The # operator

The # (single number sign) operator converts a parameter of a function-like macro into a character string literal. For example, if macro ABC is defined using the following directive:

   #define ABC(x)   #x

all subsequent invocations of the macro ABC would be expanded into a character string literal containing the argument passed to ABC. For example:

Invocation	Result of macro expansion
`ABC(1)`	"1"
`ABC(Hello there)`	"Hello there"

The # operator should not be confused with the null directive.

Use the # operator in a function-like macro definition according to the following rules:

A parameter following # operator in a function- like macro is converted into a character string literal containing the argument passed to the macro.
White-space characters that appear before or after the argument passed to the macro are deleted.
Multiple white-space characters imbedded within the argument passed to the macro are replaced by a single space character.
If the argument passed to the macro contains a string literal and if a \ (backslash) character appears within the literal, a second \ character is inserted before the original \ when the macro is expanded.
If the argument passed to the macro contains a " (double quotation mark) character, a \ character is inserted before the " when the macro is expanded.
The conversion of an argument into a string literal occurs before macro expansion on that argument.
If more than one ## operator or # operator appears in the replacement list of a macro definition, the order of evaluation of the operators is not defined.
If the result of the macro expansion is not a valid character string literal, the behavior is undefined.

The following examples demonstrate the use of the # operator:

   #define STR(x)        #x
   #define XSTR(x)       STR(x)
   #define ONE           1

Invocation	Result of macro expansion
`STR(\n "\n" '\n')`	`"\n \"\\n\" '\\n'"`
`STR(ONE)`	"ONE"
`XSTR(ONE)`	"1"
`XSTR("hello")`	"\"hello\""

Related information

The null directive (#)