NAME¶

std::basic_ostream::flush - std::basic_ostream::flush

Synopsis¶

basic_ostream& flush();

Writes uncommitted changes to the underlying output sequence. Behaves as an
UnformattedOutputFunction.

If rdbuf() is a null pointer, the sentry object is not constructed.

Otherwise, after constructing and checking the sentry object, calls
rdbuf()->pubsync(). If the call returns -1, calls setstate(badbit).

Parameters¶

(none)

Return value¶

*this

Exceptions¶

May throw std::ios_base::failure if (exceptions() & badbit) != 0.

Example¶

// Run this code

#include <chrono>
#include <iostream>
#include <thread>

using namespace std::chrono_literals;

void f()
{
std::cout << "Output from thread... ";
for (int i{1}; i != 10; ++i)
{
std::this_thread::sleep_for(250ms);
std::cout << i << ' ';

// output three numbers at once;
// the effect is observable only in real-time
if (0 == (i % 3))
std::cout.flush();
}
std::cout << std::endl; // flushes as well
}

int main()
{
std::thread tr{f};
std::this_thread::sleep_for(150ms);
std::clog << "Output from main\n";
tr.join();
}

Output:¶

Output from main
Output from thread... 1 2 3 4 5 6 7 8 9

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
flush() did not behave as an
LWG 581 C++98 UnformattedOutputFunction behaves as an
because of the resolution of LWG issue UnformattedOutputFunction
60

See also¶

pubsync invokes sync()
(public member function of std::basic_streambuf<CharT,Traits>)
sync synchronizes the buffers with the associated character sequence
[virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)

flush flushes the output stream
(function template)
endl outputs '\n' and flushes the output stream
(function template)
sync synchronizes with the underlying storage device
(public member function of std::basic_istream<CharT,Traits>)