15 Preprocessing directives [cpp]

15.12 Pragma operator [cpp.pragma.op]

1
#
A unary operator expression of the form: 
_Pragma ( string-literal )
is processed as follows: The string-literal is destringized by deleting the L prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash.
The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive.
The original four preprocessing tokens in the unary operator expression are removed.
2
#
[ Example
: 
#pragma listing on "..\listing.dir"
can also be expressed as: 
_Pragma ( "listing on \"..\\listing.dir\"" )
The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in: 
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)

LISTING( ..\listing.dir )
— end example
 ]