std::istream_iterator| Member functions |
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std::istream_iterator is a single-pass input iterator that reads successive objects of typeT from thestd::basic_istream object for which it was constructed, by calling the appropriateoperator>>. 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-constructedstd::istream_iterator is known as theend-of-stream iterator. When a validstd::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 ofstd::istream_iterator holds two data members: a pointer to the associatedstd::basic_istream object and the most recently read value of typeT.
T must meet theDefaultConstructible,CopyConstructible, andCopyAssignable requirements.
[edit]Member types
Member typesiterator_category,value_type,difference_type,pointer andreference are required to be obtained by inheriting fromstd::iterator<std::input_iterator_tag, T, Distance,const T*,const T&>. | (until C++17) |
[edit]Member functions
| constructs a newistream_iterator
(public member function)[edit] |
| destructs anistream_iterator, including the cached value
(public member function)[edit] |
| returns the current element
(public member function)[edit] |
| advances the iterator
(public member function)[edit] |
[edit]Non-member functions
| compares twoistream_iterators
(function template)[edit] |
When reading characters,std::istream_iterator skips whitespace by default (unless disabled withstd::noskipws or equivalent), whilestd::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.
[edit]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}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 |
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| P0738R2 | C++98 | the first read might be deferred to the first dereference | always performed in the constructor |
[edit]See also
