cpp/io/basic filebuf/setbuf

If is a null pointer and  is zero, the filebuf becomes unbuffered for output, meaning  and  are null and any output is immediately sent to file.

Otherwise, a call to replaces the internal buffer (the controlled character sequence) with the user-supplied character array whose first element is pointed to by  and allows this std object to use up to  bytes in that array for buffering.

This function is protected virtual, it may only be called through or from member functions of a user-defined class derived from.