizing) and ## (token pasting) are powerful tools for metaprogramming in C. The # operator converts macro arguments to strings, while ## concatenates tokens to create new identifiers dynamically.