It is used to set buffer and virtual function called by the public member function pubsetbuf to influence the object in a way that is dependent on the library implementation.
Following is the declaration for std::stringbuf::setbuf.
streambuf* setbuf (char* s, streamsize n);
s − The pointer to the sequence of characters to be written.
n − The number of characters to write.
It returns this.
Basic guarantee − if an exception is thrown, the object is in a valid state.
if s,n both arguments are zero, it introduces no data races. Otherwise, unspecified.