std::istream_iterator

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std::istream_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 default-constructed std::istream_iterator is known as the end-of-stream iterator. When a valid std::istream_iterator

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

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)

Member functions

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

Non-member functions

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

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

Example

Run this code
#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <sstream>
 
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 might be deferred to the first dereference always performed in the constructor

See also

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