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std::strstr...::underflow(3)   C++ Standard Libary  std::strstr...::underflow(3)

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

Synopsis
	protected:		       (deprecated in C++98)
	virtual int_type underflow();  (removed in C++26)

	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 <iostream>
      #include <strstream>

      struct mybuf : std::strstreambuf
      {
	  int_type overflow(int_type c)
	  {
	      std::cout << "Before overflow(): size of the get area is " <<  eg-
     ptr()-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 "  <<  eg-
     ptr()-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 " << eg-
     ptr()-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 " << eg-
     ptr()-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_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 from the associated file
	[virtual]  (virtual  protected	member	function   of	std::basic_file-
     buf<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>)

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