Subject: Re: [boost] rationale for aux_/vector0.hpp
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-04-13 06:43:01
On 04/12/09 01:55, Aleksey Gurtovoy wrote:
> On Mon, 06 Apr 2009 08:11:06 -0500, Larry Evans
> <cppljevans_at_[hidden]> wrote:
>
[snip]
>> I'm wondering why the implementation here:
>>
>>
https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/vector/aux_/preprocessed/typeof_based/vector10.hpp#L43
>>
>> adds from the back instead of the front. IOW, instead of:
>>
>> 26 template<
>> 27 typename T0, typename T1
>> 28 >
>> 29 struct vector2
>> 30 : v_item<
>> 31 T1
>> 32 , vector1<T0>
>> 33 >
>> 34 {
>> 35 typedef vector2 type;
>> 36 };
>>
>> why not:
>>
>> 26 template<
>> 27 typename T0, typename T1
>> 28 >
>> 29 struct vector2
>> 30 : v_item<
>> 31 T0
>> 32 , vector1<T1>
>> 33 >
>> 34 {
>> 35 typedef vector2 type;
>> 36 };
>
> The former has potential for better compilation times due to
> memoization gains on reusing a likely existing vector(n-1)
> instantiation in vector(n), e.g.
>
> vector2<T0,T1> -> vector2<T0,T1>, vector1<T0>
> vector3<T0,T1,T2> -> vector3<T0,T1,T2>, vector2<T0,T1>*, vector1<T0>*
>
> vs.
>
> vector2<T0,T1> -> vector2<T0,T1>, vector1<T1>
> vector3<T0,T1,T2> -> vector3<T0,T1,T2>, vector2<T1,T2>, vector1<T2>
>
> (*) reused instantiation
Ah! I see. Hmm... what about:
vector2<T1,T2> -> vector2<T1,T2>, vector1<T2>
vector3<T0,T1,T2> -> vector3<T0,T1,T2>, vector2<T1,T2>*, vector1<T2>*
(*) reused instantiation
IOW, if the vector3 differs from the vector2 by push_front, then the
latter method (using v_item starting from the head,i.e. the proposed
method) would have the memoization advantage, but if the vector3
differs from the vector2 by a push_back then the former method (using
v_item starting from the tail,i.e., the existing method) would have
the memoization advantage.
If that's right, then maybe there should be a vec_back and vec_front
where vec_back is the existing vector and vec_front is the proposed
vector. Then the docs for vec_back and vec_front would highlight why
one might be preferred over the other. But OOPS, vec_back couldn't
take advantage of a variadic compiler since (as mentioned previously)
the existing variadic compiler only allows parameter packs at the
end. Instead, it would have to resort to the existing preprocessor
:(.
[snip]
> HTH,
Yes, thanks very much.
-regards,
Larry