Standard alternative to GCC's ##__VA_ARGS__ tr

问题:

There is a well-known problem with empty args for variadic macros in C99.

example:

#define FOO(...)       printf(__VA_ARGS__)
#define BAR(fmt, ...)  printf(fmt, __VA_ARGS__)

FOO("this works fine");
BAR("this breaks!");

The use of BAR() above is indeed incorrect according to the C99 standard, since it will expand to:

printf("this breaks!",);

Note the trailing comma - not workable.

Some compilers (eg: Visual Studio 2010) will quietly get rid of that trailing comma for you. Other compilers (eg: GCC) support putting ## in front of __VA_ARGS__, like so:

#define BAR(fmt, ...)  printf(fmt, ##__VA_ARGS__)

But is there a standards-compliant way to get this behavior? Perhaps using multiple macros?

Right now, the ## version seems fairly well-supported (at least on my platforms), but I'd really rather use a standards-compliant solution.

Pre-emptive: I know I could just write a small function. I'm trying to do this using macros.

Edit: Here is an example (though simple) of why I would want to use BAR():

#define BAR(fmt, ...)  printf(fmt "\n", ##__VA_ARGS__)

BAR("here is a log message");
BAR("here is a log message with a param: %d", 42);

This automatically adds a newline to my BAR() logging statements, assuming fmt is always a double-quoted C-string. It does NOT print the newline as a separate printf(), which is advantageous if the logging is line-buffered and coming from multiple sources asynchronously.

回答1:

It is possible to avoid the use of GCC's ,##__VA_ARGS__ extension if you are willing to accept some hardcoded upper limit on the number of arguments you can pass to your variadic macro, as described in Richard Hansen's answer to this question. If you do not want to have any such limit, however, to the best of my knowledge it is not possible using only C99-specified preprocessor features; you must use some extension to the language. clang and icc have adopted this GCC extension, but MSVC has not.

Back in 2001 I wrote up the GCC extension for standardization (and the related extension that lets you use a name other than __VA_ARGS__ for the rest-parameter) in document N976, but that received no response whatsoever from the committee; I don't even know if anyone read it. In 2016 it was proposed again in N2023, and I encourage anyone who knows how that proposal is going to let us know in the comments.

回答2:

There is an argument counting trick that you can use.

Here is one standard-compliant way to implement the second BAR() example in jwd's question:

#include <stdio.h>

#define BAR(...) printf(FIRST(__VA_ARGS__) "\n" REST(__VA_ARGS__))

/* expands to the first argument */
#define FIRST(...) FIRST_HELPER(__VA_ARGS__, throwaway)
#define FIRST_HELPER(first, ...) first

/*
 * if there's only one argument, expands to nothing.  if there is more
 * than one argument, expands to a comma followed by everything but
 * the first argument.  only supports up to 9 arguments but can be
 * trivially expanded.
 */
#define REST(...) REST_HELPER(NUM(__VA_ARGS__), __VA_ARGS__)
#define REST_HELPER(qty, ...) REST_HELPER2(qty, __VA_ARGS__)
#define REST_HELPER2(qty, ...) REST_HELPER_##qty(__VA_ARGS__)
#define REST_HELPER_ONE(first)
#define REST_HELPER_TWOORMORE(first, ...) , __VA_ARGS__
#define NUM(...) \
    SELECT_10TH(__VA_ARGS__, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE,\
                TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, ONE, throwaway)
#define SELECT_10TH(a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, ...) a10

int
main(int argc, char *argv[])
{
    BAR("first test");
    BAR("second test: %s", "a string");
    return 0;
}

This same trick is used to:

• count the number of arguments
• expand differently depending on the number of arguments
• append to __VA_ARGS__

Explanation

The strategy is to separate __VA_ARGS__ into the first argument and the rest (if any). This makes it possible to insert stuff after the first argument but before the second (if present).

FIRST()

This macro simply expands to the first argument, discarding the rest.

The implementation is straightforward. The throwaway argument ensures that FIRST_HELPER() gets two arguments, which is required because the ... needs at least one. With one argument, it expands as follows:

1. FIRST(firstarg)
2. FIRST_HELPER(firstarg, throwaway)
3. firstarg

With two or more, it expands as follows:

1. FIRST(firstarg, secondarg, thirdarg)
2. FIRST_HELPER(firstarg, secondarg, thirdarg, throwaway)
3. firstarg

REST()

This macro expands to everything but the first argument (including the comma after the first argument, if there is more than one argument).

The implementation of this macro is far more complicated. The general strategy is to count the number of arguments (one or more than one) and then expand to either REST_HELPER_ONE() (if only one argument given) or REST_HELPER_TWOORMORE() (if two or more arguments given). REST_HELPER_ONE() simply expands to nothing -- there are no arguments after the first, so the remaining arguments is the empty set. REST_HELPER_TWOORMORE() is also straightforward -- it expands to a comma followed by everything except the first argument.

