NAME¶

std::unitbuf,std::nounitbuf - std::unitbuf,std::nounitbuf

Synopsis¶

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)
<< " ... ";
}

int main()
{
volatile int sink = 0;
std::cout << std::unitbuf; // enable automatic flushing

const auto start = 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
log_progress(std::chrono::high_resolution_clock::now() - start);
}
std::cout << '\n';
}

Output:¶

571ms ... 1146ms ... 1722ms ... 2294ms ... 2865ms ...

See also¶

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