これがその日の宿題です。これはマクロトリックに基づいており、今日は特に__VA_NARG__
ローラン・デニオーによって発明されたことを学びました。とにかく、次のサンプル コードは、わかりやすくするために最大 8 つのフィールドで動作します。さらに必要な場合は、複製してコードを拡張してください (これは、プリプロセッサがファイルを 1 回しか読み取らないため、再帰機能を備えていないためです)。
#include <stdio.h>
#include <stddef.h>
struct a
{
int a;
int b;
int c;
};
struct b
{
int a;
int b;
int c;
int d;
};
#define STRINGIZE(arg) STRINGIZE1(arg)
#define STRINGIZE1(arg) STRINGIZE2(arg)
#define STRINGIZE2(arg) #arg
#define CONCATENATE(arg1, arg2) CONCATENATE1(arg1, arg2)
#define CONCATENATE1(arg1, arg2) CONCATENATE2(arg1, arg2)
#define CONCATENATE2(arg1, arg2) arg1##arg2
/* PRN_STRUCT_OFFSETS will print offset of each of the fields
within structure passed as the first argument.
*/
#define PRN_STRUCT_OFFSETS_1(structure, field, ...) printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));
#define PRN_STRUCT_OFFSETS_2(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_1(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_3(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_2(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_4(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_3(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_5(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_4(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_6(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_5(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_7(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_6(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_8(structure, field, ...)\
printf(STRINGIZE(structure)":"STRINGIZE(field)"-%d\n", offsetof(structure, field));\
PRN_STRUCT_OFFSETS_7(structure, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_NARG(...) PRN_STRUCT_OFFSETS_NARG_(__VA_ARGS__, PRN_STRUCT_OFFSETS_RSEQ_N())
#define PRN_STRUCT_OFFSETS_NARG_(...) PRN_STRUCT_OFFSETS_ARG_N(__VA_ARGS__)
#define PRN_STRUCT_OFFSETS_ARG_N(_1, _2, _3, _4, _5, _6, _7, _8, N, ...) N
#define PRN_STRUCT_OFFSETS_RSEQ_N() 8, 7, 6, 5, 4, 3, 2, 1, 0
#define PRN_STRUCT_OFFSETS_(N, structure, field, ...) CONCATENATE(PRN_STRUCT_OFFSETS_, N)(structure, field, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS(structure, field, ...) PRN_STRUCT_OFFSETS_(PRN_STRUCT_OFFSETS_NARG(field, __VA_ARGS__), structure, field, __VA_ARGS__)
int main(int argc, char *argv[])
{
PRN_STRUCT_OFFSETS(struct a, a, b, c);
printf("\n");
PRN_STRUCT_OFFSETS(struct b, a, b, c, d);
return 0;
}
これは出力します:
struct a:a-0
struct a:b-4
struct a:c-8
struct b:a-0
struct b:b-4
struct b:c-8
struct b:d-12
編集:これは、より一般的になろうとするわずかに異なるバージョンです。FOR_EACH(what, ...)
マクロはwhat
、可変引数リスト内の他のすべての引数に適用されます。
したがって、次のように引数を 1 つ取るマクロを定義するだけです。
#define DO_STUFF(x) foo(x)
リスト内のすべての引数に適用されます。したがって、典型的な例では、少しハックする必要がありますが、それでも簡潔です。
#define PRN_STRUCT_OFFSETS_(structure, field) printf(STRINGIZE(structure)":"STRINGIZE(field)" - offset = %d\n", offsetof(structure, field));
#define PRN_STRUCT_OFFSETS(field) PRN_STRUCT_OFFSETS_(struct a, field)
そして、次のように適用します。
FOR_EACH(PRN_STRUCT_OFFSETS, a, b, c);
最後に、完全なサンプル プログラム:
#include <stdio.h>
#include <stddef.h>
struct a
{
int a;
int b;
int c;
};
#define STRINGIZE(arg) STRINGIZE1(arg)
#define STRINGIZE1(arg) STRINGIZE2(arg)
#define STRINGIZE2(arg) #arg
#define CONCATENATE(arg1, arg2) CONCATENATE1(arg1, arg2)
#define CONCATENATE1(arg1, arg2) CONCATENATE2(arg1, arg2)
#define CONCATENATE2(arg1, arg2) arg1##arg2
#define FOR_EACH_1(what, x, ...) what(x)
#define FOR_EACH_2(what, x, ...)\
what(x);\
FOR_EACH_1(what, __VA_ARGS__);
#define FOR_EACH_3(what, x, ...)\
what(x);\
FOR_EACH_2(what, __VA_ARGS__);
#define FOR_EACH_4(what, x, ...)\
what(x);\
FOR_EACH_3(what, __VA_ARGS__);
#define FOR_EACH_5(what, x, ...)\
what(x);\
FOR_EACH_4(what, __VA_ARGS__);
#define FOR_EACH_6(what, x, ...)\
what(x);\
FOR_EACH_5(what, __VA_ARGS__);
#define FOR_EACH_7(what, x, ...)\
what(x);\
FOR_EACH_6(what, __VA_ARGS__);
#define FOR_EACH_8(what, x, ...)\
what(x);\
FOR_EACH_7(what, __VA_ARGS__);
#define FOR_EACH_NARG(...) FOR_EACH_NARG_(__VA_ARGS__, FOR_EACH_RSEQ_N())
#define FOR_EACH_NARG_(...) FOR_EACH_ARG_N(__VA_ARGS__)
#define FOR_EACH_ARG_N(_1, _2, _3, _4, _5, _6, _7, _8, N, ...) N
#define FOR_EACH_RSEQ_N() 8, 7, 6, 5, 4, 3, 2, 1, 0
#define FOR_EACH_(N, what, x, ...) CONCATENATE(FOR_EACH_, N)(what, x, __VA_ARGS__)
#define FOR_EACH(what, x, ...) FOR_EACH_(FOR_EACH_NARG(x, __VA_ARGS__), what, x, __VA_ARGS__)
#define PRN_STRUCT_OFFSETS_(structure, field) printf(STRINGIZE(structure)":"STRINGIZE(field)" - offset = %d\n", offsetof(structure, field));
#define PRN_STRUCT_OFFSETS(field) PRN_STRUCT_OFFSETS_(struct a, field)
int main(int argc, char *argv[])
{
FOR_EACH(PRN_STRUCT_OFFSETS, a, b, c);
printf("\n");
return 0;
}