24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.10 Join view [range.join]

24.7.10.1 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{E}.
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[ Example
: 
vector<string> ss{"hello", " ", "world", "!"};
join_view greeting{ss};
for (char ch : greeting)
  cout << ch;                                   // prints: hello world!
— end example
 ]