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\"" |
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