std::istream_iterator
Defined in header <iterator>
|
||
template< class T, class CharT = char, |
(until C++17) | |
template< class T, class CharT = char, |
(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_iterator s (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
#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) |