Stringizing Operator (#)
The number-sign or “stringizing” operator (#) converts macro
parameters (after expansion) to string constants. It is used only with
macros that take arguments. If it precedes a formal parameter in the macro
definition, the actual argument passed by the macro invocation is enclosed
in quotation marks and treated as a string literal. The string literal
then replaces each occurrence of a combination of the stringizing operator
and formal parameter within the macro definition.
White space preceding the first token of the actual argument and following
the last token of the actual argument is ignored. Any white space between
the tokens in the actual argument is reduced to a single white space in
the resulting string literal. Thus, if a comment occurs between two tokens
in the actual argument, it is reduced to a single white space. The
resulting string literal is automatically concatenated with any adjacent
string literals from which it is separated only by white space.
Further, if a character contained in the argument usually requires an
escape sequence when used in a string literal (for example, the quotation
mark (") or backslash (\) character), the necessary escape
backslash is automatically inserted before the character. The following
example shows a macro definition that includes the stringizing operator
and a main function that invokes the macro:
#define stringer( x ) printf( #x "\n" )
void main()
{
stringer( In quotes in the printf function call\n );
stringer( "In quotes when printed to the screen"\n );
stringer( "This: \" prints an escaped double quote" );
}
Such invocations would be expanded during preprocessing, producing the
following code:
void main()
{
printf( "In quotes in the printf function call\n" "\n" );
printf( "\"In quotes when printed to the screen\"\n" "\n" );
printf( "\"This: \\\" prints an escaped double quote\"" "\n" );
}
When the program is run, screen output for each line is as follows:
In quotes in the printf function call
"In quotes when printed to the screen"
"This: \" prints an escaped double quotation mark"
Microsoft Specific
The Microsoft C (versions 6.0 and earlier) extension to the ANSI C
standard that previously expanded macro formal arguments appearing inside
string literals and character constants is no longer supported. Code that
relied on this extension should be rewritten using the stringizing (#)
operator. |
مدرسة
الكمبيوتر
