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Difference between revisions of "cpp/ranges/drop view"

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< cpp‎ | ranges
m (→‎Data members: TODO)
 
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−
{{cpp/ranges/view title | drop}}
+
{{cpp/ranges/view title|drop}}
 
{{cpp/ranges/drop_view/navbar}}
 
{{cpp/ranges/drop_view/navbar}}
  
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++20 |1=
+
{{dcl header|ranges}}
 +
{{dcl|num=1|since=c++20|1=
 
template< ranges::view V >
 
template< ranges::view V >
−
class drop_view : public ranges::view_interface<drop_view<V>>
+
class drop_view
 +
    : public ranges::view_interface<drop_view<V>>
 
}}
 
}}
−
{{dcl | num=2 | since=c++20 |1=
+
{{dcl|num=2|since=c++20|1=
 
namespace views {
 
namespace views {
−
     inline constexpr /*unspecified*/ drop = /*unspecified*/;
+
     inline constexpr /* unspecified */ drop = /* unspecified */;
 
}
 
}
 +
}}
 +
{{dcl h|Call signature}}
 +
{{dcl|since=c++20|1=
 +
template< ranges::viewable_range R >
 +
    requires /* see below */
 +
constexpr ranges::view auto
 +
    drop( R&& r, ranges::range_difference_t<R> count );
 +
}}
 +
{{dcl|since=c++20|1=
 +
template< class DifferenceType >
 +
constexpr /* range adaptor closure */ drop( DifferenceType&& count );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
Line 16: Line 29:
 
@1@ A range adaptor consisting of elements of the underlying sequence, skipping the first ''N'' elements.
 
@1@ A range adaptor consisting of elements of the underlying sequence, skipping the first ''N'' elements.
  
−
@2@ The expression {{c|views::drop(E,F)}} is ''expression-equivalent'' to (where {{tt|T}} is {{c|std::remove_cvref_t<decltype((E))>}} and {{tt|D}} is {{c|ranges::range_difference_t<decltype((E))>}}):
+
@2@ {{named req|RangeAdaptorObject}}. Given {{tt|T}} is {{c/core|std::remove_cvref_t<decltype((e))>}} and {{tt|D}} is {{c/core|ranges::range_difference_t<decltype((e))>}}), the expression {{c|views::drop(e, f)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to:
 
<!---->
 
<!---->
−
* {{c|((void)F, static_cast<T>(E))}}, if {{tt|T}} is a {{lc|ranges::empty_view}};
+
* {{box|{{c/core|((void)f,}}{{nbspt}}{{lti|cpp/standard library/decay-copy}}{{c/core|(e))}}}}, if {{tt|T}} is a {{l2tt|cpp/ranges/empty_view}}, except that the evaluations of {{c|e}} and {{c|f}} are indeterminately sequenced;<!-- LWG 3524 -->
−
* {{c|T{ranges::begin(E) + std::min<D>(ranges::size(E), F)}, ranges::end(E) }}, if {{tt|T}} models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}, and {{tt|T}} is a specialization of
+
* otherwise, {{c multi|
−
:* {{lc|std::span}} where {{tt|1=T::extent == std::dynamic_extent}},
+
T(ranges::begin(e) + inc, ranges::end(e),|
−
:* {{lc|std::basic_string_view}},
+
    /*to-unsigned-like*/(ranges::distance(e) - inc))
−
:* {{lc|ranges::iota_view}}, or
+
}}, if {{tt|T}} is a specialization of {{c/core|ranges::subrange}} that models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}, and {{tt|T}} needs to store the size (see {{l2tf|cpp/ranges/subrange/subrange|subrange|ranges::subrange}} for details), where {{c|inc}} is {{c|std::min<D>(ranges::distance(e), f)}};
−
:* {{lc|ranges::subrange}};
+
* otherwise, {{c|U(ranges::begin(e) + inc, ranges::end(e))}}, if {{tt|T}} is a specialization of {{c/core|std::span}}, {{lc|std::basic_string_view}}, {{c/core|ranges::iota_view}}, or {{c/core|ranges::subrange}} that models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}, where {{tt|U}} is
−
* otherwise, {{c|drop_view{E, F} }}.
+
:* {{c/core|std::span<typename T::element_type>}}, if {{tt|T}} is a specialization of {{c/core|std::span}};
 +
:* {{tt|T}} otherwise;
 +
{{rrev|since=c++23|
 +
* otherwise, if {{tt|T}} is a specialization of {{l2tt|cpp/ranges/repeat_view}}:
 +
:* {{c|views::repeat(*e.value_, ranges::distance(e) - inc)}}, if {{tt|T}} models {{lconcept|sized_range}}; in such case {{c|e}} is evaluated only once;
 +
:* {{c|((void)e, auto(f))}} otherwise, except that the evaluations of {{c|e}} and {{c|f}} are indeterminately sequenced;
 +
}}
 +
* otherwise, {{c|drop_view(e, f)}}.
 
