问题
I would like to do the equivalent of the following:
#define print_max(TYPE) \
# ifdef TYPE##_MAX \
printf("%lld\n", TYPE##_MAX); \
# endif
print_max(INT);
Now the #ifdef or any nested preprocessor directive is
not allowed as far as I can see in a function macro.
Any ideas?
Update: So it seems like this is not possible. Even a hack to check at runtime seems unachievable. So I think I'll go with something like:
#ifndef BLAH_MAX
# define BLAH_MAX 0
#endif
# etc... for each type I'm interested in
#define print_max(TYPE) \
if (TYPE##_MAX) \
printf("%lld\n", TYPE##_MAX);
print_max(INT);
print_max(BLAH);
回答1:
The Boost Preprocessor (which works for C as well as C++, even though Boost as a whole is a C++ library) library can help with this kind of task. Instead of using an #ifdef within a macro (which isn't permitted), it helps you include a file multiple times, with different macros defined each time, so that the file can use #ifdef.
The following code, if saved to max.c, should do what you want for each of the words listed in the MAXES #define at the top of the file. However, it won't work if any of the _MAX values are floating point, since the preprocessor can't handle floating point.
(Boost Processor is a handy tool, but it's not exactly straightforward; you can decide whether or not this approach is an improvement over copy-and-paste.)
#define MAXES (SHRT)(INT)(LONG)(PATH)(DOESNT_EXIST)
#if !BOOST_PP_IS_ITERATING
/* This portion of the file (from here to #else) is the "main" file */
#include <values.h>
#include <stdio.h>
#include <boost/preprocessor.hpp>
/* Define a function print_maxes that iterates over the bottom portion of this
* file for each word in MAXES */
#define BOOST_PP_FILENAME_1 "max.c"
#define BOOST_PP_ITERATION_LIMITS (0,BOOST_PP_DEC(BOOST_PP_SEQ_SIZE(MAXES)))
void print_maxes(void) {
#include BOOST_PP_ITERATE()
}
int main(int argc, char *argv[])
{
print_maxes();
}
#else
/* This portion of the file is evaluated multiple times, with
* BOOST_PP_ITERATION() resolving to a different number every time */
/* Use BOOST_PP_ITERATION() to look up the current word in MAXES */
#define CURRENT BOOST_PP_SEQ_ELEM(BOOST_PP_ITERATION(), MAXES)
#define CURRENT_MAX BOOST_PP_CAT(CURRENT, _MAX)
#if CURRENT_MAX
printf("The max of " BOOST_PP_STRINGIZE(CURRENT) " is %lld\n", (long long) CURRENT_MAX);
#else
printf("The max of " BOOST_PP_STRINGIZE(CURRENT) " is undefined\n");
#endif
#undef CURRENT
#undef CURRENT_MAX
#endif
回答2:
I've tried that before. The problem is that # is already reserved to stringize a macro parameter. It isn't parsed as a preprocessor token like the one in #define.
回答3:
The only solution I have is cheating - produce a list of types that have an _XXX_MAX as a set of defines, and then use that. I don't know how to produce the list in automated fashion in preprocessor, so I don't try. The assumption is that the list is not too long and will not be maintained too intensively.
#define PRINT_MAX(type) printf("%lld\n", _TYPE##_MAX);
#define HAVE_MAX(type) _TYPE##_MAX // not sure if this works
/* a repetitious block of code that I cannot factor out - this is the cheat */
#ifdef HAVE_MAX(INT)
#define PRINT_INT_MAX PRINT_MAX(INT)
#endif
#ifdef HAVE_MAX(LONG)
#define PRINT_LONG_MAX PRINT_MAX(LONG)
#endif
/* end of cheat */
#define print_max(type) PRINT_##TYPE##_MAX
回答4:
I don't think it's a case of the ## operator not being allowed in an #ifdef. I tried this:
#define _print_max(TYPE) \
#ifdef TYPE \
printf("%lld\n", _TYPE); \
#endif
#define print_max(TYPE) _print_max(MAX##_TYPE)
void main()
{
print_max(INT)
}
and it still didn't work (it didn't like #ifdef TYPE). The problem is that #ifdef will only accept #defined symbols, not #define arguments. Those are two different things.
