拥有一个在编译时接受定义行格式的结构的库。不同的用户有不同的结构变体。
一个例子：
typedef struct { INT16U a, INT32S b } logrow_t;
在运行时，我想遍历这个结构并打印每个元素及其类型。
想到的第一个解决方案是建表：
typedef struct { char var_name[16], char var_type[16], int bytelen } logrow_desc_t;
logrow_desc_t descriptions[] = { { "a", "INT16U", 2 }, { "b", "INT32S", 4 } };
是否有更好的解决方案允许库的任何用户指定不同的行结构？有没有办法利用预处理器/编译器在编译时构建描述表？
【问题讨论】：
标签： c gcc struct c-preprocessor
有没有办法利用预处理器/编译器来构建 编译时的描述表？
如果您和您的用户不介意在宏中指定行格式（比如LOGROW）而不是直接在logrow_t 的定义中指定，则可以使用此answer from luser droog to "Foreach macro on macros arguments" 中描述的方法的变体用过。
我把通用的、可重用的部分放在文件ppnarg.h：
/*
* The PP_NARG macro evaluates to the number of arguments that have been
* passed to it.
* Laurent Deniau, "__VA_NARG__," 17 January 2006, <comp.std.c> (29 November 2007).
*/
#define PP_NARG(...) PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N( \
_1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
_11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
_21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
_31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
_41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
_51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
_61,_62,_63,N,...) N
#define PP_RSEQ_N() \
63,62,61,60, \
59,58,57,56,55,54,53,52,51,50, \
49,48,47,46,45,44,43,42,41,40, \
39,38,37,36,35,34,33,32,31,30, \
29,28,27,26,25,24,23,22,21,20, \
19,18,17,16,15,14,13,12,11,10, \
9,8,7,6,5,4,3,2,1,0
/* need extra level to force extra eval */
#define Paste(a,b) a ## b
#define XPASTE(a,b) Paste(a,b)
/* APPLYXn variadic X-Macro by M Joshua Ryan */
/* Free for all uses. Don't be a jerk. */
/* I got bored after typing 15 of these. */
/* You could keep going upto 64 (PPNARG's limit). */
#define APPLYX1(a) X(a)
#define APPLYX2(a,b) X(a) X(b)
#define APPLYX3(a,b,c) X(a) X(b) X(c)
#define APPLYX4(a,b,c,d) X(a) X(b) X(c) X(d)
#define APPLYX5(a,b,c,d,e) X(a) X(b) X(c) X(d) X(e)
#define APPLYX6(a,b,c,d,e,f) X(a) X(b) X(c) X(d) X(e) X(f)
#define APPLYX7(a,b,c,d,e,f,g) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g)
#define APPLYX8(a,b,c,d,e,f,g,h) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h)
#define APPLYX9(a,b,c,d,e,f,g,h,i) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i)
#define APPLYX10(a,b,c,d,e,f,g,h,i,j) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j)
#define APPLYX11(a,b,c,d,e,f,g,h,i,j,k) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k)
#define APPLYX12(a,b,c,d,e,f,g,h,i,j,k,l) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l)
#define APPLYX13(a,b,c,d,e,f,g,h,i,j,k,l,m) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m)
#define APPLYX14(a,b,c,d,e,f,g,h,i,j,k,l,m,n) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m) X(n)
#define APPLYX15(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o) \
X(a) X(b) X(c) X(d) X(e) X(f) X(g) X(h) X(i) X(j) X(k) X(l) X(m) X(n) X(o)
#define APPLYX_(M, ...) M(__VA_ARGS__)
#define APPLYXn(...) APPLYX_(XPASTE(APPLYX, PP_NARG(__VA_ARGS__)), __VA_ARGS__)
//#define ZIP2(a,b) /* to be defined by application */
#define ZIP4(a,b,c,d) ZIP2(a,b) ZIP2(c,d)
#define ZIP6(a,b,c,d,e,f) ZIP4(a,b,c,d) ZIP2(e,f)
