std::strstreambuf::underflow(3)

NAME
std::strstreambuf::underflow - std::strstreambuf::underflow

Synopsis
   protected:
   virtual int_type underflow();
   Reads the next character from the get area of the buffer.
   If the input sequence has a read position available (gptr() < egptr(), returns
   (unsigned char)(*gptr()).
   Otherwise, if pptr() is not null and pptr() > egptr() (there is a put area and it is
   located after the get area), extends the end of the get area to include the
   characters that were recently written into the put area by incrementing egptr() to
   some value between gptr() and pptr(), and then returns (unsigned char)(*gptr()).
   Otherwise, returns EOF to indicate failure.

Parameters
   (none)

Return value
   The next character in the get area, (unsigned char)(*gptr()) on success, EOF on
   failure

Example
// Run this code
 #include <strstream>
 #include <iostream>
 struct mybuf : std::strstreambuf
 {
     int_type overflow(int_type c)
     {
         std::cout << "Before overflow(): size of the get area is " << egptr()-eback()
                   << " size of the put area is " << epptr()-pbase() << '\n';
         int_type rc = std::strstreambuf::overflow(c);
         std::cout << "After overflow(): size of the get area is " << egptr()-eback()
                   << " size of the put area is " << epptr()-pbase() << '\n';
         return rc;
     }
     int_type underflow()
     {
         std::cout << "Before underflow(): size of the get area is " << egptr()-eback()
                   << " size of the put area is " << epptr()-pbase() << '\n';
         int_type ch = std::strstreambuf::underflow();
         std::cout << "After underflow(): size of the get area is " << egptr()-eback()
                   << " size of the put area 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; // read-write dynamic strstreambuf
     std::iostream stream(&sbuf);
     int n;
     stream >> n;
     stream.clear();
     stream << "123";
     stream >> n;
     std::cout << n << '\n';
 }

Possible output:
 Before underflow(): size of the get area is 0 size of the put area is 0
 After underflow(): size of the get area is 0 size of the put area is 0
 underflow() returns EOF
 Before overflow(): size of the get area is 0 size of the put area is 0
 After overflow(): size of the get area is 0 size of the put area is 32
 Before underflow(): size of the get area is 0 size of the put area is 32
 After underflow(): size of the get area is 3 size of the put area is 32
 underflow() returns '1'
 Before underflow(): size of the get area is 3 size of the put area is 32
 After underflow(): size of the get area is 3 size of the put area is 32
 underflow() returns EOF
 123

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 from the associated file
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
   sgetc reads one character from the input sequence without advancing the sequence
             (public member function of std::basic_streambuf<CharT,Traits>)
   get extracts characters
             (public member function of std::basic_istream<CharT,Traits>)