<!---->
 
<!---->
−
In all cases, {{tt|decltype((F))}} must model {{tt|std::convertible_to<D>}}.
+
In all cases, {{c/core|decltype((f))}} must model {{c|std::convertible_to<D>}}.
  
−
{{c|drop_view}} models the concepts {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}}, {{lconcept|input_range}}, and {{lconcept|common_range}} when the underlying view {{c|V}} models respective concepts.
+
{{tt|drop_view}} models the concepts {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}}, {{lconcept|input_range}}, {{lconcept|common_range}}, and {{lconcept|sized_range}} when the underlying view {{tt|V}} models respective concepts.
  
−
{{cpp/expr-eq}}
+
===Data members===
 +
{{dsc begin}}
 +
{{dsc hitem|Member name|Definition}}
 +
{{dsc expos mem obj|base_|private=yes|the underlying (adapted) {{lconcept|view}} of type {{tt|V}}}}
 +
{{dsc expos mem obj|count_|private=yes|the number of elements to skip, of type {{c/core|ranges::range_difference_t<V>}}}}
 +
{{dsc expos mem obj|cache_|private=yes|TODO: mention L4$ <!-- like this: If V is not a random_access_range or a sized_range, in order to provide the amortized constant time complexity required by the range concept, the overload (1) caches the result within the drop_view object for use on subsequent calls.-->|maybe=yes}}
 +
{{dsc end}}
  
 
===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/ranges/adaptor/dsc constructor | drop_view }}
+
{{dsc inc|cpp/ranges/adaptor/dsc constructor|drop_view}}
−
{{dsc inc | cpp/ranges/adaptor/dsc base | drop_view }}
+
{{dsc inc|cpp/ranges/adaptor/dsc base|drop_view}}
−
{{dsc inc | cpp/ranges/adaptor/dsc begin | drop_view }}
+
{{dsc inc|cpp/ranges/adaptor/dsc begin|drop_view}}
−
{{dsc inc | cpp/ranges/adaptor/dsc end | drop_view }}
+
{{dsc inc|cpp/ranges/adaptor/dsc end|drop_view}}
−
{{dsc inc | cpp/ranges/adaptor/dsc size | drop_view }}
+
{{dsc inc|cpp/ranges/adaptor/dsc size|drop_view}}
 +
 
 +
{{cpp/ranges/view_interface/inherit|embedded=yes|size=yes}}
 
{{dsc end}}
 
{{dsc end}}
  
 
==={{rl|deduction guides|Deduction guides}}===
 
==={{rl|deduction guides|Deduction guides}}===
 +
 +
===Helper templates===
 +
{{ddcl|since=c++20|1=
 +
template< class T >
 +
constexpr bool enable_borrowed_range<std::ranges::drop_view<T>> =
 +
    ranges::enable_borrowed_range<T>;
 +
}}
 +
This specialization of {{c/core|ranges::enable_borrowed_range}} makes {{tt|drop_view}} satisfy {{lconcept|borrowed_range}} when the underlying view satisfies it.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| code=
+
|code=
−
| output=
+
#include <iostream>
 +
#include <ranges>
 +
 
 +
int main()
 +
{
 +
    const auto nums = {1, 2, 3, 4, 5, 6, 7};
 +
 
 +
    std::cout << "drop " << 2 << ": ";
 +
    for (int i : std::ranges::drop_view{nums, 2})
 +
        std::cout << i << ' ';
 +
    std::cout << '\n';
 +
 
 +
    std::cout << "drop " << 3 << ": ";
 +
    for (int i : nums {{!}} std::views::drop(3))
 +
        std::cout << i << ' ';
 +
    std::cout << '\n';
 +
 
 +
    std::cout << "drop " << 4 << ": ";
 +
    for (int i : std::views::iota(1, 8) {{!}} std::views::drop(4))
 +
        std::cout << i << ' ';
 +
    std::cout << '\n';
 +
 
