std::basic_stringbuf::underflow(3)

NAME
std::basic_stringbuf::underflow - std::basic_stringbuf::underflow

Synopsis
   protected:
   virtual int_type underflow()
   Reads the next character from the get area of the buffer.
   Specifically:
   1) If the input sequence has a read position available (egptr() > gptr()), returns
   Traits::to_int_type(*gptr())
   2) Otherwise, if pptr() > egptr() (some characters were inserted into the stream
   since the last time overflow() changed egptr()) then extends the end of the get area
   to include the most recently inserted characters, by changing egptr() to equal
   pptr(), and then returns Traits::to_int_type(*gptr())
   3) Otherwise, returns Traits::eof().
   Any character in the buffer which has been initialized, regardless of whether it
   originated from the string passed in the constructor or was appended by overflow(),
   is considered to be part of the input sequence.

Parameters
   (none)

Return value
   Traits::to_int_type(*gptr()) (the next character to read in the get area) in case of
   success, or Traits::eof() in case of failure.

Example
// Run this code
 #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';
 }

Possible output:
 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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      it was unclear whether characters appended by
   LWG 432 C++98 overflow() made clear
                      are considered to be part of the input sequence

See also
   underflow reads characters from the associated input sequence to the get area
   [virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)
   underflow reads from the associated file
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
   underflow reads a character from the input sequence without advancing the next
   [virtual] pointer
             (virtual protected member function of std::strstreambuf)
   sgetc reads one character from the input sequence without advancing the sequence
             (public member function of std::basic_streambuf<CharT,Traits>)