Skip to content

Sorter traits

Jump to bottom
Morwenn edited this page Apr 10, 2016 · 17 revisions 
#include <cpp-sort/sorter_traits.h>

is_projection and is_projection_iterator

The goal is these type traits is to check whether a projection function can be applied on the value_type of an iterable or an iterator. An additional template parameter Compare may be specified, in which case the traits will also check whether the given binary comparison function can be called with two projected values.

template<
    typename Projection,
    typename Iterable,
    typename Compare = std::less<>
>
struct is_projection;

template<
    typename Projection,
    typename Iterator,
    typename Compare = std::less<>
>
struct is_projection_iterator;

These traits are modeled after the standard library's type traits (that is, they inherit from either std::true_type or std::false_type), so you can expect them to provide the exact same member types, functions and static variables. Also, just like the standard library type traits, they come along with associated variable templates to reduce the boilerplate:

template<typename Projection, typename Iterable, typename Compare=std::less<>>
constexpr bool is_projection_v
    = is_projection<Projection, Iterable, Compare>::value;

template<typename Projection, typename Iterator, typename Compare=std::less<>>
constexpr bool is_projection_iterator_v
    = is_projection_iterator<Projection, Iterator, Compare>::value;

is_sorter and friends

The is_sorter family of type traits is meant to check whether a given sorter satisfies some properties: being able to be called with of variety of parameters and checking whether the parameters play nice together. For example, is_comparison_sorter inherits from std::true_type if Sorter is a type that can be called with a collection Iterable& and a comparison function Compare, and inherits from std::false_type otherwise. These traits mainly exist for SFINAE purpose and concept checking.

template<typename Sorter, typename Iterable>
struct is_sorter;

template<typename Sorter, typename Iterable, typename Compare>
struct is_comparison_sorter;

template<typename Sorter, typename Iterable, typename Projection>
struct is_projection_sorter;

template<typename Sorter, typename Iterable, typename Compare, typename Projection>
struct is_comparison_projection_sorter;

There are also variants of these traits which take a potential sorter type and an iterator type (instead of an iterable type). They exist to check whether the sorter can be called with a pair of iterators.

template<typename Sorter, typename Iterator>
struct is_sorter_iterator;

template<typename Sorter, typename Iterator, typename Compare>
struct is_comparison_sorter_iterator;

template<typename Sorter, typename Iterator, typename Projection>
struct is_projection_sorter_iterator;

template<typename Sorter, typename Iterator, typename Compare, typename Projection>
struct is_comparison_projection_sorter_iterator;

These traits are modeled after the standard library's type traits (that is, they inherit from either std::true_type or std::false_type), so you can expect them to provide the exact same member types, functions and static variables. Also, just like the standard library type traits, they come along with associated variable templates to reduce the boilerplate:

template<typename Sorter, typename Iterable>
constexpr bool is_sorter_v
    = is_sorter<Sorter, Iterable>::value;

template<typename Sorter, typename Iterable, typename Compare>
constexpr bool is_comparison_sorter_v
    = is_comparison_sorter<Sorter, Iterable, Compare>::value;

template<typename Sorter, typename Iterable, typename Projection>
constexpr bool is_projection_sorter_v
    = is_projection_sorter<Sorter, Iterable, Projection>::value;

template<typename Sorter, typename Iterable, typename Compare, typename Projection>
constexpr bool is_comparison_projection_sorter_v
    = is_comparison_projection_sorter<Sorter, Iterable, Compare, Projection>::value;

template<typename Sorter, typename Iterator>
constexpr bool is_sorter_iterator_v
    = is_sorter_iterator<Sorter, Iterator>::value;

template<typename Sorter, typename Iterator, typename Compare>
constexpr bool is_comparison_sorter_iterator_v
    = is_comparison_sorter_iterator<Sorter, Iterator, Compare>::value;

template<typename Sorter, typename Iterator, typename Projection>
constexpr bool is_projection_sorter_iterator_v
    = is_projection_sorter_iterator<Sorter, Iterator, Projection>::value;

template<typename Sorter, typename Iterator, typename Compare, typename Projection>
constexpr bool is_comparison_projection_sorter_iterator_v
    = is_comparison_projection_sorter_iterator<Sorter, Iterator, Compare, Projection>::value;

sorter_traits

The class template sorter_traits<Sorter> contains information about sorters and sorter adapters such as the kind of iterators accepted by a sorter and whether it implements or not a stable sorting algorithm.

