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Difference between revisions of "cpp/ranges/as const view"

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< cpp‎ | ranges
m (→‎Notes: lc => c/core as a temporary measure to generate link.)
 
(15 intermediate revisions by 3 users not shown)
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−
{{cpp/ranges/view title | as_const}}
+
{{cpp/ranges/view title|as_const}}
 
{{cpp/ranges/navbar}}
 
{{cpp/ranges/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | ranges}}
+
{{dcl header|ranges}}
−
{{dcl | num=1 | since=c++23 |1=
+
{{dcl|num=1|since=c++23|1=
 
template< ranges::view V >
 
template< ranges::view V >
 
     requires ranges::input_range<V>
 
     requires ranges::input_range<V>
−
class as_const_view : public ranges::view_interface<as_const_view<V>>
+
class as_const_view
 +
    : public ranges::view_interface<as_const_view<V>>
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |1=
+
{{dcl|num=2|since=c++23|1=
 
namespace views {
 
namespace views {
 
     inline constexpr /* unspecified */ as_const = /* unspecified */;
 
     inline constexpr /* unspecified */ as_const = /* unspecified */;
Line 14: Line 15:
 
}}
 
}}
 
{{dcl h|Call signature}}
 
{{dcl h|Call signature}}
−
{{dcl | since=c++23 |1=
+
{{dcl|since=c++23|1=
 
template< ranges::viewable_range R >
 
template< ranges::viewable_range R >
 
     requires /* see below */
 
     requires /* see below */
Line 23: Line 24:
 
@1@ A range adaptor that represents a view of underlying {{lconcept|view}} that is also a {{lconcept|constant_range}}. An {{tt|as_const_view}} always has read-only elements (if not empty).
 
@1@ A range adaptor that represents a view of underlying {{lconcept|view}} that is also a {{lconcept|constant_range}}. An {{tt|as_const_view}} always has read-only elements (if not empty).
  
−
@2@ {{named req|RangeAdaptorObject}}. Let {{c|e}} be a subexpression, let {{c|T}} be {{c|decltype((e))}}, and let {{c|U}} be {{c|std::remove_cvref_t<T>}}. Then the expression {{c|views::as_const(e)}} is ''expression-equivalent'' to:
+
@2@ {{named req|RangeAdaptorObject}}. Let {{c|e}} be a subexpression, let {{c|T}} be {{c|decltype((e))}}, and let {{c|U}} be {{c|std::remove_cvref_t<T>}}. Then the expression {{c|views::as_const(e)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to:
 
* {{c|views::all(e)}}, if it is a well-formed expression and {{c|views::all_t<T>}} models {{lconcept|constant_range}};
 
* {{c|views::all(e)}}, if it is a well-formed expression and {{c|views::all_t<T>}} models {{lconcept|constant_range}};
 
* otherwise, {{c|std::span<const X, Extent>(e)}} for some type {{tt|X}} and some extent {{tt|Extent}} if {{tt|U}} denotes {{c|std::span<X, Extent>}};
 
* otherwise, {{c|std::span<const X, Extent>(e)}} for some type {{tt|X}} and some extent {{tt|Extent}} if {{tt|U}} denotes {{c|std::span<X, Extent>}};
−
* otherwise, {{c|ranges::ref_view(static_cast<const U&>(e))}} if {{tt|e}} is an lvalue, {{c|const U}} models {{lconcept|constant_range}}, and {{c|U}} does not model {{lconcept|view}}.
+
* otherwise, {{c|ranges::ref_view(static_cast<const X&>(e.base()))}} if {{tt|U}} denotes {{c|ranges::ref_view<X>}} for some type {{tt|X}} and {{c|const X}} models {{lconcept|constant_range}};
 +
* otherwise, {{c|ranges::ref_view(static_cast<const U&>(e))}} if {{tt|e}} is an lvalue, {{c|const U}} models {{lconcept|constant_range}}, and {{tt|U}} does not model {{lconcept|view}}.
 
* otherwise, {{c|as_const_view{e} }}.
 
* otherwise, {{c|as_const_view{e} }}.
  
 
{{tt|as_const_view}} always models {{lconcept|constant_range}}, and it models the {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}}, {{lconcept|borrowed_range}}, {{lconcept|common_range}}, and {{lconcept|sized_range}} when the underlying view {{tt|V}} models respective concepts.
 
