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.

The specification formally permits std::istream_iterator to delay the first read to take place when the first dereferencing is done. This was reaffirmed by lwg245, but contested by p0738 as unimplementable. All existing implementations perform the first read immediately, 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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