我想知道是否可以迭代传递给 C99 中的可变参数宏或使用任何 GCC 扩展的参数？
例如，是否可以编写一个通用宏，将结构及其字段作为参数传递并打印结构内每个字段的偏移量？
像这样的东西：
结构一个{
诠释一个;
诠释 b;
诠释 c;
};
/* PRN_STRUCT_OFFSETS 将打印每个字段的偏移量
作为第一个参数传递的结构内。
*/
int main(int argc, char *argv[])
{
PRN_STRUCT_OFFSETS(结构 a, a, b, c);
返回0；
}
这是我今天的作业，它基于宏观技巧，今天我特别了解__VA_NARG__了由 Laurent Deniau 发明的。无论如何，为了清楚起见，以下示例代码最多可以使用 8 个字段。如果您需要更多，只需通过复制来扩展代码（这是因为预处理器不具有递归功能，因为它只读取文件一次）。
#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;
}
打印出来：
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
编辑：这是一个稍微不同的版本，它试图更通用。该FOR_EACH(what, ...)宏适用what于变量参数列表中的所有其他参数。
因此，您只需要定义一个带有单个参数的宏，如下所示：
#define DO_STUFF(x) foo(x)
这将应用于列表中的每个参数。因此，对于您的典型示例，您需要稍微修改一下，但它仍然保持简洁：
#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)
你像这样应用它：
FOR_EACH(PRN_STRUCT_OFFSETS, a, b, c);
最后，一个完整的示例程序：
#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;
}
冒着获得考古学家徽章的风险，我认为使用Overloading Macro on Number of Arguments中的技术对上面 Gregory 的回答有一个小的改进
使用 foo.h：
// Make a FOREACH macro
#define FE_0(WHAT)
#define FE_1(WHAT, X) WHAT(X)
#define FE_2(WHAT, X, ...) WHAT(X)FE_1(WHAT, __VA_ARGS__)
#define FE_3(WHAT, X, ...) WHAT(X)FE_2(WHAT, __VA_ARGS__)
#define FE_4(WHAT, X, ...) WHAT(X)FE_3(WHAT, __VA_ARGS__)
#define FE_5(WHAT, X, ...) WHAT(X)FE_4(WHAT, __VA_ARGS__)
//... repeat as needed
#define GET_MACRO(_0,_1,_2,_3,_4,_5,NAME,...) NAME
#define FOR_EACH(action,...) \
GET_MACRO(_0,__VA_ARGS__,FE_5,FE_4,FE_3,FE_2,FE_1,FE_0)(action,__VA_ARGS__)
// Example
// Some actions
#define QUALIFIER(X) X::
#define OPEN_NS(X) namespace X {
#define CLOSE_NS(X) }
// Helper function
#define QUALIFIED(NAME,...) FOR_EACH(QUALIFIER,__VA_ARGS__)NAME
// Emit some code
QUALIFIED(MyFoo,Outer,Next,Inner) foo();
FOR_EACH(OPEN_NS,Outer,Next,Inner)
class Foo;
FOR_EACH(CLOSE_NS,Outer,Next,Inner)
cpp foo.h 生成：
Outer::Next::Inner::MyFoo foo();
namespace Outer {namespace Next {namespace Inner {
class Foo;
}}}
如果您的结构是用X-Macros描述的，那么可以编写一个函数或一个宏来迭代结构的所有字段并打印它们的偏移量。
#include <stddef.h> // offsetof macro
//--- first describe the structure, the fields, their types
#define X_FIELDS \
X(int, field1) \
X(int, field2) \
X(char, field3) \
X(char *, field4)
//--- define the structure, the X macro will be expanded once per field
typedef struct {
#define X(type, name) type name;
X_FIELDS
#undef X
} mystruct;
//--- "iterate" over all fields of the structure and print out their offset
void print_offset(mystruct *aStruct)
{
#define X(type, name) printf("offset of %s is %d\n", #name, offsetof(mystruct, name));
X_FIELDS
#undef X
}
//--- demonstrate
int main(int ac, char**av)
{
mystruct a = { 0, 1, 'a', "hello"};
print_offset(&a);
return 0;
}
Gregory Pakosz 的解决方案效果很好。但我有两个小问题：
使用 pedantic 选项编译时，我收到警告：“ISO99 需要使用其余参数”。这是由第一个 FOR_EACH_1 宏中的可变参数引起的。删除这些并在 FOR_EACH_2 中更改对 FOR_EACH_1 的调用删除了此警告。
#define FOR_EACH_1(what, x)
#define FOR_EACH_2(what, x, ...)\
what(x); \
FOR_EACH_1(what);
由于我以非常通用的方式使用它，我有时不得不只用 1 个参数调用重复宏。（我知道重复一个项目 1 次是没有意义的；））。幸运的是，这个问题的解决方案非常简单。只需从 FOR_EACH 宏中删除 x 参数。
#define FOR_EACH(what, ...) FOR_EACH_(FOR_EACH_NARG(__VA_ARGS__), what, __VA_ARGS__)
