# and ##

The # and ## preprocessor operators are used with the #define preprocessor directive.

• Using # causes the first argument after the # to be returned as a string in quotes.
• Using ## concatenates what's before the ## with what's after it.

For example, the command

#define to_string( s ) # s

will make the compiler turn this command

<< to_string( Hello World! ) << endl;

into

<< "Hello World!" << endl;

Here is an example of the ## command:

#define concatenate( a, b ) a ## b ... int xy = 10; ...

This code will make the compiler turn

<< concatenate( x, y ) << endl;

into

<< xy << endl;

which will, of course, display '10' to standard output.