std::istream_iterator

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C++
 
Iterator library
 
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 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
char_type CharT
traits_type Traits
istream_type std::basic_istream<CharT, Traits>

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

 compares two istream_iterators
(function template)

Member types

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

These member types are required to be obtained by inheriting from std::iterator<std::input_iterator_tag, T, Distance, const T*, const T&>.

(until C++17)

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.

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 first dereferencing the first read is performed in the constructor

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");
    std::cout << "\nThe first even number is " <<
        *std::find_if(std::istream_iterator<int>(str2),
                      std::istream_iterator<int>(),
                      [](int i){return i%2 == 0;})
        << ".\n";
    //" 9 10" left in the stream
}

Output: 

0.1 0.3 0.6 1 
The first even number is 8.

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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