Re: [Boost-users] [Preprocessor] Is it possible to expand recursively?

Subject: Re: [Boost-users] [Preprocessor] Is it possible to expand recursively?
From: TONGARI (tongari95_at_[hidden])
Date: 2011-07-19 10:51:59

2011/7/19 Edward Diener <eldiener_at_[hidden]>

> On 7/18/2011 4:04 AM, TONGARI wrote:
>
>> 2011/7/18 Edward Diener <eldiener_at_[hidden]
>> <mailto:eldiener_at_tropicsoft.**com <eldiener_at_[hidden]>>>
>>
>>
>>    On 7/17/2011 1:29 PM, Edward Diener wrote:
>>
>>        On 7/17/2011 9:21 AM, TONGARI wrote:
>>
>>            Hello,
>>
>>            I'd like to have a macro like:
>>
>>            BUILD_HIERARCHY
>>            (a,
>>            (b,)
>>            (c,
>>            (d,)
>>            (e,)
>>            )
>>            )
>>
>>            which should expand to:
>>
>>            struct a {};
>>            struct b: a {};
>>            struct c: a {};
>>            struct d: c {};
>>            struct e: c {};
>>
>>            Is this possible at all?
>>            I have tried but with no luck, if this is impossible, please
>>            tell me so.
>>
>>
>>        I think you want above:
>>
>>        (a,
>>        (b)
>>        (c,
>>        (d)
>>        (e)
>>        )
>>        )
>>
>>        This is a tuple with a possible second element as a seq. Each
>>        element of
>>        your seq is a tuple with the same rule as the top tuple.
>>
>>        Following internal logic, perhaps with nested BOOST_PP_WHILEs, you
>>        should be able to cycle through each tuple and nested tuple
>>        forming the
>>        relationships you have created for your structs above. I can not
>>        guarantee this will be doable but at least you should have some
>>        idea of
>>        what to do from the description of what you have.
>>
>>
>>    I have to correct the above. The syntax for what I described must be:
>>
>>    (a,
>>       ((b))
>>       ((c,
>>          (d)
>>          (e))
>>       )
>>
>>    )
>>
>>
>> I think I need the comma since I don't assume variadic macro.
>>
>
> I do not understand what you mean by that, and what some extra comma has to
> do with variadic macros
>

Without variadic macro we can just leave the 2nd arg empty (e.g. MACRO(x, )
where MACRO requires 2 args) as no derived classes.

> I still do not have the above right. It should be:
>
>
> (a,
>   ((b))
>   ((c,
>       ((d))
>       ((e))
>    )
>   )
> )
>

IIUC, your surrounding ()s here are automatically packed by what I had:

#define PACK_2A( a1, a2 ) ((a1, a2)) PACK_2B
#define PACK_2B( a1, a2 ) ((a1, a2)) PACK_2A
#define PACK_2A_END
#define PACK_2B_END

and used like: BOOST_PP_CAT(PACK_2A children, _END)

BTW, if variadic macro available, they'd be

#define PACK_A( VARG... ) (( VARG )) PACK_B
#define PACK_B( VARG... ) (( VARG )) PACK_A
#define PACK_A_END
#define PACK_B_END

[...]

Just in case you do not know, you can use the Wave trace facility to see how
> your macros above are expanding when you invoke BUILD_HIERARCHY with your
> two parameters.

I used to use BOOST_PP_STRINGIZE to show me the result.
It turns out that BOOST_PP_SEQ_FOR_EACH cannot be used recursively.

...and I have to say I must give up on this temporarily, if I have time to
back to this again, I'll see if Wave can help me figure out what happened.

Thanks for your patience.