The arguments are counted using the NUM() macro. This macro expands to ONE if only one argument is given, TWOORMORE if between two and nine arguments are given, and breaks if 10 or more arguments are given (because it expands to the 10th argument).

The NUM() macro uses the SELECT_10TH() macro to determine the number of arguments. As its name implies, SELECT_10TH() simply expands to its 10th argument. Because of the ellipsis, SELECT_10TH() needs to be passed at least 11 arguments (the standard says that there must be at least one argument for the ellipsis). This is why NUM() passes throwaway as the last argument (without it, passing one argument to NUM() would result in only 10 arguments being passed to SELECT_10TH(), which would violate the standard).

Selection of either REST_HELPER_ONE() or REST_HELPER_TWOORMORE() is done by concatenating REST_HELPER_ with the expansion of NUM(__VA_ARGS__) in REST_HELPER2(). Note that the purpose of REST_HELPER() is to ensure that NUM(__VA_ARGS__) is fully expanded before being concatenated with REST_HELPER_.

Expansion with one argument goes as follows:

1. REST(firstarg)
2. REST_HELPER(NUM(firstarg), firstarg)
3. REST_HELPER2(SELECT_10TH(firstarg, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, ONE, throwaway), firstarg)
4. REST_HELPER2(ONE, firstarg)
5. REST_HELPER_ONE(firstarg)
6. (empty)

Expansion with two or more arguments goes as follows:

1. REST(firstarg, secondarg, thirdarg)
2. REST_HELPER(NUM(firstarg, secondarg, thirdarg), firstarg, secondarg, thirdarg)
3. REST_HELPER2(SELECT_10TH(firstarg, secondarg, thirdarg, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, TWOORMORE, ONE, throwaway), firstarg, secondarg, thirdarg)
4. REST_HELPER2(TWOORMORE, firstarg, secondarg, thirdarg)
5. REST_HELPER_TWOORMORE(firstarg, secondarg, thirdarg)
6. , secondarg, thirdarg

回答3:

Not a general solution, but in the case of printf you could append a newline like:

#define BAR_HELPER(fmt, ...) printf(fmt "\n%s", __VA_ARGS__)
#define BAR(...) BAR_HELPER(__VA_ARGS__, "")

I believe it ignores any extra args that aren't referenced in the format string. So you could probably even get away with:

#define BAR_HELPER(fmt, ...) printf(fmt "\n", __VA_ARGS__)
#define BAR(...) BAR_HELPER(__VA_ARGS__, 0)

I can't believe C99 was approved without a standard way to do this. AFAICT the problem exists in C++11 too.

回答4:

There is a way to handle this specific case using something like Boost.Preprocessor. You can use BOOST_PP_VARIADIC_SIZE to check the size of the argument list, and then conditionaly expand to another macro. The one shortcoming of this is that it can't distinguish between 0 and 1 argument, and the reason for this becomes clear once you consider the following:

BOOST_PP_VARIADIC_SIZE()      // expands to 1
BOOST_PP_VARIADIC_SIZE(,)     // expands to 2
BOOST_PP_VARIADIC_SIZE(,,)    // expands to 3
BOOST_PP_VARIADIC_SIZE(a)     // expands to 1
BOOST_PP_VARIADIC_SIZE(a,)    // expands to 2
BOOST_PP_VARIADIC_SIZE(,b)    // expands to 2
BOOST_PP_VARIADIC_SIZE(a,b)   // expands to 2
BOOST_PP_VARIADIC_SIZE(a, ,c) // expands to 3

The empty macro argument list actually consists of one argument that happens to be empty.

In this case, we are lucky since your desired macro always has at least 1 argument, we can implement it as two "overload" macros:

#define BAR_0(fmt) printf(fmt "\n")
#define BAR_1(fmt, ...) printf(fmt "\n", __VA_ARGS__)

And then another macro to switch between them, such as:

#define BAR(...) \
    BOOST_PP_CAT(BAR_, BOOST_PP_GREATER(
        BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), 1))(__VA_ARGS__) \
    /**/

or

#define BAR(...) BOOST_PP_IIF( \
    BOOST_PP_GREATER(BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), 1), \
        BAR_1, BAR_0)(__VA_ARGS__) \
    /**/

Whichever you find more readable (I prefer the first as it gives you a general form for overloading macros on the number of arguments).

It is also possible to do this with a single macro by accessing and mutating the variable arguments list, but it is way less readable, and is very specific to this problem:

#define BAR(...) printf( \
    BOOST_PP_VARIADIC_ELEM(0, __VA_ARGS__) "\n" \
    BOOST_PP_COMMA_IF( \
        BOOST_PP_GREATER(BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), 1)) \
    BOOST_PP_ARRAY_ENUM(BOOST_PP_ARRAY_POP_FRONT( \
        BOOST_PP_VARIADIC_TO_ARRAY(__VA_ARGS__)))) \
    /**/

Also, why is there no BOOST_PP_ARRAY_ENUM_TRAILING? It would make this solution much less horrible.

