std:: 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. 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 functions

Non-member functions

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 <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
}

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.

See also