std::basic_stringbuf<CharT,Traits,Allocator>::underflow

#include <iostream>
#include <sstream>
 
struct mybuf : std::stringbuf
{
 mybuf(const std::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";
 else
 std::cout << "underflow() returns '" << char(ch) << "'\n";
 return ch;
 }
};
 
int main()
{
 mybuf sbuf("123"); // read-write stream
 std::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 EOF
 std::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 15
After underflow(): get area size is 3, put area size is 15
underflow() returns EOF
n = 123
Before underflow(): get area size is 3, put area size is 15
After underflow(): get area size is 6, put area size is 15
underflow() returns '4'
Before underflow(): get area size is 6, put area size is 15
After underflow(): get area size is 6, put area size is 15
underflow() returns EOF
n = 456