You can use stdio with C++ programs, but problems can occur when you mix iostreams and stdio in the same standard stream within a program. For example, if you write to both stdout and cout, independent buffering occurs and produces unexpected results. The problem is worse if you input from both stdin and cin because independent buffering could render the input unusable.
To eliminate this problem with standard input, standard output, and standard error, use the following instruction before performing any input or output. It connects all the predefined iostreams with the corresponding predefined stdio FILEs.
ios::sync_with_stdio();
This type of a connection is not the default because a significant performance penalty occurs when the predefined streams are made unbuffered as part of the connection. You can use both stdio and iostreams in the same program applied to different files, that is, you can write to stdout using stdio routines and write to other files attached to iostreams. You can open stdio FILEs for input and also read from cin so long as you don’t also try to read from stdin.