std::basic_ostream<CharT,Traits>::flush

From cppreference.com
< cpp‎ | io‎ | basic ostream
 
 
 
 
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

#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
LWG 581 C++98 flush() did not behave as an UnformattedOutputFunction
because of the resolution of LWG issue 60
behaves as an
UnformattedOutputFunction

See also

invokes sync()
(public member function of std::basic_streambuf<CharT,Traits>)
[virtual]
synchronizes the buffers with the associated character sequence
(virtual protected member function of std::basic_streambuf<CharT,Traits>)
flushes the output stream
(function template)
outputs '\n' and flushes the output stream
(function template)
synchronizes with the underlying storage device
(public member function of std::basic_istream<CharT,Traits>)