std::istream_iterator

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

std::istream_iterator

Member functions

istream_iterator::istream_iterator

istream_iterator::~istream_iterator

istream_iterator::operator*istream_iterator::operator->

istream_iterator::operator++istream_iterator::operator++(int)

Non-member functions

operator==operator!= (until C++20)

 

Defined in header <iterator>
template<class T,           class CharT =char,           class Traits =std::char_traits<CharT>,           class Distance =std::ptrdiff_t> class istream_iterator     :publicstd::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. An end-of-stream iterator remains in the end-of-stream state even if the underlying stream changes state. Absent a reassignment, it cannot become a non-end-of-stream iterator anymore.

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 std::iterator<std::input_iterator_tag, T, Distance, const T*, const T&>.

(until C++17)

Member functions

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

Non-member functions

operator==operator!= (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 is more efficient, since it avoids the overhead of constructing and destructing the sentry object once per character.

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

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