Edit: Alright, here is a BOOST_PP_ARRAY_ENUM_TRAILING, and a version that uses it (this is now my favourite solution):

#define BOOST_PP_ARRAY_ENUM_TRAILING(array) \
    BOOST_PP_COMMA_IF(BOOST_PP_ARRAY_SIZE(array)) BOOST_PP_ARRAY_ENUM(array) \
    /**/

#define BAR(...) printf( \
    BOOST_PP_VARIADIC_ELEM(0, __VA_ARGS__) "\n" \
    BOOST_PP_ARRAY_ENUM_TRAILING(BOOST_PP_ARRAY_POP_FRONT( \
        BOOST_PP_VARIADIC_TO_ARRAY(__VA_ARGS__)))) \
    /**/

回答5:

I ran into a similar problem recently, and I do believe there's a solution.

The key idea is that there's a way to write a macro NUM_ARGS to count the number of arguments which a variadic macro is given. You can use a variation of NUM_ARGS to build NUM_ARGS_CEILING2, which can tell you whether a variadic macro is given 1 argument or 2-or-more arguments. Then you can write your Bar macro so that it uses NUM_ARGS_CEILING2 and CONCAT to send its arguments to one of two helper macros: one which expects exactly 1 argument, and another which expects a variable number of arguments greater than 1.

Here's an example where I use this trick to write the macro UNIMPLEMENTED, which is very similar to BAR:

STEP 1:

/** 
 * A variadic macro which counts the number of arguments which it is
 * passed. Or, more precisely, it counts the number of commas which it is
 * passed, plus one.
 *
 * Danger: It can't count higher than 20. If it's given 0 arguments, then it
 * will evaluate to 1, rather than to 0.
 */

#define NUM_ARGS(...)                                                   \
    NUM_ARGS_COUNTER(__VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13,       \
                     12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)    

#define NUM_ARGS_COUNTER(a1, a2, a3, a4, a5, a6, a7,        \
                         a8, a9, a10, a11, a12, a13,        \
                         a14, a15, a16, a17, a18, a19, a20, \
                         N, ...)                            \
    N

STEP 1.5:

/*
 * A variant of NUM_ARGS that evaluates to 1 if given 1 or 0 args, or
 * evaluates to 2 if given more than 1 arg. Behavior is nasty and undefined if
 * it's given more than 20 args.
 */

#define NUM_ARGS_CEIL2(...)                                           \
    NUM_ARGS_COUNTER(__VA_ARGS__, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, \
                     2, 2, 2, 2, 2, 2, 2, 1)

Step 2:

#define _UNIMPLEMENTED1(msg)                                        \
    log("My creator has forsaken me. %s:%s:%d." msg, __FILE__,      \
        __func__, __LINE__)

#define _UNIMPLEMENTED2(msg, ...)                                   \
    log("My creator has forsaken me. %s:%s:%d." msg, __FILE__,      \
        __func__, __LINE__, __VA_ARGS__)

STEP 3:

#define UNIMPLEMENTED(...)                                              \
    CONCAT(_UNIMPLEMENTED, NUM_ARGS_CEIL2(__VA_ARGS__))(__VA_ARGS__)

Where CONCAT is implemented in the usual way. As a quick hint, if the above seems confusing: the goal of CONCAT there is to expand to another macro "call".

Note that NUM_ARGS itself isn't used. I just included it to illustrate the basic trick here. See Jens Gustedt's P99 blog for a nice treatment of it.

Two notes:

• NUM_ARGS is limited in the number of arguments that it handles. Mine can only handle up to 20, although the number is totally arbitrary.

• NUM_ARGS, as shown, has a pitfall in that it returns 1 when given 0 arguments. The gist of it is that NUM_ARGS is technically counting [commas + 1], and not args. In this particular case, it actually works to our advantage. _UNIMPLEMENTED1 will handle an empty token just fine and it saves us from having to write _UNIMPLEMENTED0. Gustedt has a workaround for that as well, although I haven't used it and I'm not sure if it would work for what we're doing here.

回答6:

A very simple macro I'm using for debug printing:

#define __DBG_INT(fmt, ...) printf(fmt "%s", __VA_ARGS__);
#define DBG(...) __DBG_INT(__VA_ARGS__, "\n")

int main() {
        DBG("No warning here");
        DBG("and we can add as many arguments as needed. %s", "nice!");
        return 0;
}

No matter how many arguments are passed to DBG there are no c99 warning.

