Re: [boost] [MPL][vector] Is there interest in mpl::vector using variadic templates?
On Mar 3, 2017 17:58, "Peter Dimov via Boost" <boost@lists.boost.org> wrote: Bruno Dutra wrote: OTOH the void* trick requires that a pack of M void pointers be
instantiated when the element at position M is retrieved from a list, so, for a list of N elements, the compiler must instantiate N *distinct* packs of void pointers, because each of these packs have a different size and is thus a distinct type.
Not just that, but the helper function that is required to obtain the Mth element is a template instantiation as well. Saying that we shouldn't use map_from_list because it consumes memory for the instantiation is misleading; the alternatives consume memory for instantiations as well. Except for __type_pack_element, of course. Right, the trick is usually to figure out what consumes less memory, i.e. what leads to the least amount of distinct template instantiations. Alias templates help a lot with that, since they are not actually instantiated, but there is only so much one can do with alias templates alone. Bruno
On Mar 3, 2017 17:58, "Peter Dimov via Boost" <boost@lists.boost.org> wrote:
Bruno Dutra wrote:
OTOH the void* trick requires that a pack of M void pointers be
instantiated when the element at position M is retrieved from a
for a list of N elements, the compiler must instantiate N *distinct*
On 03/03/2017 11:21 AM, Bruno Dutra via Boost wrote: list, so, packs
of void pointers, because each of these packs have a different size and is thus a distinct type.
Not just that, but the helper function that is required to obtain the Mth element is a template instantiation as well.
Saying that we shouldn't use map_from_list because it consumes memory for the instantiation is misleading; the alternatives consume memory for instantiations as well. Except for __type_pack_element, of course.
I see your point. It seems obvious now that you mention the helper function. Thanks Peter. More explicitly, the helper functions (would metafunction be more accurate?) for the: // mp_repeat_c method from: simple_cxx11_metaprogramming_2.html are: struct mp_repeat_c_impl struct mp_at_c_impl OTOH, for the: // mp_map_from_list method, the metafunctions are: struct mp_map_from_list_impl struct mp_at_c_impl in addition, there's the: class integer_sequence Hence, just based on the number of class template's used, mp_repeat_c wins (although, since the cost of instantiating integer_sequence can be amortized, it may not be by much). A more difficult but accurate analysis would, somehow, calculate the number of distinct class template instantiations. Anyone care to try that? OOPS. Bruno did that, sortof, when he said (in the Date: Fri, 3 Mar 2017 17:51:26 +0100 post): OTOH the void* trick requires that a pack of M void pointers be instantiated when the element at position M is retrieved from a list, so, for a list of N elements, the compiler must instantiate N *distinct* packs of void pointers, because each of these packs have a different size and is thus a distinct type. This means the compiler does not benefit from memoization at all. So, I guess to retrieve N elements, something of the order of N^2 instantiations are needed? That because, for each of the N elements, a different instantiation of N elements( M void* and N-M other elements) is needed? OOPS. Looking at the times in the table, it appears almost linear, not quadratic. Attached is a libreoffice spreadsheet with the graph of times vs. number of elements. It does clearly show the void* method does show some non-linearity, but it's not very pronounced.
Right, the trick is usually to figure out what consumes less memory, i.e. what leads to the least amount of distinct template instantiations.
Which is more expensive, function template instantiations and class template instantiations? (I'd assumed class templates were more expensive). Thanks to you both for helping me understand the code complexity. -regards, Larry
participants (2)
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Bruno Dutra -
Larry Evans