{{tt|as_const_view}} always models {{lconcept|constant_range}}, and it models the {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}}, {{lconcept|borrowed_range}}, {{lconcept|common_range}}, and {{lconcept|sized_range}} when the underlying view {{tt|V}} models respective concepts.
−
 
−
{{cpp/expr-eq}}
 
  
 
===Data members===
 
===Data members===
−
Typical implementations of {{tt|as_const_view}} only hold one non-static data member:
+
{{dsc begin}}
−
* the underlying view of type {{tt|V}} (shown here as {{tt|''base_''}}, the name is exposition only).
+
{{dsc hitem|Member object|Definition}}
 +
{{dsc expos mem obj|base_|private=yes|The underlying view of type {{tt|V}}.}}
 +
{{dsc end}}
  
 
===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc mem ctor | nolink=true | constructs an {{tt|as_const_view}} | notes={{mark c++23}} }}
+
{{dsc mem ctor|{{PAGENAME}}#ctor|constructs an {{tt|as_const_view}}}}
−
{{dsc mem fun | nolink=true | base | returns the underlying view {{tt|V}} | notes={{mark c++23}} }}
+
{{dsc mem fun|{{PAGENAME}}#base|title=base|returns the underlying view {{tt|V}}}}
−
{{dsc mem fun | nolink=true | begin | returns the beginning iterator of the {{tt|as_const_view}} | notes={{mark c++23}} }}
+
{{dsc mem fun|{{PAGENAME}}#begin|title=begin|returns the beginning iterator of the {{tt|as_const_view}}}}
−
{{dsc mem fun | nolink=true | end | returns the end iterator of the {{tt|as_const_view}} | notes={{mark c++23}} }}
+
{{dsc mem fun|{{PAGENAME}}#end|title=end|returns the end iterator of the {{tt|as_const_view}}}}
−
{{dsc mem fun | nolink=true | size | returns the size of the view if it is bounded | notes={{mark c++23}} }}
+
{{dsc mem fun|{{PAGENAME}}#size|title=size|returns the size of the view if it is bounded}}
  
 
{{cpp/ranges/view_interface/inherit|embedded=yes|size=yes}}
 
{{cpp/ranges/view_interface/inherit|embedded=yes|size=yes}}
 
{{dsc end}}
 
{{dsc end}}
  
−
{{member | {{small|std::ranges::as_const_view::}}as_const_view |
+
{{anchor|ctor}}
 +
{{member|{{small|std::ranges::as_const_view::}}as_const_view|
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++23 |1=
+
{{dcl|num=1|since=c++23|1=
 
as_const_view() requires std::default_initializable<V> = default;
 
as_const_view() requires std::default_initializable<V> = default;
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |
+
{{dcl|num=2|since=c++23|
−
constexpr explicit as_const_view(V base);
+
constexpr explicit as_const_view( V base );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
@1@ Value-initializes {{tt|''base_''}} via its default member initializer ({{c|1== V()}}).
+
@1@ Value-initializes {{tti|base_}} via its default member initializer ({{c|1== V()}}).
−
@2@ Initializes {{tt|''base_''}} with {{c|std::move(base)}}.
+
@2@ Initializes {{tti|base_}} with {{c|std::move(base)}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | base | a view }}
+
{{par|base|a view}}
 
{{par end}}
 
{{par end}}
 
}}
 
}}
  
−
{{member | {{small|std::ranges::as_const_view::}}base|
+
{{anchor|base}}
 +
{{member|{{small|std::ranges::as_const_view::}}base|
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++23 |
+
{{dcl|num=1|since=c++23|
 
constexpr V base() const& requires std::copy_constructible<V>;
 
constexpr V base() const& requires std::copy_constructible<V>;
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |
+
{{dcl|num=2|since=c++23|
 
constexpr V base() &&;
 
constexpr V base() &&;
 
}}
 
}}
Line 82: Line 86:
 
}}
 
}}
  
−
{{member | {{small|std::ranges::as_const_view::}}begin|
+
{{anchor|begin}}
 +
{{member|{{small|std::ranges::as_const_view::}}begin|
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++23 |
+
{{dcl|num=1|since=c++23|
−
constexpr auto begin() requires (!/*simple-view*/<V>) {
+
constexpr auto begin() requires (!__simple_view<V>);
−
  return ranges::cbegin(base_);  
+
−
}
+
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |
+
{{dcl|num=2|since=c++23|
−
constexpr auto begin() const requires ranges::range<const V> {
+
constexpr auto begin() const requires ranges::range<const V>;
−
  return ranges::cbegin(base_);  
+
−
}
+
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
 
Returns the constant iterator of the view.
 
