升压预处理器不扩大

Question

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)()) 
) 

当我此进行预处理用锵3.2或g ++ 4.6.3，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,,)); 

（I加入行分解为清晰度）

问题是，为什么内部BOOST_PP_SEQ_FOR_EACH没有扩展？

再次传递此输出扩展了预期结果。

编辑：经过大量的搜索后，我读了一个宏不会展开，如果它被调用两次，我想这就是为什么。

编辑：我应该使用PP_SEQ_FOR_EACH_I，R不意味着被用作下标。

Answer 1

BOOST_PP_SEQ_FOR_EACH不是reentrant。在Boost.PP中只有几个可重入的宏（BOOST_PP_FOR,BOOST_PP_WHILE和BOOST_PP_REPEAT）。但是，您可以通过使用延迟表达式，这样的解决办法是：

#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)()) 
) 

Answer 2

#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)()) 
)