#include <iostream>#include <sstream> struct mybuf:std::stringbuf{    mybuf(conststd::string& new_str,std::ios_base::openmode which=std::ios_base::in|std::ios_base::out):std::stringbuf(new_str, which){}     int_type overflow(int_type c){std::cout<<"Before overflow():  get area size is "<< egptr()- eback()<<", the put area size is "<< epptr()- pbase()<<'\n';        int_type rc= std::stringbuf::overflow(c);std::cout<<"After overflow():   get area size is "<< egptr()- eback()<<", put area size is "<< epptr()- pbase()<<'\n';return rc;}     int_type underflow(){std::cout<<"Before underflow(): get area size is "<< egptr()- eback()<<", put area size is "<< epptr()- pbase()<<'\n';        int_type ch= std::stringbuf::underflow();std::cout<<"After underflow():  get area size is "<< egptr()- eback()<<", put area size is "<< epptr()- pbase()<<'\n'; if(ch==EOF)std::cout<<"underflow() returns EOF\n";elsestd::cout<<"underflow() returns '"<<char(ch)<<"'\n";return ch;}}; int main(){    mybuf sbuf("123");// read-write streamstd::iostream stream(&sbuf);int n;    stream>> n;// calls sgetc() four times// three calls return the characters '1', '2', '3'// the fourth call, gptr() == egptr() and underflow() is called// underflow returns EOFstd::cout<<"n = "<< n<<'\n';    stream.clear();// clear the eofbit     stream<<"123456";// sputc() is called 6 times// first three calls store "123" in the existing buffer// 4th call finds that pptr() == epptr() and calls overflow()// overflow() grows the buffer and sets egptr() to 4// 5th and 6th calls store '5' and '6', advancing pptr()     stream>> n;// calls sgetc() 4 times// 1st call returns the '4' that was made available by overflow()// on the 2nd call, egptr() == egptr() and underflow() is called// underflow advances egptr() to equal pptr() (which is 6)// 3rd sgetc() returns '6'// 4th sgetc() finds gptr() == egptr(), calls underflow()// underflow() returns EOF std::cout<<"n = "<< n<<'\n';}

Before underflow(): get area size is 3, put area size is 15After underflow():  get area size is 3, put area size is 15underflow() returns EOFn = 123Before underflow(): get area size is 3, put area size is 15After underflow():  get area size is 6, put area size is 15underflow() returns '4'Before underflow(): get area size is 6, put area size is 15After underflow():  get area size is 6, put area size is 15underflow() returns EOFn = 456