90 lines
5.5 KiB
Markdown
90 lines
5.5 KiB
Markdown
[range.as.const]
|
||
|
||
# 25 Ranges library [[ranges]](./#ranges)
|
||
|
||
## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.as.const)
|
||
|
||
### 25.7.22 As const view [range.as.const]
|
||
|
||
#### [25.7.22.1](#overview) Overview [[range.as.const.overview]](range.as.const.overview)
|
||
|
||
[1](#overview-1)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10028)
|
||
|
||
as_const_view presents a view of an underlying sequence as constant[.](#overview-1.sentence-1)
|
||
|
||
That is, the elements of an as_const_view cannot be modified[.](#overview-1.sentence-2)
|
||
|
||
[2](#overview-2)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10032)
|
||
|
||
The name views::as_const denotes
|
||
a range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1)
|
||
|
||
Let E be an expression,
|
||
let T be decltype((E)), and
|
||
let U be remove_cvref_t<T>[.](#overview-2.sentence-2)
|
||
|
||
The expression views::as_const(E) is expression-equivalent to:
|
||
|
||
- [(2.1)](#overview-2.1)
|
||
|
||
If views::all_t<T> models [constant_range](range.refinements#concept:constant_range "25.4.6 Other range refinements [range.refinements]"),
|
||
then views::all(E)[.](#overview-2.1.sentence-1)
|
||
|
||
- [(2.2)](#overview-2.2)
|
||
|
||
Otherwise,
|
||
if U denotes empty_view<X> for some type X, then auto(views::empty<const X>)[.](#overview-2.2.sentence-1)
|
||
|
||
- [(2.3)](#overview-2.3)
|
||
|
||
Otherwise,
|
||
if U denotes span<X, Extent> for some type X and some extent Extent,
|
||
then span<const X, Extent>(E)[.](#overview-2.3.sentence-1)
|
||
|
||
- [(2.4)](#overview-2.4)
|
||
|
||
Otherwise,
|
||
if U denotes ref_view<X> for some type X andconst X models [constant_range](range.refinements#concept:constant_range "25.4.6 Other range refinements [range.refinements]"),
|
||
then ref_view(static_cast<const X&>(E.base()))[.](#overview-2.4.sentence-1)
|
||
|
||
- [(2.5)](#overview-2.5)
|
||
|
||
Otherwise,
|
||
if E is an lvalue,const U models [constant_range](range.refinements#concept:constant_range "25.4.6 Other range refinements [range.refinements]"), andU does not model [view](range.view#concept:view "25.4.5 Views [range.view]"),
|
||
then ref_view(static_cast<const U&>(E))[.](#overview-2.5.sentence-1)
|
||
|
||
- [(2.6)](#overview-2.6)
|
||
|
||
Otherwise, as_const_view(E)[.](#overview-2.6.sentence-1)
|
||
|
||
[3](#overview-3)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10067)
|
||
|
||
[*Example [1](#overview-example-1)*: template<[constant_range](range.refinements#concept:constant_range "25.4.6 Other range refinements [range.refinements]") R>void cant_touch_this(R&&);
|
||
|
||
vector<char> hammer = {'m', 'c'};
|
||
span<char> beat = hammer;
|
||
cant_touch_this(views::as_const(beat)); // will not modify the elements of hammer â *end example*]
|
||
|
||
#### [25.7.22.2](#view) Class template as_const_view [[range.as.const.view]](range.as.const.view)
|
||
|
||
[ð](#lib:as_const_view::iterator)
|
||
|
||
namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>requires [input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]")<V>class as_const_view : public view_interface<as_const_view<V>> { V *base_* = V(); // *exposition only*public: as_const_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_initializable [concept.default.init]")<V> = default; constexpr explicit as_const_view(V base); constexpr V base() const & requires [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_constructible [concept.copyconstructible]")<V> { return *base_*; }constexpr V base() && { return std::move(*base_*); }constexpr auto begin() requires (<V>) { return ranges::cbegin(*base_*); }constexpr auto begin() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> { return ranges::cbegin(*base_*); }constexpr auto end() requires (<V>) { return ranges::cend(*base_*); }constexpr auto end() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> { return ranges::cend(*base_*); }constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<V> { return ranges::size(*base_*); }constexpr auto size() const requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V> { return ranges::size(*base_*); }constexpr auto reserve_hint() requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<V>{ return ranges::reserve_hint(*base_*); }constexpr auto reserve_hint() const requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<const V>{ return ranges::reserve_hint(*base_*); }}; template<class R> as_const_view(R&&) -> as_const_view<views::all_t<R>>;}
|
||
|
||
[ð](#lib:as_const_view::iterator,constructor)
|
||
|
||
`constexpr explicit as_const_view(V base);
|
||
`
|
||
|
||
[1](#view-1)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L10122)
|
||
|
||
*Effects*: Initializes *base_* with std::move(base)[.](#view-1.sentence-1)
|