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std::basic_...f::underflow(3)  C++ Standard Libary std::basic_...f::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  se-
     quence) 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 avail-
     able
      underflow() returns 73.
      After the call, size of the get area is 110 with 110 read positions avail-
     able
      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_stream-
     buf<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>)

http://cppreference.com 	   2024.06.10	   std::basic_...f::underflow(3)
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