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_iterator_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 constructed, by
   calling the appropriate operator>>. The actual read operation is performed when the
   iterator is incremented, not when it is dereferenced. The first object 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. Dereferencing 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 CopyAssignable
   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::istreambuf_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 retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   P0738R2 C++98 the first read may be deferred to the the first read is performed
                      first dereferencing in the constructor

See also
   ostream_iterator output iterator that writes to std::basic_ostream
                       (class template)
   istreambuf_iterator input iterator that reads from std::basic_streambuf
                       (class template)