std::basic_filebuf<CharT,Traits>::underflow (3) - Linux Manuals

std::basic_filebuf<CharT,Traits>::underflow: std::basic_filebuf<CharT,Traits>::underflow

NAME

std::basic_filebuf<CharT,Traits>::underflow - std::basic_filebuf<CharT,Traits>::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