29 Input/output library [input.output]

29.10 Synchronized output streams [syncstream]

29.10.3 Class template basic_­osyncstream [syncstream.osyncstream]

29.10.3.1 Overview [syncstream.osyncstream.overview]

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>; // [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; ~basic_osyncstream(); // 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)); } private: syncbuf_type sb; // exposition only }; }
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Allocator shall meet the Cpp17Allocator requirements (Table 36).
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[Example 1:
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]
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[Example 2:
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]

29.10.3.2 Construction and destruction [syncstream.osyncstream.cons]

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basic_osyncstream(streambuf_type* buf, const Allocator& allocator);
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Effects: Initializes sb from buf and allocator.
Initializes the base class with basic_­ostream<charT, traits>(addressof(sb)).
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[Note 1:
The member functions of the provided stream buffer can be called from emit() while a lock is held, which might result in a deadlock if used incautiously.
— end note]
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Postconditions: get_­wrapped() == buf is true.
🔗
basic_osyncstream(basic_osyncstream&& other) noexcept;
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Effects: Move constructs the base class and sb from the corresponding subobjects of other, and calls basic_­ostream<charT, traits>​::​set_­rdbuf(addressof(sb)).
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Postconditions: The value returned by get_­wrapped() is the value returned by os.get_­wrapped() prior to calling this constructor.
nullptr == other.get_­wrapped() is true.

29.10.3.3 Member functions [syncstream.osyncstream.members]

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void emit();
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Effects: Calls sb.emit().
If that call returns false, calls setstate(ios_­base​::​badbit).
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[Example 1:
A flush on a basic_­osyncstream does not flush immediately: { osyncstream bout(cout); bout << "Hello," << '\n'; // no flush bout.emit(); // characters transferred; cout not flushed bout << "World!" << endl; // flush noted; cout not flushed bout.emit(); // characters transferred; cout flushed bout << "Greetings." << '\n'; // no flush } // characters transferred; cout not flushed
— end example]
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[Example 2:
The function emit() can be used to handle exceptions from operations on the underlying stream.
{ osyncstream bout(cout); bout << "Hello, " << "World!" << '\n'; try { bout.emit(); } catch (...) { // handle exception } } — end example]
🔗
streambuf_type* get_wrapped() const noexcept;
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Returns: sb.get_­wrapped().
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[Example 3:
Obtaining the wrapped stream buffer with get_­wrapped() allows wrapping it again with an osyncstream.
For example, { osyncstream bout1(cout); bout1 << "Hello, "; { osyncstream(bout1.get_wrapped()) << "Goodbye, " << "Planet!" << '\n'; } bout1 << "World!" << '\n'; } produces the uninterleaved output 
Goodbye, Planet!
Hello, World!
— end example]