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 |