Stringize (#) and token pasting (##) operator in C language

C programming supports two special preprocessor directive for string operations. Stringize (#) and token pasting (##) are C preprocessor string manipulation operators.

In previous article we learned about basic and conditional preprocessor directives in C language. In this article we will move further and learn string manipulation preprocessor operators.

C programming supports two string manipulation operators.

• Stringize operator (#)
• Token pasting operator (##)

Stringize operator (#)

We use stringize operator in C to convert a token to string. It transforms the macro parameter to string. It causes the macro parameter to enclose in double quoted string.

Syntax:

#define MACRO_NAME(param) #param

Example:

#include <stdio.h>

// Macro definition with stringize operator
#define PRINT(msg) #msg

int main()
{
	printf(PRINT(C Programming in Codeforwin));
	
	return 0;
}

Output:

C Programming in Codeforwin

Note: In the above code printf() function accepts a pointer to constant character (string). However, we are passing a macro to the printf() function. The macro executes prior to the compilation of program. In the process of macro expansion during preprocessing, the C preprocessor expands PRINT(C Programming in Codeforwin) to "C Programming in Codeforwin". After which our printf() function is expanded to printf("C Programming in Codeforwin");.

Token pasting operator (##)

Token pasting operator (##) combines or concatenates tokens together. The ## operator concatenates two tokens. During macro expansion parameter next to ## is combined with the parameter before ## and returned as a single token.

Read more about What are tokens in programming?

Syntax:

#define MACRO_NAME(param1, param2) param1##param2

Where MACRO_NAME is an identifier i.e. name of the macro. It accepts two param param1 and param2 and returns concatenation of param1 and param2.

Note: It is not necessary to pass two params to macro for token pasting. You can pass any number of parameter.

Example:

#include <stdio.h>

// Macro definition with token pasting
#define CONCAT(a, b) a##b

int main()
{
	printf("CONCAT(10, 20) = %d\n", CONCAT(10, 20));
	
	return 0;
}

Output:

CONCAT(10, 20) = 1020

Feel free to drop your queries and suggestions.

Happy coding 😉