BOOST_PP_SEQ_ENUM空序列

Question

How could I write a macro that is like BOOST_PP_SEQ_ENUM , but handles the empty sequence, ie

#define SEQ (a)(b)
MY_BOOST_PP_SEQ_ENUM(SEQ) // expands to a,b
#define SEQ
MY_BOOST_PP_SEQ_ENUM(SEQ) // expands to nothing

Answer 1

There was a solution to this problem proposed here , named add #define BOOST_PP_SEQ_ENUM_0 . It was rejected with the comment "no such thing as an empty sequence", but that seems to solve my problem.

Answer 2

You just need to define SEQ as an empty sequence:

#define SEQ ()

and you can use BOOST_PP_SEQ_ENUM directly.

Works here with Boost 1.47 using G++ 4.7.1 and VC 2010.

Answer 3

You can use this:

#define PP_SEQ_ENUM(x) BOOST_PP_CAT(PP_SEQ_ENUM_0 x, _END)
#define PP_SEQ_ENUM_0(x) x PP_SEQ_ENUM_1
#define PP_SEQ_ENUM_1(x) , x PP_SEQ_ENUM_2
#define PP_SEQ_ENUM_2(x) , x PP_SEQ_ENUM_1
#define PP_SEQ_ENUM_0_END
#define PP_SEQ_ENUM_1_END
#define PP_SEQ_ENUM_2_END 

This should work for sequences of any size(even ones greater than 256)