Standard library header <syncstream>

#include <ostream>
 
namespace std {
  template<class CharT, class Traits = char_traits<CharT>,
           class Allocator = allocator<CharT>>
    class basic_syncbuf;
 
  using syncbuf = basic_syncbuf<char>;
  using wsyncbuf = basic_syncbuf<wchar_t>;
 
  template<class CharT, class Traits = char_traits<CharT>,
           class Allocator = allocator<CharT>>
    class basic_osyncstream;
 
  using osyncstream = basic_osyncstream<char>;
  using wosyncstream = basic_osyncstream<wchar_t>;
}

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>;
 
    // 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();
 
    // assignment and swap
    basic_syncbuf& operator=(basic_syncbuf&&);
    void swap(basic_syncbuf&);
 
    // member functions
    bool emit();
    streambuf_type* get_wrapped() const noexcept;
    allocator_type get_allocator() const noexcept;
    void set_emit_on_sync(bool) noexcept;
 
  protected:
    // overridden virtual functions
    int sync() override;
 
  private:
    streambuf_type* wrapped;    // exposition only
    bool emit_on_sync{};        // exposition only
  };
 
  // specialized algorithms
  template<class CharT, class Traits, class Allocator>
    void swap(basic_syncbuf<CharT, Traits, Allocator>&,
              basic_syncbuf<CharT, Traits, Allocator>&);
}

namespace std {
  template<class CharT, class Traits = char_traits<CharT>,
           class Allocator = allocator<CharT>>
  class basic_osyncstream : public basic_ostream<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>;
    using syncbuf_type   = basic_syncbuf<CharT, Traits, Allocator>;
 
    // 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;
    ~basic_osyncstream();
 
    // assignment
    basic_osyncstream& operator=(basic_osyncstream&&) noexcept;
 
    // member functions
    void emit();
    streambuf_type* get_wrapped() const noexcept;
    syncbuf_type* rdbuf() const noexcept
    { return const_cast<syncbuf_type*>(addressof(sb)); }
 
  private:
    syncbuf_type sb;    // exposition only
  };
}