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\"" |