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

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< cpp‎ | ranges
m (→‎Example: +)
m (→‎Notes: lc => c/core as a temporary measure to generate link.)
 
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{{dsc begin}}
 
{{dsc begin}}
 
{{dsc hitem|Member object|Definition}}
 
{{dsc hitem|Member object|Definition}}
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{{dsc|{{tti|base_}} {{mark|private}}|The underlying view of type {{tt|V}}.<br>{{mark expos mem obj}}}}
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{{dsc expos mem obj|base_|private=yes|The underlying view of type {{tt|V}}.}}
 
{{dsc end}}
 
{{dsc end}}
  
 
===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc mem ctor|{{PAGENAME}}#ctor|constructs an {{tt|as_const_view}}|notes={{mark c++23}}}}
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{{dsc mem ctor|{{PAGENAME}}#ctor|constructs an {{tt|as_const_view}}}}
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{{dsc mem fun|{{PAGENAME}}#base|title=base|returns the underlying view {{tt|V}}|notes={{mark c++23}}}}
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{{dsc mem fun|{{PAGENAME}}#base|title=base|returns the underlying view {{tt|V}}}}
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{{dsc mem fun|{{PAGENAME}}#begin|title=begin|returns the beginning iterator of the {{tt|as_const_view}}|notes={{mark c++23}}}}
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{{dsc mem fun|{{PAGENAME}}#begin|title=begin|returns the beginning iterator of the {{tt|as_const_view}}}}
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{{dsc mem fun|{{PAGENAME}}#end|title=end|returns the end iterator of the {{tt|as_const_view}}|notes={{mark c++23}}}}
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{{dsc mem fun|{{PAGENAME}}#end|title=end|returns the end iterator of the {{tt|as_const_view}}}}
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{{dsc mem fun|{{PAGENAME}}#size|title=size|returns the size of the view if it is bounded|notes={{mark c++23}}}}
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{{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}}
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{{ddcl|since=c++23|1=
 
{{ddcl|since=c++23|1=
 
template< class T >
 
template< class T >
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inline constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =
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constexpr bool enable_borrowed_range<std::ranges::as_const_view<T>> =
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     std::ranges::enable_borrowed_range<T>;
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     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===
 
===Notes===
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{{feature_test_macro|__cpp_lib_ranges_as_const|value=202207L|std=C++23|{{ttt|std::ranges::as_const_view}}, {{lc|std::const_iterator}}}}
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{{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===
 
{{example|code=
 
{{example|code=
 +
#include <cassert>
 
#include <ranges>
 
#include <ranges>
−
+
 
 
int main()
 
int main()
 
{
 
{
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     int x[] {0, 1, 2};
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     int x[]{1, 2, 3, 4, 5};
−
   
+
−
    auto v1 = std::views::all(x);
+
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    v1[0]++; // OK, can modify non-const object
+
  
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     [[maybe_unused]]
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     auto v1 = x {{!}} std::views::drop(2);
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     auto v2 = std::views::as_const(x);
+
    assert(v1.back() == 5);
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     // v2[0]++; // Error, all elements of v2 are constant
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    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
 
}
 
}
 
}}
 
}}
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{{dsc inc|cpp/iterator/dsc basic_const_iterator}}
 
{{dsc inc|cpp/iterator/dsc basic_const_iterator}}
 
{{dsc end}}
 
{{dsc end}}
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 +
{{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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