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std::istream_iterator(3)      C++ Standard Libary     std::istream_iterator(3)

NAME
       std::istream_iterator - std::istream_iterator

Synopsis
	  Defined in header <iterator>
	  template< class T,

	  class	CharT =	char,
	  class		  Traits	   =	      std::char_traits<CharT>,
       (until C++17)
	  class	Distance = std::ptrdiff_t >
	  class	  istream_iterator:   public   std::iterator<std::input_itera-
       tor_tag,

	  T, Distance, const T*, const T&>
	  template< class T,

	  class	CharT =	char,
	  class		  Traits	   =	      std::char_traits<CharT>,
       (since C++17)
	  class	Distance = std::ptrdiff_t >

	  class	istream_iterator;

	  std::istream_iterator	is a single-pass  input	 iterator  that	 reads
       successive objects
	  of  type  T from the std::basic_istream object for which it was con-
       structed, by
	  calling the appropriate operator>>. The  actual  read	 operation  is
       performed when the
	  iterator  is incremented, not	when it	is dereferenced. The first ob-
       ject is read when
	  the iterator is constructed. Dereferencing only returns  a  copy  of
       the most	recently
	  read object.

	  The  default-constructed  std::istream_iterator is known as the end-
       of-stream
	  iterator. When a valid std::istream_iterator reaches the end of  the
       underlying
	  stream,  it becomes equal to the end-of-stream iterator. Dereferenc-
       ing or
	  incrementing it further invokes undefined behavior.

	  A typical implementation of  std::istream_iterator  holds  two  data
       members:	a pointer
	  to  the  associated  std::basic_istream object and the most recently
       read value of type
	  T.

	  T must meet the DefaultConstructible,	CopyConstructible, and CopyAs-
       signable
	  requirements.

Member types
	  Member type	    Definition
	  iterator_category std::input_iterator_tag
	  value_type	    T
	  difference_type   Distance
	  pointer	    const T*
	  reference	    const T&
	  char_type	    CharT
	  traits_type	    Traits
	  istream_type	    std::basic_istream<CharT, Traits>

	  Member types iterator_category, value_type, difference_type, pointer
	  and reference	 are  required	to  be	obtained  by  inheriting  from
       (until C++17)
	  std::iterator<std::input_iterator_tag, T, Distance, const T*,	const
	  T&>.

Member functions
	  constructor	  constructs a new istream_iterator
			  (public member function)
	  destructor	   destructs an	istream_iterator, including the	cached
       value
			  (public member function)
	  operator*	  returns the current element
	  operator->	  (public member function)
	  operator++	  advances the iterator
	  operator++(int) (public member function)

Non-member functions
	  operator==	     compares two istream_iterators
	  operator!=	     (function template)
	  (removed in C++20)

Notes
	  When reading characters, std::istream_iterator skips	whitespace  by
       default (unless
	  disabled  with  std::noskipws	 or  equivalent),  while std::istream-
       buf_iterator does not.
	  In addition, std::istreambuf_iterator	is more	 efficient,  since  it
       avoids the
	  overhead  of constructing and	destructing the	sentry object once per
       character.

Example
       // Run this code

	#include <iostream>
	#include <sstream>
	#include <iterator>
	#include <numeric>
	#include <algorithm>

	int main()
	{
	    std::istringstream str("0.1	0.2 0.3	0.4");
	    std::partial_sum(std::istream_iterator<double>(str),
			     std::istream_iterator<double>(),
			     std::ostream_iterator<double>(std::cout, "	"));

	    std::istringstream str2("1 3 5 7 8 9 10");
	    auto it = std::find_if(std::istream_iterator<int>(str2),
			      std::istream_iterator<int>(),
			      [](int i){return i%2 == 0;});
	    if (it != std::istream_iterator<int>())
		std::cout << "\nThe first even number is " << *it << ".\n";
	    //"	9 10" left in the stream
	}

Output:
	0.1 0.3	0.6 1
	The first even number is 8.

	 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
	  P0738R2  C++98       the first read may be deferred to the the first
       read is performed
			     first dereferencing		   in the con-
       structor

See also
	  ostream_iterator    output iterator that  writes  to	std::basic_os-
       tream
			      (class template)
	  istreambuf_iterator	input	iterator   that	 reads	from  std::ba-
       sic_streambuf
			      (class template)

http://cppreference.com		  2022.07.31	      std::istream_iterator(3)

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