29 Input/output library [input.output]

29.10 Synchronized output streams [syncstream]

29.10.3 Class template basic_­osyncstream [syncstream.osyncstream] Overview [syncstream.osyncstream.overview]

namespace std {
  template<class charT, class traits, class Allocator>
  class basic_osyncstream : public basic_ostream<charT, traits> {
    using char_type   = charT;
    using int_type    = typename traits::int_type;
    using pos_type    = typename traits::pos_type;
    using off_type    = typename traits::off_type;
    using traits_type = traits;

    using allocator_type = Allocator;
    using streambuf_type = basic_streambuf<charT, traits>;
    using syncbuf_type   = basic_syncbuf<charT, traits, Allocator>;

    // [syncstream.osyncstream.cons], construction and destruction
    basic_osyncstream(streambuf_type*, const Allocator&);
    explicit basic_osyncstream(streambuf_type* obuf)
      : basic_osyncstream(obuf, Allocator()) {}
    basic_osyncstream(basic_ostream<charT, traits>& os, const Allocator& allocator)
      : basic_osyncstream(os.rdbuf(), allocator) {}
    explicit basic_osyncstream(basic_ostream<charT, traits>& os)
      : basic_osyncstream(os, Allocator()) {}
    basic_osyncstream(basic_osyncstream&&) noexcept;

    // [syncstream.osyncstream.assign], assignment
    basic_osyncstream& operator=(basic_osyncstream&&) noexcept;

    // [syncstream.osyncstream.members], member functions
    void emit();
    streambuf_type* get_wrapped() const noexcept;
    syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(addressof(sb)); }

    syncbuf_type sb;    // exposition only
Allocator shall satisfy the Cpp17Allocator requirements ([tab:utilities.allocator.requirements]Table *tab:utilities.allocator.requirements).
A named variable can be used within a block statement for streaming.
  osyncstream bout(cout);
  bout << "Hello, ";
  bout << "World!";
  bout << endl; // flush is noted
  bout << "and more!\n";
}   // characters are transferred and cout is flushed
end example
A temporary object can be used for streaming within a single statement.
osyncstream(cout) << "Hello, " << "World!" << '\n';
In this example, cout is not flushed.
end example