template<typename Sorter>
struct sorter_traits;

This class template peeks into Sorter to extract the following types:

  • using iterator_category = typename Sorter::iterator_category;
  • using is_stable = typename Sorter::is_stable;

This class is a bit different than the trait classes in the standard library: if one of the types above doesn't exist in the passed sorter, it won't exist in sorter_traits either. That means that the traits are not tightly coupled: if a sorter doesn't define is_stable but defines iterator_category, it can still be used in hybrid_adapter; instantiating the corresponding sorter_traits won't cause a compile-time error because of the missing is_stable.

iterator_category

template<typename Sorter>
using iterator_category = typename sorter_traits<Sorter>::iterator_category;

Some tools need to know which category of iterators a sorting algorithm can work with. Therefore, a well-defined sorter shall provide one of the standard library iterator tags in order to document that.

When a sorter is adapted so that it may be used with several categories of iterators, the resulting sorter's iterator category will correspond to the most permissive among the original sorters. For example, if an hybrid_adapter merges sorting algorithms with std::forward_iterator_tag and std::random_access_iterator_tag, the resulting sorter's category will be std::forward_iterator_tag since it is guaranteed to work with any iterable type which has at least forward iterators.

is_stable

template<typename Sorter>
using is_stable = typename sorter_traits<Sorter>::is_stable;

This type trait is always either std::true_type or std::false_type and tells whether a sorting algorithm is stable or not. This information may be useful in some contexts, even though it is currently unused by the library.

When a sorter adapter is used, the resulting sorter is stable if and only if its stability can be guaranteed and unstable otherwise, even when the adapted sorter may be stable (for example, self_sort_adapter is always considered unstable since it is impossible to guarantee the stability of any sort method).

rebind_iterator_category

template<typename Sorter, typename Category>
struct rebind_iterator_category;

This class allows to get a sorter similar to the one passed but with a stricter iterator category. For example, it can generate a sorter whose iterator category is std::bidirectional_iterator_tag from another sorter whose iterator category is std::forward_iterator_tag. It is mostly useful to make several sorters with the same iterator category work for different iterator categories in an hybrid_adapter. Let's say we have two sorters, foo_sorter and bar_sorter; both have the iterator category std::forward_iterator_tag, but foo_sorter is the best to sort forward iterators while bar_sorter benefits from some optimizations for bidirectional and random-access iterators. It is possible to use the best of both with the following sorter:

using sorter = cppsort::hybrid_adapter<
    foo_sorter,
    cppsort::rebind_iterator_category<
        bar_sorter,
        std::bidirectional_iterator_tag
    >
>;

The sorter above will pick foo_sorter for forward iterators, but will pick bar_sorter for bidirectional and random-access iterators. A compile-time error will occur if one tries to rebind to an iterator category that is not derived from the sorter's original iterator category.

fixed_sorter_traits

The class template fixed_sorter_traits<FixedSizeSorter> contains information about fixed-size sorters such as their stability or the sizes for which a valid specialization of the fixed-size sorter exists.

template<
    template<std::size_t N> class FixedSizeSorter,
    typename Indices
>
struct fixed_sorter_traits;

This class template can be specialized for any fixed-size sorter and exposes the following properties:

  • domain: a specialization of std::index_sequence containing all the sizes for which a specialization of the fixed-size sorter exists. If this trait isn't specified, it is assumed that the fixed-size sorter can be specialized for any value of N.
  • iterator_category: the category of iterators every specialization of the fixed-size sorter is guaranteed to work with. Individual specializations may work with less strict iterator categories.
  • is_stable: an alias for std::true_type if every specialization of the fixed-size sorter is guaranteed to be stable, and std::false_type otherwise.
Clone this wiki locally