cpp.info: Concatenation

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Concatenation

   It is often useful to merge two tokens into one while expanding
macros.  This is called "token pasting" or "token concatenation".  The
`##' preprocessing operator performs token pasting.  When a macro is
expanded, the two tokens on either side of each `##' operator are
combined into a single token, which then replaces the `##' and the two
original tokens in the macro expansion.  Usually both will be
identifiers, or one will be an identifier and the other a preprocessing
number.  When pasted, they make a longer identifier.  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 token pasting.
   However, two tokens that don't together form a valid token cannot be
pasted together.  For example, you cannot concatenate `x' with `+' in
either order.  If you try, the preprocessor issues a warning and emits
the two tokens.  Whether it puts white space between the tokens is
undefined.  It is common to find unnecessary uses of `##' in complex
macros.  If you get this warning, it is likely that you can simply
remove the `##'.
   Both the tokens combined by `##' could come from the macro body, but
you could just as well write them as one token in the first place.
Token pasting is most useful when one or both of the tokens comes from a
macro argument.  If either of the tokens next to an `##' is a parameter
name, it is replaced by its actual argument before `##' executes.  As
with stringification, the actual argument is not macro-expanded first.
If the argument is empty, that `##' has no effect.
   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 `*'.  You can put as much whitespace
between `##' and its operands as you like, including comments, and you
can put comments in arguments that will be concatenated.  However, it
is an error if `##' appears at either end of a macro body.
   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) (void);
     };
     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),
       ...
     };