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Difference between revisions of "cpp/ranges/lazy split view"

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< cpp‎ | ranges
m (→‎Synopsis: + "since C++20")
(→‎Defect reports: + P2210R2)
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{{dcl h | Helper concepts}}
 
{{dcl h | Helper concepts}}
 
{{dcl | num=3 | since=c++20 |1=
 
{{dcl | num=3 | since=c++20 |1=
−
namespace ranges {
 
−
  template< auto > struct /*require_constant*/;    // exposition only
 
−
}
 
−
}}
 
−
{{dcl | num=4 | since=c++20 |1=
 
 
template< class R >
 
template< class R >
 
concept /*tiny_range*/ =      // exposition only
 
concept /*tiny_range*/ =      // exposition only
 
   ranges::sized_range<R> &&
 
   ranges::sized_range<R> &&
−
     requires { typename /*require_constant*/<std::remove_reference_t<R>::size()>; } &&
+
     requires /*is-constexpr-sized*/<R> &&
 
       (std::remove_reference_t<R>::size() <= 1);
 
       (std::remove_reference_t<R>::size() <= 1);
 
}}
 
}}
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@2@ A [[cpp/ranges#Range adaptor objects|range adaptor object]]. The expression {{c|views::lazy_split(e, f)}} is ''expression-equivalent'' to {{c|lazy_split_view(e, f)}}<!-- LWG 3524 -->.
 
@2@ A [[cpp/ranges#Range adaptor objects|range adaptor object]]. The expression {{c|views::lazy_split(e, f)}} is ''expression-equivalent'' to {{c|lazy_split_view(e, f)}}<!-- LWG 3524 -->.
  
−
@3@ The exposition-only type {{tt|''require_constant''}}, such that the requirement {{c|requires { typename /*require_constant*/<E>; } }} is satisfied if and only if {{tt|E}} is {{tt|constexpr}} qualified.<!--O_o-->
+
@3@ The exposition-only concept {{c|/*tiny_range*/<Pattern>}} is satisfied if {{c|Pattern}} satisfies {{lconcept|sized_range}}, {{c|Pattern::size()}} is a constant expression that are suitable as a template non-type argument, and the value of {{c|Pattern::size()}} is less than or equal to {{tt|1}}.<!-- The standard uses `std::remove_reference_t<R>::size()`, but since `Pattern` is a `view`, it cannot be a reference. --> Notably, {{rlpt|empty_view}} and {{rlpt|single_view}} satisfy this concept.
−
 
+
−
@4@ The exposition-only concept {{c|/*tiny_range*/<Pattern>}} is satisfied if {{c|Pattern}} satisfies {{lconcept|sized_range}}, {{c|Pattern::size()}} is a constant expression, and the value of {{c|Pattern::size()}} is less than or equal to {{tt|1}}.<!-- The standard uses `std::remove_reference_t<R>::size()`, but since `Pattern` is a `view`, it cannot be a reference. --> Notably, {{rlpt|empty_view}} and {{rlpt|single_view}} satisfy this concept.
+
  
 
{{tt|lazy_split_view}} models the concepts {{lconcept|forward_range}} and {{lconcept|input_range}} when the underlying {{lconcept|view}} {{tt|V}} models respective concepts, and models {{lconcept|common_range}} when {{tt|V}} models both {{lconcept|forward_range}} and {{lconcept|common_range}}.
 
{{tt|lazy_split_view}} models the concepts {{lconcept|forward_range}} and {{lconcept|input_range}} when the underlying {{lconcept|view}} {{tt|V}} models respective concepts, and models {{lconcept|common_range}} when {{tt|V}} models both {{lconcept|forward_range}} and {{lconcept|common_range}}.
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==={{rl|deduction guides|Deduction guides}}===
 
==={{rl|deduction guides|Deduction guides}}===
−
 
−
===Notes===
 
−
{{todo|R2210R2 changes to be reflected.}}
 
  
 
===Example===
 
===Example===
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substrings: { H e l l o } { C + + } { 2 0 } { ! }
 
substrings: { H e l l o } { C + + } { 2 0 } { ! }
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|paper=P2210R2|std=C++20|before=the old {{tt|split_view}} was too lazy to be easily used|after=moves its functionality to {{tt|lazy_split_view}}}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
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{{dsc end}}
 
{{dsc end}}
  
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{{langlinks|es|ja|ru|zh}}
+
{{langlinks|de|es|ja|ru|zh}}

