What does the # and ## means in C preprocessor macro of c++

c macros c-preprocessor stringification
user2131316 picture user2131316 · Sep 18, 2013 · Viewed 11.7k times · Source

I read the following code:

#define MACRO(abc, def) {#def ## #abc}

char result[10] = MARCO(abc, def);

I know the ## operator is used to merge the two string to one, but what about the # in front of def and abc?

Answer

Pierre Fourgeaud picture Pierre Fourgeaud · Sep 18, 2013

From the standard (emphasis mine):

16.3.2 The # operator [cpp.stringize]

2/ A character string literal is a string-literal with no prefix. If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument. [...]

It "stringifies" the token following the #.

Example:

#define STRINGIFY(x) #x

STRINGIFY(foo)  // will be replaced by "foo"

Related questions

C Macros to create strings

Alternative Titles (to aid search) Convert a preprocessor token to a string How to make a char string from a C macro's value? Original Question I would like to use C #define to build literal strings at compile time. The …

Read More
Error when defining a stringising macro with __VA_ARGS__

I have been trying to implement a function macro in C that prepends "DEBUG: ", to the argument, and passes its arguments to printf: #define DBG(format, ...) printf("DEBUG: " #format "\n", __VA_ARGS__) This gives me this error in gcc: src/…

Read More
Opposite of C preprocessor "stringification"

When using C preprocessor one can stringify macro argument like this: #define TO_STRING(x) "a string with " #x and so when used, the result is as follows: TO_STRING(test) will expand to: "a string with test" Is there …

Read More