Returns the constant iterator of the view.
 +
 +
@1,2@ Equivalent to {{c|return ranges::cbegin(base_);}}
 
}}
 
}}
  
−
{{member | {{small|std::ranges::as_const_view::}}end|
+
{{anchor|end}}
 +
{{member|{{small|std::ranges::as_const_view::}}end|
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++23 |
+
{{dcl|num=1|since=c++23|
−
constexpr auto end() requires (!/*simple-view*/<V>) {
+
constexpr auto end() requires (!__simple_view<V>);
−
  return ranges::cend(base_);  
+
−
}
+
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |
+
{{dcl|num=2|since=c++23|
−
constexpr auto end() const requires ranges::range<const V> {
+
constexpr auto end() const requires ranges::range<const V>;
−
  return ranges::cend(base_);  
+
−
}
+
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
 
Returns the constant sentinel of the view.
 
Returns the constant sentinel of the view.
 +
 +
@1,2@ Equivalent to {{c|return ranges::cend(base_);}}
 
}}
 
}}
  
−
{{member | {{small|std::ranges::as_const_view::}}size|
+
{{anchor|size}}
 +
{{member|{{small|std::ranges::as_const_view::}}size|
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl | num=1 | since=c++23 |
+
{{dcl|num=1|since=c++23|
−
constexpr auto size() requires ranges::sized_range<V> {
+
constexpr auto size() requires ranges::sized_range<V>;
−
    return ranges::size(base_);
+
−
}
+
 
}}
 
}}
−
{{dcl | num=2 | since=c++23 |
+
{{dcl|num=2|since=c++23|
−
constexpr auto size() const requires ranges::sized_range<const V> {
+
constexpr auto size() const requires ranges::sized_range<const V>;
−
    return ranges::size(base_);
+
−
}
+
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
 
Returns the size of the view if the view is bounded.
 
Returns the size of the view if the view is bounded.
 +
 +
@1,2@ Equivalent to {{c|return ranges::size(base_);}}
 
}}
 
}}
  
 
===Deduction guides===
 
===Deduction guides===
−
{{ddcl | since=c++23 |
+
{{ddcl|since=c++23|
−
template<class R>
+
template< class R >
−
as_const_view(R&&) -> as_const_view<views::all_t<R>>;
+
as_const_view( R&& ) -> as_const_view<views::all_t<R>>;
 
}}
 
}}
  
 
===Helper templates===
 
===Helper templates===
 
{{ddcl|since=c++23|1=
 
{{ddcl|since=c++23|1=
−
template<class T>
+
template< class T >
−
inline constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =
+
constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =
−
     std::ranges::enable_borrowed_range<T>;
+
     ranges::enable_borrowed_range<T>;
 
}}
 
}}
 
This specialization of {{ltt|cpp/ranges/borrowed_range|std::ranges::enable_borrowed_range}} makes {{tt|as_const_view}} satisfy {{lconcept|borrowed_range}} when the underlying view satisfies it.
 
This specialization of {{ltt|cpp/ranges/borrowed_range|std::ranges::enable_borrowed_range}} makes {{tt|as_const_view}} satisfy {{lconcept|borrowed_range}} when the underlying view satisfies it.
 +
 +
===Notes===
 +
{{feature_test_macro|__cpp_lib_ranges_as_const|value=202207L|std=C++23|{{tt|ranges::as_const_view}}, {{c/core|std::const_iterator}}}}
  
 
===Example===
 
===Example===
−
{{todo|example}}
+
{{example|code=
 +
#include <cassert>
 +
#include <ranges>
 +
 
 +
int main()
 +
{
 +
    int x[]{1, 2, 3, 4, 5};
 +
 
 +
    auto v1 = x {{!}} std::views::drop(2);
 +
    assert(v1.back() == 5);
 +
    v1[0]++; // OK, can modify non-const element
 +
 
 +
    auto v2 = x {{!}} std::views::drop(2) {{!}} std::views::as_const;
 +
    assert(v2.back() == 5);
 +
    // v2[0]++; // Compile-time error, cannot modify read-only element
 +
}
 +
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/ranges/dsc cbegin}}
+
{{dsc inc|cpp/ranges/dsc as_rvalue_view}}
−
{{dsc inc | cpp/ranges/dsc cend}}
+
{{dsc inc|cpp/ranges/dsc cbegin}}
−
<!--
+
{{dsc inc|cpp/ranges/dsc cend}}
−
{{dsc inc | cpp/iterator/dsc basic_const_iterator}}
+
{{dsc inc|cpp/utility/dsc as_const}}
−
-->
+
{{dsc inc|cpp/iterator/dsc basic_const_iterator}}
 