Revision as of 01:06, 20 January 2022

 
C++
 
Ranges library
Range adaptors
 
std::ranges::lazy_split_view
 
Defined in header <ranges>
template< ranges::input_range V, ranges::forward_range Pattern >

requires ranges::view<V> && ranges::view<Pattern> &&
  std::indirectly_comparable<ranges::iterator_t<V>, ranges::iterator_t<Pattern>,
                             ranges::equal_to> &&
  (ranges::forward_range<V> || /*tiny_range*/<Pattern>)

class lazy_split_view : public ranges::view_interface<lazy_split_view<V, Pattern>>
 (1) (since C++20)
namespace views {

    inline constexpr /*unspecified*/ lazy_split = /*unspecified*/;

}
 (2) (since C++20)
Call signature
template< ranges::viewable_range R, class Pattern >

    requires /* see below */

constexpr ranges::view auto lazy_split( R&& r, Pattern&& pattern );
 (since C++20)
template< class Pattern >
constexpr /*range adaptor closure*/ lazy_split( Pattern&& pattern );
 (since C++20)
Helper concepts
template< class R >

concept /*tiny_range*/ =      // exposition only
  ranges::sized_range<R> &&
    requires /*is-constexpr-sized*/<R> &&

      (std::remove_reference_t<R>::size() <= 1);
 (3) (since C++20)
1) lazy_split_view takes a view and a delimiter, and splits the view into subranges on the delimiter.

Two major scenarios are supported:

2) A range adaptor object. The expression views::lazy_split(e, f) is expression-equivalent to lazy_split_view(e, f).
3) The exposition-only concept /*tiny_range*/<Pattern> is satisfied if Pattern satisfies sized_range, Pattern::size() is a constant expression that are suitable as a template non-type argument, and the value of Pattern::size() is less than or equal to 1. Notably, empty_view and single_view satisfy this concept.

lazy_split_view models the concepts forward_range and input_range when the underlying view V models respective concepts, and models common_range when V models both forward_range and common_range.

The inner range (ranges::range_reference_t<lazy_split_view>) models the concepts forward_range and input_range when the underlying view V models respective concepts. It does not model common_range, and cannot be used with algorithms that expect a bidirectional_range or higher.

Contents

Data members

Typical implementations of lazy_split_view hold two or three non-static data members:

  • the underlying view of type V (shown here as base_ for exposition only),
  • the pattern (shown here as pattern_ for exposition only) that is used as a delimiter to split the underlying view, and
  • the caching object (shown here as current_ for exposition only) of the std::optional-like exposition-only type /*non_propagating_cache*/<ranges::iterator_t<V>>. The current_ caches the result of calls to begin(). Present only if the expression ranges::forward_range<V> evaluates to false.

Member functions

 constructs a lazy_split_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
Inherited from std::ranges::view_interface
returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>) [edit]
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]

Nested classes

(C++20)
 the iterator type
(exposition-only member class template*)
(C++20)
 the iterator type of the inner range
(exposition-only member class template*)

Deduction guides

Example

Run this code
#include <algorithm>
#include <iostream>
#include <ranges>
#include <string_view>
 
// P2210R2: a temporary patch until online g++ >= 12
#define lazy_split_view split_view
#define lazy_split split
 
auto print = [](auto const& view)
{
    // `view` is a std::views::lazy_split_view::/*outer_iterator*/::value_type
 
    for (std::cout << "{ "; const auto element : view)
        std::cout << element << ' ';
    std::cout << "} ";
};
 
int main()
{
    constexpr static auto source = { 0, 1,0, 2,3,0, 4,5,6,0, 7,8,9 };
    constexpr int delimiter {0};
    constexpr std::ranges::lazy_split_view outer_view{source, delimiter};
    std::cout << "splits[" << std::ranges::distance(outer_view) << "]:  ";
    for (auto const& inner_view: outer_view)
        print(inner_view);
 
    constexpr std::string_view hello { "Hello C++ 20 !" };
    std::cout << "\n" "substrings: ";
    std::ranges::for_each(hello | std::views::lazy_split(' '), print);
 
    constexpr std::string_view text { "Hello-+-C++-+-20-+-!" };
    constexpr std::string_view delim { "-+-" };
    std::cout << "\n" "substrings: ";
    std::ranges::for_each(text | std::views::lazy_split(delim), print);
}

Output: 

splits[5]:  { } { 1 } { 2 3 } { 4 5 6 } { 7 8 9 }
substrings: { H e l l o } { C + + } { 2 0 } { ! }
substrings: { H e l l o } { C + + } { 2 0 } { ! }

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2210R2 C++20 the old split_view was too lazy to be easily used moves its functionality to lazy_split_view

See also

(C++20)
 a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object)[edit]
(C++20)
 a view consisting of the sequence obtained from flattening a view of ranges
(class template) (range adaptor object)[edit]
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