std::flush
| I/O manipulators | ||||
| Print functions (C++23) | ||||
| C-style I/O | ||||
| Buffers | ||||
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(C++98/26*) | ||||
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| Streams | ||||
| Abstractions | ||||
| File I/O | ||||
| String I/O | ||||
| Array I/O | ||||
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(C++23) | ||||
(C++23) | ||||
(C++98/26*) | ||||
(C++98/26*) | ||||
(C++98/26*) | ||||
| Synchronized Output | ||||
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| Types | ||||
| Error category interface | ||||
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(C++11) |
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| Output flushing | |||||||||||||||||||||||||||||||
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| Quoted manipulator | |||||||||||||||||||||||||||||||
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| Defined in header <ostream>
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| template< class CharT, class Traits > std::basic_ostream<CharT, Traits>& flush( std::basic_ostream<CharT, Traits>& os ); |
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Flushes the output sequence os as if by calling os.flush().
This is an output-only I/O manipulator, it may be called with an expression such as out << std::flush for any out of type std::basic_ostream.
Contents |
[edit] Notes
This manipulator may be used to produce an incomplete line of output immediately, e.g. when displaying output from a long-running process, logging activity of multiple threads or logging activity of a program that may crash unexpectedly. An explicit flush of std::cout is also necessary before a call to std::system, if the spawned process performs any screen I/O (a common example is std::system("pause") on Windows). In most other usual interactive I/O scenarios, std::endl is redundant when used with std::cout because any input from std::cin, output to std::cerr, or program termination forces a call to std::cout.flush().
When a complete line of output needs to be flushed, the std::endl manipulator may be used.
When every output operation needs to be flushed, the std::unitbuf manipulator may be used.
[edit] Parameters
| os | - | reference to output stream |
[edit] Return value
os (reference to the stream after manipulation).
[edit] Example
Without std::flush, the output would be the same, but may not appear in real time.
#include <chrono> #include <iostream> template<typename Diff> void log_progress(Diff d) { std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(d) << " ... " << std::flush; } int main() { volatile int sink = 0; 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'; }
Possible output:
567ms ... 1137ms ... 1707ms ... 2269ms ... 2842ms ...
[edit] See also
| controls whether output is flushed after each operation (function) [edit] | |
| outputs '\n' and flushes the output stream (function template) [edit] | |
| synchronizes with the underlying storage device (public member function of std::basic_ostream<CharT,Traits>) [edit]
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