回答5:
Unlike templates, the preprocessor is not turing-complete. An #ifdef inside a macro is not possible. Your only solution is to make sure you only call print_max on types which has a matching _MAX defined, e.g. INT_MAX. The compiler will surely tell you when they aren't.
回答6:
There's no easy way to do this. The closest you can come is to #define a large number of IFDEF macros such as:
#undef IFDEF_INT_MAX
#ifdef INT_MAX
#define IFDEF_INT_MAX(X) X
#else
#define IFDEF_INT_MAX(X)
#endif
#undef IFDEF_BLAH_MAX
#ifdef BLAH_MAX
#define IFDEF_BLAH_MAX(X) X
#else
#define IFDEF_BLAH_MAX(X)
#endif
:
since you need a lot of them (and they might be useful multiple places), it makes a lot of sense to stick all these in their own header file 'ifdefs.h' which you can include whenever you need them. You can even write a script that regenerates ifdef.h from a list of 'macros of interest'
Then, your code becomes
#include "ifdefs.h"
#define print_max(TYPE) \
IFDEF_##TYPE##_MAX( printf("%lld\n", TYPE##_MAX); )
print_max(INT);
print_max(BLAH);
回答7:
As long as you're only interested in integral values, and assuming hardware using 2's compliment and 8-bit bytes:
// Of course all this MAX/MIN stuff assumes 2's compilment, with 8-bit bytes...
#define LARGEST_INTEGRAL_TYPE long long
/* This will evaluate to TRUE for an unsigned type, and FALSE for a signed
* type. We use 'signed char' since it should be the smallest signed type
* (which will sign-extend up to <type>'s size) vs. possibly overflowing if
* going in the other direction (from a larger type to a smaller one).
*/
#define ISUNSIGNED(type) (((type) ((signed char) -1)) > (type) 0)
/* We must test for the "signed-ness" of <type> to determine how to calculate
* the minimum/maximum value.
*
* e.g., If a typedef'ed type name is passed in that is actually an unsigned
* type:
*
* typedef unsigned int Oid;
* MAXIMUM_(Oid);
*/
#define MINIMUM_(type) ((type) (ISUNSIGNED(type) ? MINIMUM_UNSIGNED_(type) \
: MINIMUM_SIGNED_( type)))
#define MAXIMUM_(type) ((type) (ISUNSIGNED(type) ? MAXIMUM_UNSIGNED_(type) \
: MAXIMUM_SIGNED_( type)))
/* Minumum unsigned value; zero, by definition -- we really only have this
* macro for symmetry.
*/
#define MINIMUM_UNSIGNED_(type) ((type) 0)
// Maximum unsigned value; all 1's.
#define MAXIMUM_UNSIGNED_(type) \
((~((unsigned LARGEST_INTEGRAL_TYPE) 0)) \
>> ((sizeof(LARGEST_INTEGRAL_TYPE) - sizeof(type)) * 8))
/* Minimum signed value; a 1 in the most-significant bit.
*
* We use LARGEST_INTEGRAL_TYPE as our base type for the initial bit-shift
* because we should never overflow (i.e., <type> should always be the same
* size or smaller than LARGEST_INTEGRAL_TYPE).
*/
#define MINIMUM_SIGNED_(type) \
((type) \
((signed LARGEST_INTEGRAL_TYPE) \
(~((unsigned LARGEST_INTEGRAL_TYPE) 0x0) << ((sizeof(type) * 8) - 1))))
// Maximum signed value; 0 in most-significant bit; remaining bits all 1's.
#define MAXIMUM_SIGNED_(type) (~MINIMUM_SIGNED_(type))
来源：https://stackoverflow.com/questions/301711/is-it-possible-for-c-preprocessor-macros-to-contain-preprocessor-directives