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(cpp-295.info)Concatenation

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Concatenation
-------------

   "Concatenation" means joining two strings into one.  In the context
of macro expansion, concatenation refers to joining two lexical units
into one longer one.  Specifically, an actual argument to the macro can
be concatenated with another actual argument or with fixed text to
produce a longer name.  The longer name might be the name of a function,
variable or type, or a C keyword; it might even be the name of another
macro, in which case it will be expanded.

   When you define a macro, you request concatenation with the special
operator `##' in the macro body.  When the macro is called, after
actual arguments are substituted, all `##' operators are deleted, and
so is any whitespace next to them (including whitespace that was part
of an actual argument).  The result is to concatenate the syntactic
tokens on either side of the `##'.

   Consider a C program that interprets named commands.  There probably
needs to be a table of commands, perhaps an array of structures
declared as follows:

     struct command
     {
       char *name;
       void (*function) ();
     };
     
     struct command commands[] =
     {
       { "quit", quit_command},
       { "help", help_command},
       ...
     };

   It would be cleaner not to have to give each command name twice,
once in the string constant and once in the function name.  A macro
which takes the name of a command as an argument can make this
unnecessary.  The string constant can be created with stringification,
and the function name by concatenating the argument with `_command'.
Here is how it is done:

     #define COMMAND(NAME)  { #NAME, NAME ## _command }
     
     struct command commands[] =
     {
       COMMAND (quit),
       COMMAND (help),
       ...
     };

   The usual case of concatenation is concatenating two names (or a
name and a number) into a longer name.  But this isn't the only valid
case.  It is also possible to concatenate two numbers (or a number and
a name, such as `1.5' and `e3') into a number.  Also, multi-character
operators such as `+=' can be formed by concatenation.  In some cases
it is even possible to piece together a string constant.  However, two
pieces of text that don't together form a valid lexical unit cannot be
concatenated.  For example, concatenation with `x' on one side and `+'
on the other is not meaningful because those two characters can't fit
together in any lexical unit of C.  The ANSI standard says that such
attempts at concatenation are undefined, but in the GNU C preprocessor
it is well defined: it puts the `x' and `+' side by side with no
particular special results.

   Keep in mind that the C preprocessor converts comments to whitespace
before macros are even considered.  Therefore, you cannot create a
comment by concatenating `/' and `*': the `/*' sequence that starts a
comment is not a lexical unit, but rather the beginning of a "long"
space character.  Also, you can freely use comments next to a `##' in a
macro definition, or in actual arguments that will be concatenated,
because the comments will be converted to spaces at first sight, and
concatenation will later discard the spaces.


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