The # and ## operators are used with the #define macro. 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.
Example code:
For example, the command
#define to_string( s ) # s
will make the compiler turn this command
cout << to_string( Hello World! ) << endl;
into
cout << "Hello World!" << endl;
Here is an example of the ## command:
#define concatenate( x, y ) x ## y
...
int xy = 10;
...
This code will make the compiler turn
cout << concatenate( x, y ) << endl;
into
cout << xy << endl;
which will, of course, display '10' to standard
output.