std::basic_istream::sync(3)

NAME
std::basic_istream::sync - std::basic_istream::sync

Synopsis
   int sync();
   Synchronizes the input buffer with the associated data source.
   Behaves as UnformattedInputFunction, except that gcount() is not affected. After
   constructing and checking the sentry object,
   If rdbuf() is a null pointer, returns -1.
   Otherwise, calls rdbuf()->pubsync(). If that function returns -1, calls
   setstate(badbit) and returns -1. Otherwise, returns 0.

Parameters
   (none)

Return value
   0 on success, -1 on failure or if the stream does not support this operation (is
   unbuffered).

Notes
   As with readsome(), it is implementation-defined whether this function does anything
   with library-supplied streams. The intent is typically for the next read operation
   to pick up any changes that may have been made to the associated input sequence
   after the stream buffer last filled its get area. To achieve that, sync() may empty
   the get area, or it may refill it, or it may do nothing. A notable exception is
   Visual Studio, where this operation discards the unprocessed input when called with
   a standard input stream.

Example
   Demonstrates the use of input stream sync() with file input. Note that output here
   is implementation-defined, since calls to std::basic_filebuf::sync are
   implementation-defined for reads. // Run this code
 #include <fstream>
 #include <iostream>
 void file_abc()
 {
     std::ofstream f("test.txt");
     f << "abc\n";
 }
 void file_123()
 {
     std::ofstream f("test.txt");
     f << "123\n";
 }
 int main()
 {
     file_abc(); // file now contains "abc"
     std::ifstream f("test.txt");
     std::cout << "Reading from the file\n";
     char c;
     f >> c;
     std::cout << c;
     file_123(); // file now contains "123"
     f >> c;
     std::cout << c;
     f >> c;
     std::cout << c << '\n';
     f.close();
     file_abc(); // file now contains "abc"
     f.open("test.txt");
     std::cout << "Reading from the file, with sync()\n";
     f >> c;
     std::cout << c;
     file_123(); // file now contains "123"
     f.sync();
     f >> c;
     std::cout << c;
     f >> c;
     std::cout << c << '\n';
 }

Possible output:
 Reading from the file
 abc
 Reading from the file, with sync()
 a23
   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 62 C++98 sync() returned traits::eof() if returns -1 in this
                     rdbuf()->pubsync() returns -1 case

See also
   sync synchronizes the buffers with the associated character sequence
   [virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)
   flush synchronizes with the underlying storage device
             (public member function of std::basic_ostream<CharT,Traits>)