C++

Reference

<iterator>

istream_iterator

class template
<iterator>

std::istream_iterator

template <class T, class charT=char, class traits=char_traits<charT>,           class Distance = ptrdiff_t>  class istream_iterator;
Istream iterator

Istream iterators are input iterators that read successive elements from an input stream (such as cin).

They are constructed from a basic_istream object, to which they become associated, so that whenever operator++ is used on the iterator, it extracts an element from the stream (using operator>>).

This kind of iterator has a special state as an end-of-stream iterator, which is acquired if an input operations fails (as returned by fail after an operation with the associated stream), and is also the resulting value of a default-constructed object.

Template parameters

T
Element type for the iterator: The type of elements extracted from the stream
charT
First template parameter of the associated basic_istream object: The type of elements the stream handles (char for istream).
traits
Second template parameter of the associated basic_istream: Character traits for the elements the stream handles.
Distance
Type to represent the difference between two iterators (generally a signed integral type, such as ptrdiff_t).
Note: The default template arguments correspond to an instantiation that uses an istream object as associated stream.

Member types

memberdefinition in istream_iteratordescription
istream_typebasic_istream<charT,traits>Type of the associated input stream
iterator_categoryinput_iterator_tagInput iterator
value_typeTType of the elements pointed by the iterator
char_typecharTType of the characters handled by the associated stream
traits_typetraitsCharacter traits for associated stream
difference_typeDistanceDistance type
pointerconst T*
referenceconst T&

Member functions

(constructor)
Construct istream iterator (public member function)
operator*
Dereference iterator (public member function)
operator->
Dereference iterator (public member function)
operator++
Increment iterator position (public member function)

Non-member function overloads

relational operators
Relational operators for istream_iterator (function template)

Example

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// istream_iterator example
#include <iostream>     // std::cin, std::cout
#include <iterator>     // std::istream_iterator

int main () {
  double value1, value2;
  std::cout << "Please, insert two values: ";

  std::istream_iterator<double> eos;              // end-of-stream iterator
  std::istream_iterator<double> iit (std::cin);   // stdin iterator

  if (iit!=eos) value1=*iit;

  ++iit;
  if (iit!=eos) value2=*iit;

  std::cout << value1 << "*" << value2 << "=" << (value1*value2) << '\n';

  return 0;
}

Possible output:

Please, insert two values: 2 32
2*32=64


Possible implementation

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template <class T, class charT=char, class traits=char_traits<charT>, class Distance=ptrdiff_t>
  class istream_iterator :
    public iterator<input_iterator_tag, T, Distance, const T*, const T&>
{
  basic_istream<charT,traits>* in_stream;
  T value;

public:
  typedef charT char_type;
  typedef traits traits_type;
  typedef basic_istream<charT,traits> istream_type;
  istream_iterator() : in_stream(0) {}
  istream_iterator(istream_type& s) : in_stream(&s) { ++*this; }
  istream_iterator(const istream_iterator<T,charT,traits,Distance>& x)
    : in_stream(x.in_stream), value(x.value) {}
  ~istream_iterator() {}

  const T& operator*() const { return value; }
  const T* operator->() const { return &value; }
  istream_iterator<T,charT,traits,Distance>& operator++() {
    if (in_stream && !(*in_stream >> value)) in_stream=0;
    return *this;
  }
  istream_iterator<T,charT,traits,Distance> operator++(int) {
    istream_iterator<T,charT,traits,Distance> tmp = *this;
    ++*this;
    return tmp;
  }
};

See also

ostream_iterator
Ostream iterator (class template)
input_iterator_tag
Input iterator category (class)
istream
Input stream (class)