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# std::istream_iterator

From cppreference.com
< cpp‎ | iterator

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 may be read when the iterator is constructed or when the first dereferencing is done. Otherwise, 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.

### [edit] Member types

Member type | Definition
`char_type` | `CharT`
`traits_type` | `Traits`
`istream_type` | std::basic_istream<CharT, Traits>

### [edit] Member functions

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

### [edit] Non-member functions

operator==operator!= | compares two `istream_iterator`s (function template) [edit]

### [edit] Member types

Member type | Definition
`value_type` | T
`difference_type` | Distance
`pointer` | const T*
`reference` | const T&
`iterator_category` | std::input_iterator_tag

Note: before C++17, these member types are required to be obtained by inheriting from std::iterator<std::input_iterator_tag,T,Distance,const T*,const T&>.

### [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

Run this code

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

    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, " "));
    }

Output:

    0.1 0.3 0.6 1

### [edit] See also

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

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