25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.22 As const view [range.as.const]

25.7.22.1 Overview [range.as.const.overview]

1
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as_const_view presents a view of an underlying sequence as constant.
That is, the elements of an as_const_view cannot be modified.
2
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The name views​::​as_const denotes a range adaptor object ([range.adaptor.object]).
Let E be an expression, let T be decltype((E)), and let U be remove_cvref_t<T>.
The expression views​::​as_const(E) is expression-equivalent to:
  • (2.1)
    If views​::​all_t<T> models constant_range, then views​::​all(E).
  • (2.2)
    Otherwise, if U denotes empty_view<X> for some type X, then auto(views​::​empty<const X>).
  • (2.3)
    Otherwise, if U denotes span<X, Extent> for some type X and some extent Extent, then span<const X, Extent>(E).
  • (2.4)
    Otherwise, if U denotes ref_view<X> for some type X and const X models constant_range, then ref_view(static_cast<const X&>(E.base())).
  • (2.5)
    Otherwise, if E is an lvalue, const U models constant_range, and U does not model view, then ref_view(static_cast<const U&>(E)).
  • (2.6)
    Otherwise, as_const_view(E).
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[Example 1: template<constant_range 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 — end example]