#define ZIP8(a,b,c,d,e,f,g,h) ZIP6(a,b,c,d,e,f) ZIP2(g,h)
#define ZIP10(a,b,c,d,e,f,g,h,i,j) ZIP8(a,b,c,d,e,f,g,h) ZIP2(i,j)
#define ZIP12(a,b,c,d,e,f,g,h,i,j,k,l) ZIP10(a,b,c,d,e,f,g,h,i,j) ZIP2(k,l)
// define more of those ZIPxx() if more arguments are needed
#define ZIP(...) APPLYX_(XPASTE(ZIP, PP_NARG(__VA_ARGS__)), __VA_ARGS__)
在您的应用程序中，可以这样使用：
// only this line is to be defined by the users:
#define LOGROW INT16U, a, INT32S, b
// the following defines `logrow_t` and `descriptions` according to the above:
#include <stddef.h>
#include "ppnarg.h"
#define ZIP2(a, b) a b;
typedef struct { ZIP(LOGROW) } logrow_t;
#undef ZIP2
typedef struct { char *name, *type; size_t size; } logrow_desc_t;
#define ZIP2(a, b) { #b, #a, sizeof (a) },
logrow_desc_t descriptions[] = { ZIP(LOGROW) };
#undef ZIP2
【讨论】：
似乎可以使用宏。比如下面的代码：
#include <stdio.h>
#include <stddef.h>
#include <stdint.h>
// first we need a foreach macro for each 2 arguments apply a function
#define FOR2EACH_2(f, _1,_2) f(_1, _2)
#define FOR2EACH_4(f, _1,_2, ...) f(_1, _2) FOR2EACH_2(f, __VA_ARGS__)
#define FOR2EACH_6(f, _1,_2, ...) f(_1, _2) FOR2EACH_4(f, __VA_ARGS__)
#define FOR2EACH_N(_8,_7,_6,_5,_4,_3,_2,_1,N,...) FOR2EACH_##N
#define FOR2EACH(f, ...) FOR2EACH_N(__VA_ARGS__,8,7,6,5,4,3,2,1)(f, __VA_ARGS__)
// structure for holding description
struct struct_desc_s {
const char *name;
const char *type;
size_t size;
};
// then a callback for struct fields
#define STRUCT_WITH_DESC_FIELD(type, name) type name;
// a callback for description values
#define STRUCT_WITH_DESC_DESC(type, name) { #name, #type, sizeof(type) },
// and finally our definition
#define STRUCT_WITH_DESC(NAME, ...) \
struct NAME { \
FOR2EACH(STRUCT_WITH_DESC_FIELD, __VA_ARGS__) \
}; \
const struct struct_desc_s NAME##_desc[] = { \
FOR2EACH(STRUCT_WITH_DESC_DESC, __VA_ARGS__) \
};
STRUCT_WITH_DESC(logrow,
uint16_t, a,
uint32_t, b
)
int main() {
for (size_t i = 0; i < sizeof(logrow_desc)/sizeof(logrow_desc[0]); ++i) {
printf("%s %s %zu\n", logrow_desc[i].name, logrow_desc[i].type, logrow_desc[i].size);
}
}
输出：
a uint16_t 2
b uint32_t 4
【讨论】：
考虑 XMacro。
#define LOWGROW_XMACRO \
X(a, INT16U) \
X(b, INT32S)
现在您可以将结构定义为：
typedef struct {
#define X(NAME,TYPE) TYPE NAME;
LOWGROW_XMACRO
#undef X
} lowgrow_t;
或创建描述：
logrow_desc_t descriptions[] = {
#define X(NAME,TYPE) { #NAME, #TYPE, sizeof(TYPE) },
LOWGROW_XMACRO
#undef X
};
可能的用途是无限的。
可以将上述宏放到一个专用的头文件中（即“lowgrow.hpp”） 并在定义 XMacro 后包含它：
#define LOWGROW_XMACRO \
...
#include "lowgrow.hpp"
或者，LOWGROW_XMACRO 可以将另一个宏作为参数。这个高阶宏用来制作更多功能的 API。
// lowgrow.hpp
#define LOWGROW_MAKE_STRUCT_MEMBER(NAME, TYPE) TYPE NAME;
#define LOWGROW_MAKE_DESCRIPTION(NAME, TYPE) { #NAME, #TYPE, sizeof (TYPE) },
#define LOWGROW_DO_STUFF(X) \
typedef struct { \
X(LOWGROW_MAKE_STRUCT_MEMBER) \
} lowgrow_t; \
\
logrow_desc_t descriptions[] = { \
X(LOWGROW_MAKE_DESCRIPTION) \
};
现在宏可以被实例化：
#include "lowgrow.hpp"
#define LOWGROW_XMACRO(X) \
X(a, INT16U) \
X(b, INT32S)
LOWGROW_DO_STUFF(LOWGROW_XMACRO)
扩展为（为清楚起见添加了一些换行符之后）：
typedef struct { INT16U a; INT32S b; } lowgrow_t;
logrow_desc_t descriptions[] = {
{ "a", "INT16U", sizeof (INT16U) },
{ "b", "INT32S", sizeof (INT32S) },
};
【讨论】：