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#include <stdio.h>
/**
** @file
**
** This plays with gcc features to define a weird macro that emulates
** anonymous functions that can be used as closures. Don't use that
** seriously, this is only meant to prove that this is syntactically
** not a big deal, and that the ideas necessary for it can be found
** in modern compilers.
**
**/
/**
** @def ANONYMOUS(RET)
** @brief Use this to define an anonymous function with return type @a RET.
**
** The @c ... argument list is the parameter list of the
** function. This the expects the code of the function in <code>({
** })</code> parenthesis. A simple example would be
**
** @code
** // Function header
** ANONYMOUS(int) (double a, unsigned b)
** // Function body
** ({
** return printf("we found %g for %u, average %g\n", a, b, a/b);
** })
** // Function arguments
** (x, 23);
** @endcode
**
** Of course you would usually put such things inside a macro. See
** MAX() for a more complicated example.
**/
#ifdef __clang__
# define ANONYMOUS(RET) (^ RET ANON_PART2
# define ANON_PART2(...) (__VA_ARGS__) ANON_PART3
# define ANON_PART3(...) __VA_ARGS__)
#else
# define ANONYMOUS(RET) ({ RET __anon_func ANON_PART2
# define ANON_PART2(...) (__VA_ARGS__) ANON_PART3
# define ANON_PART3(...) __VA_ARGS__ __anon_func; })
#endif
#define MAXU(A, B) \
ANONYMOUS(uintmax_t) (uintmax_t a, uintmax_t b) \
({ \
if (a < b) return b; \
else return a; \
}) \
((A), (B))
#define MAX(A, B) \
/* function header */ \
/* return type */ \
ANONYMOUS(__typeof__(1 ? (A) : (B))) \
/*and then argument list */ \
(__typeof__(A) a, __typeof__(B) b) \
/* function body */ \
({ \
if (a < b) return b; \
else return a; \
}) \
/* function call */ \
((A), (B))
/**
** Test to see if that even works when such functions are nested.
**/
typedef void df(unsigned);
df* Df[5] = { };
void g(unsigned level, unsigned depth) {
Df[depth%5] =
ANONYMOUS(void) (register unsigned a)
({
printf("from level %u, depth %u found in position %u\n",
level, depth,
MAX(2*a, depth)
);
return;
});
if (level)
g(level-1, depth+1);
else
for (unsigned i = 0; i < 5; ++i)
if (Df[i]) Df[i](i);
}
int main(int argc, [[maybe_unused]] char* argv[]) {
g(argc, 0);
}
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