Macro definitions and #,##

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
If the operator # is used before a parameter is used in the replacement sequence, that parameter is replaced by a string literal (as if it were enclosed between double quotes)

#define str(x) #x
cout << str(test);

This would be translated into:

cout << "test";

The operator ## concatenates two arguments leaving no blank spaces between them:

#define glue(a,b) a ## b
glue(c,out) << "test";

This would also be translated into:

cout << "test";

From http://www.cplusplus.com/doc/tutorial/preprocessor/