< cpp‎ | iterator
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Iterator adaptors
Range access
Defined in header <iterator>
template< class T,

          class CharT = char,
          class Traits = std::char_traits<CharT>,
          class Distance = std::ptrdiff_t >
class istream_iterator

    : public std::iterator<std::input_iterator_tag, T, Distance, const T*, const T&>
(until C++17)
template< class T,

          class CharT = char,
          class Traits = std::char_traits<CharT>,
          class Distance = std::ptrdiff_t >

class istream_iterator;
(since C++17)

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. An end-of-stream iterator remains in the end-of-stream state even if the underlying stream changes state. Absent a reassignment, it cannot become a non-end-of-stream iterator anymore.

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.


[edit] 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 std::iterator<std::input_iterator_tag, T, Distance, const T*, const T&>.

(until C++17)

[edit] Member functions

constructs a new istream_iterator
(public member function) [edit]
destructs an istream_iterator, including the cached value
(public member function) [edit]
returns the current element
(public member function) [edit]
advances the iterator
(public member function) [edit]

[edit] Non-member functions

(removed in C++20)
compares two istream_iterators
(function template) [edit]

[edit] 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.

[edit] Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <sstream>
int main()
    std::istringstream str("0.1 0.2 0.3 0.4");
                     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),
                           [](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


0.1 0.3 0.6 1 
The first even number is 8.

[edit] 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 might be deferred to the first dereference always performed in the constructor

[edit] See also

output iterator that writes to std::basic_ostream
(class template) [edit]
input iterator that reads from std::basic_streambuf
(class template) [edit]