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Showing content from https://cplusplus.com/reference/istream/basic_istream/sync/ below:

public member function

<istream> <iostream>

std::basic_istream::sync

Synchronize input buffer

Synchronizes the associated stream buffer with its controlled input sequence.

Specifics of the operation depend on the particular implementation of the stream buffer object associated to the stream.

Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true). Then (if good), it calls pubsync on its associated stream buffer object (if rdbuf is null, the function returns -1 instead). Finally, it destroys the sentry object before returning.

If the call to pubsync fails (i.e., it returns -1), the function sets the badbit flag, and returns -1. Otherwise it returns zero, indicating success.

Notice that the called function may succeed when no action is performed, if that is the behavior defined for the stream buffer object on synchronization.

Calling this function does not alter the value returned by gcount.



Parameters none

Return Value If the function fails, either because no stream buffer object is associated to the stream (rdbuf is null), or because the call to its pubsync member fails, it returns -1.
Otherwise, it returns zero, indicating success.

Errors are signaled by modifying the internal state flags:


flag error eofbit - failbit The construction of sentry failed (such as when the stream state was not good before the call). badbit Either the internal call to pubsync returned -1, or some other error occurred on the stream (such as when the function catches an exception thrown by an internal operation).
When set, the integrity of the stream may have been affected. Multiple flags may be set by a single operation.

If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.



Example 
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// syncing input stream
#include <iostream>     // std::cin, std::cout

int main () {
  char first, second;

  std::cout << "Please, enter a word: ";
  first = std::cin.get();
  std::cin.sync();

  std::cout << "Please, enter another word: ";
  second = std::cin.get();

  std::cout << "The first word began by " << first << '\n';
  std::cout << "The second word began by " << second << '\n';

  return 0;
}

This example demonstrates how sync behaves on certain implementations of cin, removing any unread character from the standard input queue of characters.

Possible output:


Please, enter a word: test
Please enter another word: text
The first word began by t
The second word began by t


Data races Modifies the stream object.
Concurrent access to the same stream object may cause data races, except for the standard stream objects cin and wcin when these are synchronized with stdio (in this case, no data races are initiated, although no guarantees are given on the order in which characters are extracted or synchronized between threads).

Exception safetyBasic guarantee: if an exception is thrown, the object is in a valid state.
It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state.
Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.

See also
basic_ostream::flush
Flush output stream buffer (public member function)
basic_streambuf::pubsync
Synchronize stream buffer (public member function)

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