Overload C/C++ preprocessor macro on structure of its argument

Question

I would like to write a preprocessor macro that does one thing if it's argument is a parenthesized tuple of tokens, like this:

MY_MACRO((x, y))

and something else if it's just a single token, like this:

MY_MACRO(x)

Is that possible?

How about distinguishing between the number of space-separated tokens, i.e. between MY_MACRO(x) and MY_MACRO(x y)?

Note that I am not trying to overload based on the number of arguments - it's a unary macro in all cases.

EDIT: I am willing to use variadic macros if they help

This is impossible in C/C++. You should define two different macros. Write more detailed what do you need (code example). — artyom.stv, Mar 02 '11 at 19:37
For the first question: with or without variadic macro support? For the second question: no. — James McNellis, Mar 02 '11 at 19:38
Maybe http://stackoverflow.com/questions/2632300/looping-through-macro-varargs-values has some ideas you can use — pmg, Mar 02 '11 at 19:47

Ise Wisteria · Accepted Answer · 2011-03-02T20:29:20.663

7

As for your first question, the following macros might meet your purpose:

#define CONCAT_( x, y ) x ## y
#define CONCAT( x, y ) CONCAT_( x, y )
#define IS_SINGLE_1(...) 0
#define IGNORE(...)
#define IS_SINGLE_2_0           0 IGNORE(
#define IS_SINGLE_2_IS_SINGLE_1 1 IGNORE(
#define IS_SINGLE( x ) CONCAT( IS_SINGLE_2_, IS_SINGLE_1 x ) )
IS_SINGLE((x, y)) // 0
IS_SINGLE(x)      // 1

Macro IS_SINGLE is expanded to 1 if the argument is single token, otherwise, 0.

Hope this helps

edited Mar 02 '11 at 20:29

answered Mar 02 '11 at 20:20

Ise Wisteria

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Awesome, thanks! I assume the other one (distinguishing between number of space-separated tokens) is impossible as James said... – HighCommander4 Mar 02 '11 at 20:36
2

For space-separated it's impossible. For comma-separated, there is a solution for any bounded limit on the number of arguments. – R.. GitHub STOP HELPING ICE Mar 02 '11 at 20:41
1

@HighCommander4: Thanks! As you mentioned, as for space separated arguments, unless arguments are strictly restricted(for example only `x`, `y` and `z` are allowed), I think it is generally very difficult(impossible?)... If `m4` is allowed, probably it is the way to go :-) – Ise Wisteria Mar 02 '11 at 22:20
This is the first time I hear of `m4`... it looks interesting. Thanks for the tip :) – HighCommander4 Mar 03 '11 at 00:26

score 2 · Answer 2 · answered Mar 02 '11 at 19:43

Using boost.preprocessor

#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/seq/for_each.hpp>

#define SEQ (w)(x)(y)(z)

#define MACRO(r, data, elem) BOOST_PP_CAT(elem, data)

BOOST_PP_SEQ_FOR_EACH(MACRO, _, SEQ) // expands to w_ x_ y_ z_

It's not exactly the same as even a single argument case requires parenthesis. But It does allow a variable number of parenthesized arguments.

Also a possibility: Use the BOOST_PP_IF, BOOST_PP_EQUAL, and BOOST_PP_TUPLE_ELEM to do something like:

MACRO(1, a)
MACRO(2, (a,b) )
MACRO(3, (a,b,c) )

or so.

Overload C/C++ preprocessor macro on structure of its argument

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