这是包含两个更改的完整列表：
#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, ...) CONCATENATE(FOR_EACH_, N)(what, __VA_ARGS__)
#define FOR_EACH(what, ...) FOR_EACH_(FOR_EACH_NARG(__VA_ARGS__), what, __VA_ARGS__)
也许使用可变参数作为数组初始值设定项，并迭代 countof(array)？即 sizeof(array)/sizeof(array[0])。该数组可能是 C99 匿名数组。
我想不出另一种方法来迭代宏的 var-args，因为我不知道如何对每个 var-arg 元素的文本做任何事情。var-arg 部分也可能是其中包含逗号的单个参数，因为您可以使用 CPP、AFAIK 对其执行所有操作。
但这是我对 var-args 进行迭代的想法：
#define countof(a) ( sizeof(a)/sizeof((a)[0]) )
#define MACRO(fd, format, ...) do { int ar_[] = { __VA_ARGS__ }; \
for(int i=0; i<countof(ar_) ; ++i){ \
fprintf(fd, format, ar_[i]); \
} } while(0)
这是我能想到的最好的，使用标准 C：
#include <stddef.h>
#include <stdio.h>
// prints a single offset
#define PRN_STRUCT_OFFSET(x, a) printf("&" #x "." #a " = %d\n", offsetof(x, a));
// prints a struct with one member
#define PRN_STRUCT_OFFSETS_1(x, a) PRN_STRUCT_OFFSET(x, a)
// prints a struct with two members
#define PRN_STRUCT_OFFSETS_2(x, a, b) \
PRN_STRUCT_OFFSET(x, a) \
PRN_STRUCT_OFFSET(x, b)
// and so on until some N.
// Boost.Preprocessor might help here, I'm not sure
struct some_struct
{
int a;
void* c;
};
int main(void)
{
PRN_STRUCT_OFFSETS_2(struct some_struct, a, c);
return 0;
}
我将此添加为另一个答案。这是使用 G++ 4.5.0 编译的 C++0x 的尝试
#include <iostream>
using namespace std;
template<typename L>
inline void for_each(L l)
{
}
template<typename L, typename P, typename... Q>
inline void for_each(L l, P arg, Q... args)
{
l(arg);
for_each(l, args...);
}
int main()
{
for_each([] (int x) { cout << x; }, 1, 2, 3);
return 0;
}
程序打印
123
但是，使用这种方法，您传递给 lambda 表达式的所有参数都需要具有相同的类型，int如上例所示。但是，lambdas 允许您捕获变量，例如：
int main()
{
int offset = 10;
for_each([offset] (int x) { cout << offset + x << endl; }, 1, 2, 3);
return 0;
}
打印出来：
11
12
13
要启用一个空的__VA_ARGS__，可以使用 GNU 扩展##_VA_ARGS__
https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
如果您的目标是Objective-C……查看Github 上的 AWESOME KSVarArgs
KSVarArgs 是一组宏，旨在使在 Objective-C 中处理变量参数更容易。所有宏都假定可变参数列表仅包含objective-c 对象或类似对象的结构（可分配给类型id）。基本宏 ksva_iterate_list() 迭代变量参数，为每个参数调用一个块，直到遇到终止的 nil。其他宏是为了方便转换为常用集合。
/*! @param firstNote NSString that is the only known arg
*/
- (void) observeWithBlocks:(NSString*)firstNote,...{
/*! ksva_list_to_nsarray puts varargs into
new array, `namesAndBlocks`
*/
ksva_list_to_nsarray(firstNote, namesAndBlocks);
/// Split the array into Names and Blocks
NSArray *names = [namesAndBlocks subArrayWithMembersOfKind:NSString.class],
*justBlocks = [namesAndBlocks arrayByRemovingObjectsFromArray:names];
[names eachWithIndex:^(id obj, NSInteger idx) {
[self observeName:obj usingBlock:^(NSNotification *n) {
((void(^)())justBlocks[idx])(n);
}];
}];
}
示例用法：
[NSNotificationCenter.defaultCenter observeWithBlocks:
NSViewFrameDidChangeNotification, /// first, named arg
^(NSNotification *m){ [self respondToFrameChange]; }, // vararg
NSTextViewDidChangeSelectionNotification, // vararg
^(NSNotification *z){ [z.infoDict[@"textView"] save]; }, // vararg
nil // must nil-terminate
];