 +
    // Note that dropping more than the number of elements is OK:
 +
    for (int dp : {5, 6, 7, 890, 100500})
 +
    {
 +
        std::cout << "drop " << dp << ": ";
 +
        for (int i : std::views::iota(1, 8) {{!}} std::views::drop(dp))
 +
            std::cout << i << ' ';
 +
        std::cout << '\n';
 +
    }
 +
}
 +
|output=
 +
drop 2: 3 4 5 6 7
 +
drop 3: 4 5 6 7
 +
drop 4: 5 6 7
 +
drop 5: 6 7
 +
drop 6: 7
 +
drop 7:
 +
drop 890:
 +
drop 100500:
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3407|std=C++20|before={{tt|views::drop}} sometimes fails to<br>construct a sized random access range|after=the construction is adjusted<br>so that it is always valid}}
 +
{{dr list item|wg=lwg|dr=3494|std=c++20|before={{tt|drop_view}} was never a {{tt|borrowed_range}}|after=it is a {{tt|borrowed_range}} if its underlying view is}}
 +
{{dr list end}}
 +
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/ranges/dsc drop_while_view}}
 +
{{dsc end}}
 +
 +
{{langlinks|de|es|ja|ru|zh}}

Latest revision as of 08:07, 13 July 2024

 
C++
 
Ranges library
Range adaptors
 
std::ranges::drop_view
 
Defined in header <ranges>
template< ranges::view V >

class drop_view

    : public ranges::view_interface<drop_view<V>>
 (1) (since C++20)
namespace views {

    inline constexpr /* unspecified */ drop = /* unspecified */;

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

    requires /* see below */
constexpr ranges::view auto

    drop( R&& r, ranges::range_difference_t<R> count );
 (since C++20)
template< class DifferenceType >
constexpr /* range adaptor closure */ drop( DifferenceType&& count );
 (since C++20)
1) A range adaptor consisting of elements of the underlying sequence, skipping the first N elements.
2) RangeAdaptorObject. Given T is std::remove_cvref_t<decltype((e))> and D is ranges::range_difference_t<decltype((e))>), the expression views::drop(e, f) is expression-equivalent to:
  • std::span<typename T::element_type>, if T is a specialization of std::span;
  • T otherwise;
  • views::repeat(*e.value_, ranges::distance(e) - inc), if T models sized_range; in such case e is evaluated only once;
  • ((void)e, auto(f)) otherwise, except that the evaluations of e and f are indeterminately sequenced;
(since C++23)
  • otherwise, drop_view(e, f).
In all cases, decltype((f)) must model std::convertible_to<D>.

drop_view models the concepts contiguous_range, random_access_range, bidirectional_range, forward_range, input_range, common_range, and sized_range when the underlying view V models respective concepts.

Contents

[edit] Data members

Member name Definition
base_ (private) the underlying (adapted) view of type V
(exposition-only member object*)
count_ (private) the number of elements to skip, of type ranges::range_difference_t<V>
(exposition-only member object*)
cache_ (private)
(conditionally present) 		TODO: mention L4$
(exposition-only member object*)

[edit] Member functions

 constructs a drop_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]
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(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]
gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator.
(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]
returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>) [edit]
returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Deduction guides

[edit] Helper templates

template< class T >

constexpr bool enable_borrowed_range<std::ranges::drop_view<T>> =

    ranges::enable_borrowed_range<T>;
 (since C++20)

This specialization of ranges::enable_borrowed_range makes drop_view satisfy borrowed_range when the underlying view satisfies it.

[edit] Example

Run this code
#include <iostream>
#include <ranges>
 
int main()
{
    const auto nums = {1, 2, 3, 4, 5, 6, 7};
 
    std::cout << "drop " << 2 << ": ";
    for (int i : std::ranges::drop_view{nums, 2})
        std::cout << i << ' ';
    std::cout << '\n';
 
    std::cout << "drop " << 3 << ": ";
    for (int i : nums | std::views::drop(3))
        std::cout << i << ' ';
    std::cout << '\n';
 
    std::cout << "drop " << 4 << ": ";
    for (int i : std::views::iota(1, 8) | std::views::drop(4))
        std::cout << i << ' ';
    std::cout << '\n';
 
    // Note that dropping more than the number of elements is OK:
    for (int dp : {5, 6, 7, 890, 100500})
    {
        std::cout << "drop " << dp << ": ";
        for (int i : std::views::iota(1, 8) | std::views::drop(dp))
            std::cout << i << ' ';
        std::cout << '\n';
    }
}

Output: 

drop 2: 3 4 5 6 7
drop 3: 4 5 6 7
drop 4: 5 6 7
drop 5: 6 7
drop 6: 7
drop 7:
drop 890: 
drop 100500:

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 3407 C++20 views::drop sometimes fails to
construct a sized random access range 		the construction is adjusted
so that it is always valid
LWG 3494 C++20 drop_view was never a borrowed_range it is a borrowed_range if its underlying view is

[edit] See also

(C++20)
 a view consisting of the elements of another view, skipping the initial subsequence of elements until the first element where the predicate returns false
(class template) (range adaptor object)[edit]
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