On 04/12/09 01:55, Aleksey Gurtovoy wrote:
On Mon, 06 Apr 2009 08:11:06 -0500, Larry Evans <cppljevans@suddenlink.net> wrote:
[snip]
I'm wondering why the implementation here:
https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/vector/aux_/preproc...
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