The ## operator

The ## (double number sign) operator concatenates two tokens in a macro invocation (text and/or arguments) given in a macro definition.

If a macro XY was defined using the following directive: 
   #define XY(x,y)    x##y

the last token of the argument for x is concatenated with the first token of the argument for y.

Use the ## operator according to the following rules:
  • The ## operator cannot be the very first or very last item in the replacement list of a macro definition.
  • The last token of the item in front of the ## operator is concatenated with first token of the item following the ## operator.
  • Concatenation takes place before any macros in arguments are expanded.
  • If the result of a concatenation is a valid macro name, it is available for further replacement even if it appears in a context in which it would not normally be available.
  • If more than one ## operator and/or # operator appears in the replacement list of a macro definition, the order of evaluation of the operators is not defined.
The following examples demonstrate the use of the ## operator: 
#define ArgArg(x, y)          x##y
#define ArgText(x)            x##TEXT
#define TextArg(x)            TEXT##x
#define TextText              TEXT##text
#define Jitter                1
#define bug                   2
#define Jitterbug             3
Invocation Result of macro expansion
ArgArg(lady, bug) "ladybug"
ArgText(con) "conTEXT"
TextArg(book) "TEXTbook"
TextText "TEXTtext"
ArgArg(Jitter, bug) 3
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