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Use variadic template expansion in min category deduction in zip_iterator.

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This results in marginally faster compile times in modern code bases
where tuples are variadic.
This commit is contained in:
Andrey Semashev 2025-02-05 19:37:57 +03:00
parent aadd90df45
commit 8c2fdf6053
1 changed files with 16 additions and 27 deletions
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43
include/boost/iterator/zip_iterator.hpp
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@ -107,15 +107,6 @@ struct is_trailing_null_type< tuples::null_type > : std::true_type {};
template< >
struct is_trailing_null_type< fusion::void_ > : std::true_type {};
// The trait checks if the tail is either an empty type list or begins with a "null" type, which indicates that the rest
// of the types in the tail are unused
template< typename... Tail >
struct is_tail_empty;
template< >
struct is_tail_empty< > : std::true_type {};
template< typename Front, typename... Tail >
struct is_tail_empty< Front, Tail... > : detail::is_trailing_null_type< Front > {};
// Metafunction to obtain the type of the tuple whose element types
// are the reference types of an iterator tuple.
template< typename IteratorTuple >
@ -161,30 +152,28 @@ struct minimum_traversal_category_in_iterator_list;
template< typename IteratorList >
using minimum_traversal_category_in_iterator_list_t = typename minimum_traversal_category_in_iterator_list< IteratorList >::type;
template< template< typename... > class List, typename... Iterators >
struct minimum_traversal_category_in_iterator_list< List< Iterators... > >
{
// Note: non-variadic Boost.Tuple and Boost.Fusion need special handling
// to avoid instantiating iterator traits on the trailing "null" types.
// For such types just use random_access_traversal_tag, which will not
// affect the result of min_category.
using type = min_category_t<
mp11::mp_eval_if<
detail::is_trailing_null_type< Iterators >,
random_access_traversal_tag,
pure_iterator_traversal_t, Iterators
>...
>;
};
template< typename FrontTraversal, typename Tail >
using minimum_traversal_category_in_tail_t = min_category_t<
FrontTraversal,
minimum_traversal_category_in_iterator_list_t< Tail >
>;
template< template< typename... > class List, typename Front, typename... Tail >
struct minimum_traversal_category_in_iterator_list< List< Front, Tail... > >
{
using front_traversal = pure_iterator_traversal_t< Front >;
// Note: non-variadic Boost.Tuple and Boost.Fusion need special handling
// to avoid instantiating iterator traits on the trailing "null" types.
// Note 2: we rename the List to mp_list in the process of iteration to
// avoid specifying one template parameter to std::pair, if List is std::pair.
using type = mp11::mp_eval_if<
detail::is_tail_empty< Tail... >,
front_traversal,
minimum_traversal_category_in_tail_t,
front_traversal,
mp11::mp_list< Tail... >
>;
};
template< typename Front, typename Tail >
struct minimum_traversal_category_in_iterator_list< tuples::cons< Front, Tail > >
{