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std::basic_...f::underflow(3)  C++ Standard Libary std::basic_...f::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	retroac-
     tively to
	previously published C++ standards.

	  DR	Applied to		Behavior as published		    Cor-
     rect 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_stream-
     buf<CharT,Traits>)

	underflow reads from the associated file
	[virtual]  (virtual  protected	member	function   of	std::basic_file-
     buf<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>)

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