Subject: Re: [boost] [Preprocessor] Adding variadic macros support
From: Edward Diener (eldiener_at_[hidden])
Date: 2010-11-24 18:24:16
On 11/24/2010 5:45 PM, Jeffrey Lee Hellrung, Jr. wrote:
> On 11/24/2010 2:14 PM, Edward Diener wrote:
> [...]
>> I am still not sure what you mean by "macro composition" and "argument
>> binding" in a preprocessor library. Boost PP has much support for macro
>> composition within the context of what it offers in the form of
>> repeatable constructs, data types, etc. but I have a feeling you mean
>> something else. As far as "argument binding", in terms of preprocessor
>> macros all arguments are initially in the form of tokens, so I am not
>> sure what you mean by the phrase.
>
> Imagine Boost.MPL without placeholders.
>
> As a simple example, suppose you already have a macro handy:
>
> #define macro( n ) ...insert your favorite expression involving n...
>
> and want to use it in an invocation of BOOST_PP_ENUM. This requires
> defining a forwarding macro:
>
> #define macro2( z, n, data ) macro( n )
> BOOST_PP_ENUM( N, macro2, ~ )
>
> Argument binding refers to the ability to bind the arguments of
> function-style macros to tokens or placeholders for tokens to be
> substituted later. So one would be able to do something like (making up
> the syntax from what I can remember)
>
> BOOST_PP_ENUM( N, BIND( macro, 2 ), ~ )
>
> where BIND( macro, 2 )( z, n, data ) expands (intermediately) to macro(
> n ). The tokens 1, 2, 3, ... are placeholders to be substituted later,
> and to bind literal tokens to macro arguments, you wrap them in
> parentheses, so that BIND( macro, ( foo ) )( z, n, data ) expands to
> macro( foo ).
>
> Using the fictitious BIND macro, you can also compose macros inline,
> similar to how Boost.Bind works.
>
> This, at least, is what I remember from browsing through either the
> Avalanche code or Paul's Chaos/Order code, I don't remember which. But I
> think both basically provide this functionality.
>
> Hopefully that poor and hastily typed explanation helps,
I understand the explanation. It does not have anything to do with
variadic data as I see it so I do not see it being part of what I have
done. Perhaps someone else can come up with an implementation of your
bind idea for macros.