{{dsc end}}
 
{{dsc end}}
 +
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 14:47, 19 June 2024

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

    requires ranges::input_range<V>
class as_const_view

    : public ranges::view_interface<as_const_view<V>>
 (1) (since C++23)
namespace views {

    inline constexpr /* unspecified */ as_const = /* unspecified */;

}
 (2) (since C++23)
Call signature
template< ranges::viewable_range R >

    requires /* see below */

constexpr ranges::view auto as_const( R&& r );
 (since C++23)
1) A range adaptor that represents a view of underlying view that is also a constant_range. An as_const_view always has read-only elements (if not empty).
2) RangeAdaptorObject. Let e be a subexpression, let T be decltype((e)), and let U be std::remove_cvref_t<T>. Then the expression views::as_const(e) is expression-equivalent to:

as_const_view always models constant_range, and it models the contiguous_range, random_access_range, bidirectional_range, forward_range, borrowed_range, common_range, and sized_range when the underlying view V models respective concepts.

Contents

[edit] Data members

Member object Definition
base_ (private) The underlying view of type V.
(exposition-only member object*)

[edit] Member functions

 constructs an as_const_view
(public member function)
returns the underlying view V
(public member function)
returns the beginning iterator of the as_const_view
(public member function)
returns the end iterator of the as_const_view
(public member function)
returns the size of the view if it is bounded
(public member function)
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]

std::ranges::as_const_view::as_const_view

as_const_view() requires std::default_initializable<V> = default;
 (1) (since C++23)
constexpr explicit as_const_view( V base );
 (2) (since C++23)
1) Value-initializes base_ via its default member initializer (= V()).
2) Initializes base_ with std::move(base).

Parameters

base - a view

std::ranges::as_const_view::base

constexpr V base() const& requires std::copy_constructible<V>;
 (1) (since C++23)
constexpr V base() &&;
 (2) (since C++23)

Returns the underlying view.

1) Copy-constructs the result from the underlying view. Equivalent to return base_;.
2) Move-constructs the result from the underlying view. Equivalent to return std::move(base_);.

std::ranges::as_const_view::begin

constexpr auto begin() requires (!__simple_view<V>);
 (1) (since C++23)
constexpr auto begin() const requires ranges::range<const V>;
 (2) (since C++23)

Returns the constant iterator of the view.

1,2) Equivalent to return ranges::cbegin(base_);

std::ranges::as_const_view::end

constexpr auto end() requires (!__simple_view<V>);
 (1) (since C++23)
constexpr auto end() const requires ranges::range<const V>;
 (2) (since C++23)

Returns the constant sentinel of the view.

1,2) Equivalent to return ranges::cend(base_);

std::ranges::as_const_view::size

constexpr auto size() requires ranges::sized_range<V>;
 (1) (since C++23)
constexpr auto size() const requires ranges::sized_range<const V>;
 (2) (since C++23)

Returns the size of the view if the view is bounded.

1,2) Equivalent to return ranges::size(base_);

[edit] Deduction guides

template< class R >
as_const_view( R&& ) -> as_const_view<views::all_t<R>>;
 (since C++23)

[edit] Helper templates

template< class T >

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

    ranges::enable_borrowed_range<T>;
 (since C++23)

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

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_ranges_as_const 202207L (C++23) ranges::as_const_view, std::const_iterator

[edit] Example

Run this code
#include <cassert>
#include <ranges>
 
int main()
{
    int x[]{1, 2, 3, 4, 5};
 
    auto v1 = x | std::views::drop(2);
    assert(v1.back() == 5);
    v1[0]++; // OK, can modify non-const element
 
    auto v2 = x | std::views::drop(2) | std::views::as_const;
    assert(v2.back() == 5);
    // v2[0]++; // Compile-time error, cannot modify read-only element
}

[edit] See also

(C++23)
 a view of a sequence that casts each element to an rvalue
(class template) (range adaptor object)[edit]
(C++20)
 returns an iterator to the beginning of a read-only range
(customization point object)[edit]
(C++20)
 returns a sentinel indicating the end of a read-only range
(customization point object)[edit]
(C++17)
 obtains a reference to const to its argument
(function template) [edit]
(C++23)
 iterator adaptor that converts an iterator into a constant iterator
(class template) [edit]
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