[syncstream.osyncstream]

# 31 Input/output library [[input.output]](./#input.output)

## 31.11 Synchronized output streams [[syncstream]](syncstream#osyncstream)

### 31.11.3 Class template basic_osyncstream [syncstream.osyncstream]

#### [31.11.3.1](#overview) Overview [[syncstream.osyncstream.overview]](syncstream.osyncstream.overview)

[🔗](#lib:basic_osyncstream)

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]](#cons "31.11.3.2 Construction and destruction"), 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&&); // [[syncstream.osyncstream.members]](#members "31.11.3.3 Member functions"), member functionsvoid 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*};}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12958)

Allocator shall meet
the [*Cpp17Allocator*](allocator.requirements.general#:Cpp17Allocator "16.4.4.6.1 General [allocator.requirements.general]") requirements ([[allocator.requirements.general]](allocator.requirements.general "16.4.4.6.1 General"))[.](#overview-1.sentence-1)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12962)

[*Example [1](#overview-example-1)*:

A named variable can be used within a block statement for streaming[.](#overview-2.sentence-1)

{ 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*]

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12976)

[*Example [2](#overview-example-2)*:

A temporary object can be used for streaming within a single statement[.](#overview-3.sentence-1)

osyncstream(cout) << "Hello, " << "World!" << '\n';

In this example, cout is not flushed[.](#overview-3.sentence-2)

— *end example*]

#### [31.11.3.2](#cons) Construction and destruction [[syncstream.osyncstream.cons]](syncstream.osyncstream.cons)

[🔗](#lib:basic_osyncstream,constructor)

`basic_osyncstream(streambuf_type* buf, const Allocator& allocator);
`

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12993)

*Effects*: Initializes *sb* from buf and allocator[.](#cons-1.sentence-1)

Initializes the base class with basic_ostream<charT, traits>(addressof(*sb*))[.](#cons-1.sentence-2)

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12998)

[*Note [1](#cons-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[.](#cons-2.sentence-1)

— *end note*]

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13005)

*Postconditions*: get_wrapped() == buf is true[.](#cons-3.sentence-1)

[🔗](#lib:basic_osyncstream,constructor_)

`basic_osyncstream(basic_osyncstream&& other) noexcept;
`

[4](#cons-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13016)

*Effects*: Move constructs the base class
and *sb* from the corresponding subobjects of other,
and calls basic_ostream<charT, traits>​::​set_rdbuf(addressof(*sb*))[.](#cons-4.sentence-1)

[5](#cons-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13022)

*Postconditions*: The value returned by get_wrapped() is the value returned by other.get_wrapped() prior to calling this constructor[.](#cons-5.sentence-1)

nullptr == other.get_wrapped() is true[.](#cons-5.sentence-2)

#### [31.11.3.3](#members) Member functions [[syncstream.osyncstream.members]](syncstream.osyncstream.members)

[🔗](#lib:set_emit_on_sync,basic_osyncstream)

`void emit();
`

[1](#members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13038)

*Effects*: Behaves as an unformatted output function ([[ostream.unformatted]](ostream.unformatted "31.7.6.4 Unformatted output functions"))[.](#members-1.sentence-1)

After constructing a sentry object, calls *sb*.emit()[.](#members-1.sentence-2)

If that call returns false,
calls setstate(ios_base​::​badbit)[.](#members-1.sentence-3)

[2](#members-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13045)

[*Example [1](#members-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*]

[3](#members-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13060)

[*Example [2](#members-example-2)*:

The function emit() can be used to
handle exceptions from operations on the underlying stream[.](#members-3.sentence-1)

{ osyncstream bout(cout);
 bout << "Hello, " << "World!" << '\n'; try { bout.emit(); } catch (...) {// handle exception}} — *end example*]

[🔗](#lib:set_emit_on_sync,basic_osyncstream_)

`streambuf_type* get_wrapped() const noexcept;
`

[4](#members-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13084)

*Returns*: *sb*.get_wrapped()[.](#members-4.sentence-1)

[5](#members-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L13088)

[*Example [3](#members-example-3)*:

Obtaining the wrapped stream buffer with get_wrapped() allows wrapping it again with an osyncstream[.](#members-5.sentence-1)

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*]