The trick is __DBG_INT adding a dummy param so ... will always have at least one argument and c99 is satisfied.

回答7:

This is the simplified version that I use. It is based upon the great techniques of the other answers here, so many props to them:

#define _SELECT(PREFIX,_5,_4,_3,_2,_1,SUFFIX,...) PREFIX ## _ ## SUFFIX

#define _BAR_1(fmt)      printf(fmt "\n")
#define _BAR_N(fmt, ...) printf(fmt "\n", __VA_ARGS__);
#define BAR(...) _SELECT(_BAR,__VA_ARGS__,N,N,N,N,1)(__VA_ARGS__)

int main(int argc, char *argv[]) {
    BAR("here is a log message");
    BAR("here is a log message with a param: %d", 42);
    return 0;
}

That's it.

As with other solutions this is limited to the number of arguments the macro. To support more, add more parameters to _SELECT, and more N arguments. The argument names count down (instead of up) to serve as a reminder that the count-based SUFFIX argument is provided in reverse order.

This solution treats 0 arguments as though it is 1 argument. So BAR() nominally "works", because it expands to _SELECT(_BAR,,N,N,N,N,1)(), which expands to _BAR_1()(), which expands to printf("\n").

If you want, you can get creative with the use of _SELECT and provide different macros for different number of arguments. For example, here we have a LOG macro that takes a 'level' argument before the format. If format is missing, it logs "(no message)", if there is just 1 argument, it will log it through "%s", otherwise it will treat the format argument as a printf format string for the remaining arguments.

#define _LOG_1(lvl)          printf("[%s] (no message)\n", #lvl)
#define _LOG_2(lvl,fmt)      printf("[%s] %s\n", #lvl, fmt)
#define _LOG_N(lvl,fmt, ...) printf("[%s] " fmt "\n", #lvl, __VA_ARGS__)
#define LOG(...) _SELECT(_LOG,__VA_ARGS__,N,N,N,2,1)(__VA_ARGS__)

int main(int argc, char *argv[]) {
    LOG(INFO);
    LOG(DEBUG, "here is a log message");
    LOG(WARN, "here is a log message with param: %d", 42);
    return 0;
}
/* outputs:
[INFO] (no message)
[DEBUG] here is a log message
[WARN] here is a log message with param: 42
*/

回答8:

In your situation (at least 1 argument present, never 0), you can define BAR as BAR(...), use Jens Gustedt's HAS_COMMA(...) to detect a comma and then dispatch to BAR0(Fmt) or BAR1(Fmt,...) accordingly.

This:

#define HAS_COMMA(...) HAS_COMMA_16__(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0)
#define HAS_COMMA_16__(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, ...) _15
#define CAT_(X,Y) X##Y
#define CAT(X,Y) CAT_(X,Y)
#define BAR(.../*All*/) CAT(BAR,HAS_COMMA(__VA_ARGS__))(__VA_ARGS__)
#define BAR0(X) printf(X "\n")
#define BAR1(X,...) printf(X "\n",__VA_ARGS__)


#include <stdio.h>
int main()
{
    BAR("here is a log message");
    BAR("here is a log message with a param: %d", 42);
}

compiles with -pedantic without a warning.

回答9:

C (gcc), 762 bytes

#define EMPTYFIRST(x,...) A x (B)
#define A(x) x()
#define B() ,

#define EMPTY(...) C(EMPTYFIRST(__VA_ARGS__) SINGLE(__VA_ARGS__))
#define C(...) D(__VA_ARGS__)
#define D(x,...) __VA_ARGS__

#define SINGLE(...) E(__VA_ARGS__, B)
#define E(x,y,...) C(y(),)

#define NONEMPTY(...) F(EMPTY(__VA_ARGS__) D, B)
#define F(...) G(__VA_ARGS__)
#define G(x,y,...) y()

#define STRINGIFY(...) STRINGIFY2(__VA_ARGS__)
#define STRINGIFY2(...) #__VA_ARGS__

#define BAR(fmt, ...) printf(fmt "\n" NONEMPTY(__VA_ARGS__) __VA_ARGS__)

int main() {
    puts(STRINGIFY(NONEMPTY()));
    puts(STRINGIFY(NONEMPTY(1)));
    puts(STRINGIFY(NONEMPTY(,2)));
    puts(STRINGIFY(NONEMPTY(1,2)));

    BAR("here is a log message");
    BAR("here is a log message with a param: %d", 42);
}

Try it online!

Assumes:

• No arg contain comma or bracket
• No arg contain A~G (can rename to hard_collide ones)

回答10:

The standard solution is to use FOO instead of BAR. There are a few weird cases of argument reordering it probably can't do for you (though I bet someone can come up with clever hacks to disassemble and reassemble __VA_ARGS__ conditionally based on the number of arguments in it!) but in general using FOO "usually" just works.