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.
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&>.
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.
#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.[edit] 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 [edit] See alsoRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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