25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.14 Join view [range.join]

25.7.14.1 Overview [range.join.overview]

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join_view flattens a view of ranges into a view.
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The name views​::​join denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​join(E) is expression-equivalent to join_view<views​::​all_t<decltype((E))>>{E}.
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[Example 1: vector<string> ss{"hello", " ", "world", "!"}; for (char ch : ss | views::join) cout << ch; // prints hello world! — end example]