29 Input/output library [input.output]

29.10 Synchronized output streams [syncstream]

29.10.2 Class template basic_­syncbuf [syncstream.syncbuf]

29.10.2.1 Overview [syncstream.syncbuf.overview]

namespace std {
  template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
  class basic_syncbuf : public basic_streambuf<charT, traits> {
  public:
    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>;

    // [syncstream.syncbuf.cons], construction and destruction
    basic_syncbuf()
      : basic_syncbuf(nullptr) {}
    explicit basic_syncbuf(streambuf_type* obuf)
      : basic_syncbuf(obuf, Allocator()) {}
    basic_syncbuf(streambuf_type*, const Allocator&);
    basic_syncbuf(basic_syncbuf&&);
    ~basic_syncbuf();

    // [syncstream.syncbuf.assign], assignment and swap
    basic_syncbuf& operator=(basic_syncbuf&&);
    void swap(basic_syncbuf&);

    // [syncstream.syncbuf.members], member functions
    bool emit();
    streambuf_type* get_wrapped() const noexcept;
    allocator_type get_allocator() const noexcept;
    void set_emit_on_sync(bool) noexcept;

  protected:
    // [syncstream.syncbuf.virtuals], overridden virtual functions
    int sync() override;

  private:
    streambuf_type* wrapped;    // exposition only
    bool emit_on_sync{};        // exposition only
  };

  // [syncstream.syncbuf.special], specialized algorithms
  template<class charT, class traits, class Allocator>
    void swap(basic_syncbuf<charT, traits, Allocator>&,
              basic_syncbuf<charT, traits, Allocator>&);
}
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Class template basic_­syncbuf stores character data written to it, known as the associated output, into internal buffers allocated using the object's allocator.
The associated output is transferred to the wrapped stream buffer object *wrapped when emit() is called or when the basic_­syncbuf object is destroyed.
Such transfers are atomic with respect to transfers by other basic_­syncbuf objects with the same wrapped stream buffer object.