std::unitbuf, std::nounitbuf

 

C++

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

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Diagnostics library

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Input/output library

I/O manipulators

Print functions (C++23)

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

basic_spanbuf (C++23)

strstreambuf (C++98/26*)

basic_syncbuf (C++20)

Streams

Abstractions

ios_base

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basic_ifstream

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basic_fstream

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basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

basic_ispanstream (C++23)

basic_ospanstream (C++23)

basic_spanstream (C++23)

istrstream (C++98/26*)

ostrstream (C++98/26*)

strstream (C++98/26*)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

Input/output manipulators

Floating-point formatting
showpointnoshowpoint setprecision	fixedscientifichexfloatdefaultfloat (C++11)(C++11)
Integer formatting
setbase showbasenoshowbase	dechexoct
Boolean formatting
boolalphanoboolalpha
Field width and fill control
setfill setw	internalleftright
Other formatting
showposnoshowpos	uppercasenouppercase
Whitespace processing
ws ends	skipwsnoskipws
Output flushing
flush endl flush_emit (C++20)	unitbufnounitbuf emit_on_flushnoemit_on_flush (C++20)(C++20)
Status flags manipulation
resetiosflags	setiosflags
Time and money I/O
get_money (C++11) get_time (C++11)	put_money (C++11) put_time (C++11)
Quoted manipulator
quoted (C++14)

 

Defined in header <ios>
std::ios_base& unitbuf( std::ios_base& str );	(1)
std::ios_base& nounitbuf( std::ios_base& str );	(2)

Enables or disables automatic flushing of the output stream after any output operation. Has no effect on input.

1) Enables the unitbuf flag in the stream str as if by calling str.setf(std::ios_base::unitbuf).

2) Disables the unitbuf flag in the stream str as if by calling str.unsetf(std::ios_base::unitbuf).

This is an I/O manipulator, it may be called with an expression such as out << std::unitbuf for any out of type std::basic_ostream or with an expression such as in >> std::unitbuf for any in of type std::basic_istream.

Notes

Flushing is performed in the destructor of the std::basic_ostream::sentry object, which calls str.rdbuf()->pubsync() if str.flags() & std::ios_base::unitbuf is true.

The standard output objects std::cerr and std::wcerr have their unitbuf bit set by default.

Parameters

str

-

reference to I/O stream

Return value

str (reference to the stream after manipulation).

Example

Without std::unitbuf or another explicit flush, the output is the same, but does not appear in real time.

Run this code

#include <chrono>
#include <iostream>
 
template<typename Diff>
void log_progress(Diff d)
{
    std::cout << "..("
              << std::chrono::duration_cast<std::chrono::milliseconds>(d).count()
              << " ms)..";
}
 
int main()
{
    volatile int sink = 0;
    std::cout << std::unitbuf; // enable automatic flushing
 
    auto t1 = std::chrono::high_resolution_clock::now();
    for (int j = 0; j < 5; ++j)
    {
        for (int n = 0; n < 10000; ++n)
            for (int m = 0; m < 20000; ++m)
                sink += m * n; // do some work
        auto now = std::chrono::high_resolution_clock::now();
        log_progress(now - t1);
    }
    std::cout << '\n';
}

Output:

..(450 ms)....(902 ms)....(1352 ms)....(1802 ms)....(2252 ms)..

See also

flush	flushes the output stream (function template)
endl	outputs '\n' and flushes the output stream (function template)