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

Subject: Re: [Boost-users] [preprocessor] Identity function for token concatenation
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-06-18 15:49:02

On Mon, Jun 18, 2012 at 12:34 PM, Jeremiah Willcock <jewillco_at_[hidden]>wrote:

> On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:
>
>  On Mon, Jun 18, 2012 at 12:25 PM, Jeremiah Willcock <jewillco_at_[hidden]>
>> wrote:
>>      On Mon, 18 Jun 2012, Jeffrey Lee Hellrung, Jr. wrote:
>>
>>            On Mon, Jun 18, 2012 at 10:35 AM, Jeremiah Willcock <
>> jewillco_at_[hidden]> 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<ZZZ, >(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