std::basic_filebuf::underflow(3)

NAME
std::basic_filebuf::underflow - std::basic_filebuf::underflow

Synopsis
   protected:
   virtual int_type underflow()
   Reads more data into the input area.
   Behaves like the base class std::basic_streambuf::underflow, except that to read the
   data from the associated character sequence (the file) into the get area, first
   reads the bytes from the file into a temporary buffer (allocated as large as
   necessary), then uses std::codecvt::in of the imbued locale to convert the external
   (typically, multibyte) representation to the internal form which is then used to
   populate the get area. The conversion may be skipped if the locale's
   std::codecvt::always_noconv returns true.

Parameters
   (none)

Return value
   Traits::to_int_type(*gptr()) (the first character of the pending sequence) in case
   of success, or Traits::eof() in case of failure.

Example
// Run this code
 #include <fstream>
 #include <iostream>
 struct mybuf : std::filebuf
 {
     int underflow()
     {
          std::cout << "Before underflow(): size of the get area is "
                    << egptr()-eback() << " with "
                    << egptr()-gptr() << " read positions available\n";
          int rc = std::filebuf::underflow();
          std::cout << "underflow() returns " << rc << ".\nAfter the call, "
                    << "size of the get area is "
                    << egptr()-eback() << " with "
                    << egptr()-gptr() << " read positions available\n";
         return rc;
     }
 };
 int main()
 {
     mybuf buf;
     buf.open("test.txt", std::ios_base::in);
     std::istream stream(&buf);
     while (stream.get()) ;
 }

Possible output:
 Before underflow(): size of the get area is 0 with 0 read positions available
 underflow() returns 73.
 After the call, size of the get area is 110 with 110 read positions available
 Before underflow(): size of the get area is 110 with 0 read positions available
 underflow() returns -1.
 After the call, size of the get area is 0 with 0 read positions available

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 returns the next character available in the input sequence
   [virtual] (virtual protected member function of
             std::basic_stringbuf<CharT,Traits,Allocator>)
   underflow reads a character from the input sequence without advancing the next
   [virtual] pointer
             (virtual protected member function of std::strstreambuf)
   uflow reads from the associated file and advances the next pointer in the get
   [virtual] area
             (virtual protected member function)
   overflow writes characters to the associated file from the put area
   [virtual] (virtual protected member function)
   sgetc reads one character from the input sequence without advancing the sequence
             (public member function of std::basic_streambuf<CharT,Traits>)