3

我有以下代码：

#include <boost/preprocessor.hpp>

#define ARGS(r, data, elem) \
    BOOST_PP_COMMA_IF(BOOST_PP_SUB(r, 2)) \
    BOOST_PP_SEQ_ELEM(0, elem) BOOST_PP_SEQ_ELEM(1, elem)

#define DEF_FUN(name, args) void name(BOOST_PP_SEQ_FOR_EACH(ARGS,,args));

#define DEF_FUNCTIONS_ELEM(r, data, elem) DEF_FUN(BOOST_PP_SEQ_ELEM(0, elem), BOOST_PP_SEQ_ELEM(1, elem))

#define DEF_FUNCTIONS(funSeqs) \
    BOOST_PP_SEQ_FOR_EACH(DEF_FUNCTIONS_ELEM,, funSeqs)


DEF_FUNCTIONS_ELEM(2,, (fun0) (((int)(arg0))   ((char)(arg1))))

DEF_FUNCTIONS
(
    ((fun0) (((int)(arg0))   ((char)(arg1))))
    ((fun1) (((char)(arg0))  ((long)(arg1)) ((short)(arg2))))
    ((fun3) ())
)

当我使用 Clang 3.2 或 g++ 4.6.3 对其进行预处理时，我得到：

void fun0( int arg0 , char arg1 );

void fun0(BOOST_PP_SEQ_FOR_EACH(ARGS,,((int)(arg0)) ((char)(arg1)))); 
void fun1(BOOST_PP_SEQ_FOR_EACH(ARGS,,((char)(arg0)) ((long)(arg1)) ((short)(arg2)))); 
void fun3(BOOST_PP_SEQ_FOR_EACH(ARGS,,));

（为了清楚起见，我添加了换行符）

问题是，为什么内部的 BOOST_PP_SEQ_FOR_EACH 没有展开？

再次传递此输出会扩展预期结果。

编辑：经过大量搜索，我读到一个宏如果被调用两次就不会扩展，我认为这就是原因。

编辑：我应该使用 PP_SEQ_FOR_EACH_I，R 并不意味着用作下标。

4

2 回答 2

4

BOOST_PP_SEQ_FOR_EACH是不可重入的。Boost.PP 中只有少数宏是可重入的（BOOST_PP_FOR、、BOOST_PP_WHILE和BOOST_PP_REPEAT）。但是，您可以通过使用延迟表达式来解决它，如下所示：

#include <boost/preprocessor.hpp>

#define EXPAND(...) __VA_ARGS__
#define EMPTY()
#define DEFER(x) x EMPTY()
// An indirection macro to avoid direct recursion
#define BOOST_PP_SEQ_FOR_EACH_ID() BOOST_PP_SEQ_FOR_EACH

#define ARGS(r, data, elem) \
    BOOST_PP_COMMA_IF(BOOST_PP_SUB(r, 2)) \
    BOOST_PP_SEQ_ELEM(0, elem) BOOST_PP_SEQ_ELEM(1, elem)

// Defer BOOST_PP_SEQ_FOR_EACH_ID here
#define DEF_FUN(name, args) void name(DEFER(BOOST_PP_SEQ_FOR_EACH_ID)()(ARGS,,args));

#define DEF_FUNCTIONS_ELEM(r, data, elem) DEF_FUN(BOOST_PP_SEQ_ELEM(0, elem), BOOST_PP_SEQ_ELEM(1, elem))

// Add EXPAND here to apply another scan to expand the deferred expression
#define DEF_FUNCTIONS(funSeqs) \
    EXPAND(BOOST_PP_SEQ_FOR_EACH(DEF_FUNCTIONS_ELEM,, funSeqs))


DEF_FUNCTIONS
(
    ((fun0) (((int)(arg0))   ((char)(arg1))))
    ((fun1) (((char)(arg0))  ((long)(arg1)) ((short)(arg2))))
    ((fun3) ())
)
于 2013-03-28T14:01:10.700 回答
1 
#include <boost/preprocessor.hpp>

#define ARGS(r, data, index, elem) \
    BOOST_PP_SEQ_ELEM(0, elem) BOOST_PP_SEQ_ELEM(1, elem) BOOST_PP_COMMA_IF(BOOST_PP_NOT_EQUAL(index, BOOST_PP_DEC(data)))

#define DEF_FUN(name, args) void name(BOOST_PP_SEQ_FOR_EACH_I(ARGS,BOOST_PP_SEQ_SIZE(args),args));

#define DEF_FUNCTIONS_ELEM(r, data, elem) DEF_FUN(BOOST_PP_SEQ_ELEM(0, elem), BOOST_PP_SEQ_ELEM(1, elem))

#define DEF_FUNCTIONS(funSeqs) \
    BOOST_PP_SEQ_FOR_EACH(DEF_FUNCTIONS_ELEM,, funSeqs)


DEF_FUNCTIONS_ELEM(2,, (fun0) (((int)(arg0))   ((char)(arg1))))

DEF_FUNCTIONS
(
    ((fun0) (((int)(arg0))   ((char)(arg1))))
    ((fun1) (((char)(arg0))  ((long)(arg1)) ((short)(arg2))))
    ((fun3) ())
)
于 2013-03-27T23:41:18.043 回答