NAME

std::istream_iterator - std::istream_iterator

Synopsis

Defined in header <iterator>
template< class T,

class CharT = char,
class Traits = std::char_traits<CharT>, (until C++17)
class Distance = std::ptrdiff_t >
class istream_iterator: public std::iterator<std::input_iterator_tag,

T, Distance, const T*, const T&>
template< class T,

class CharT = char,
class Traits = std::char_traits<CharT>, (since C++17)
class Distance = std::ptrdiff_t >

class istream_iterator;

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

Member functions

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

Non-member functions

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

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.

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");
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 may be deferred to the the first read is performed
first dereferencing in the constructor