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 |
---|---|
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 |
(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_iterator s (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
#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 first dereferencing | the first read is 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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