What does a double hash (##) in a macro mean?

Question

In the below code, what does the ## do?

 #define MAKE_TYPE(myname) \\
 typedef int myname ## Id; \\

Answer 1

The ## in a macro is concatenation. Here, MAKE_TYPE(test) will expand to : typedef int testId.

From 16.3.3 (The ## operator) :

For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token

Answer 2

icecrime is correct, but something important to point out in the definition is that the tokens need to be valid preprocessing tokens. Examples:

#define CONCAT(a,b) a ## b
CONCAT(ClassyClass, <int>); // bad, <int> is not a valid preprocessing token
CONCAT(Symbol, __LINE__); // valid as both are valid tokens