Re: [Boost-users] [preprocessor] Identity function for token concatenation

On Mon, Jun 18, 2012 at 12:34 PM, Jeremiah Willcock wrote:

On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:

On Mon, Jun 18, 2012 at 12:25 PM, Jeremiah Willcock

...

wrote: On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:

On Mon, Jun 18, 2012 at 10:35 AM, Jeremiah Willcock < jewillco@osl.iu.edu> wrote: Boost.Preprocessor has a macro BOOST_PP_INTERCEPT to eat a numeric value that it is token-concatenated onto. Is there a similar macro that just returns the value? I.e., some BOOST_PP_EMPTY_FOR_CONCAT such that BOOST_PP_EMPTY_FOR_CONCAT ## 3 turns into 3? As with BOOST_PP_INTERCEPT, it only needs to work for small integer values. Is there some other technique I can use for this? I am using it to generate std::get<> invocations in BOOST_PP_ENUM_BINARY_PARAMS. Thank you for your help.

I don't know any such facility in Boost.PP, but it's pretty trivial to generate:

#define X0 0 #define X1 1 #define X2 2 // etc.

...which makes me think I might be overlooking something in Boost.PP :/

Somewhat related: what do your ENUM_BINARY_PARAMS invocations look like? I know I've gotten away with cat'ing template parameter delimiters ("<" and ">") against preprocessor integer tokens.

I'm not using this code anymore, but it was basically:

BOOST_PP_ENUM_BINARY_PARAMS(**nparams, std::get(tup) BOOST_PP_INTERCEPT)

where ZZZ is the identity-type macro discussed in the email. Even using something that I would expect to work like + as ZZZ breaks since apparently signs aren't allowed at the beginnings of pp-tokens. I'm using GCC 4.7, and it seems to be strict about concatenations.

So, just to confirm,

BOOST_PP_ENUM_BINARY_PARAMS( nparams, std::get<, >(tup) BOOST_PP_INTERCEPT )

(i.e., with ZZZ being an empty token) does not work with gcc 4.7?

Unless "empty token" means something other than putting nothing there, it expands to what I want but produces errors:

foo.cpp:3:1: error: pasting "<" and "0" does not give a valid preprocessing token foo.cpp:3:1: error: pasting "<" and "1" does not give a valid preprocessing token foo.cpp:3:1: error: pasting "<" and "2" does not give a valid preprocessing token foo.cpp:3:1: error: pasting "<" and "3" does not give a valid preprocessing token foo.cpp:3:1: error: pasting "<" and "4" does not give a valid preprocessing token std::get< 0 >(tup) , std::get< 1 >(tup) , std::get< 2 >(tup) , std::get< 3 >(tup) , std::get< 4 >(tup)

When trying to expand:

BOOST_PP_ENUM_BINARY_PARAMS(5, std::get<, >(tup) BOOST_PP_INTERCEPT)

Sounds like I might need to change my preprocessor programming habits :( How about using a leading 0? BOOST_PP_ENUM_BINARY_PARAMS( nparams, std::get< 0, >(tup) BOOST_PP_INTERCEPT ) - Jeff