| Iterator concepts | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Iterator primitives | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Algorithm concepts and utilities | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Indirect callable concepts | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Common algorithm requirements | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Utilities | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Iterator adaptors | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Defined in header <iterator> | ||
struct input_iterator_tag{}; | (1) | |
struct output_iterator_tag{}; | (2) | |
struct forward_iterator_tag:public input_iterator_tag{}; | (3) | |
struct bidirectional_iterator_tag:public forward_iterator_tag{}; | (4) | |
struct random_access_iterator_tag:public bidirectional_iterator_tag{}; | (5) | |
struct contiguous_iterator_tag:public random_access_iterator_tag{}; | (6) | (since C++20) |
Defines the category of an iterator. Each tag is an empty type.
Contents |
For everyLegacyIterator typeIt, atypedefstd::iterator_traits<It>::iterator_category must be defined to be an alias to one of these tag types, to indicate the most specific category thatIt is in.
input_iterator_tag corresponds toLegacyInputIterator.output_iterator_tag corresponds toLegacyOutputIterator.forward_iterator_tag corresponds toLegacyForwardIterator.bidirectional_iterator_tag corresponds toLegacyBidirectionalIterator.random_access_iterator_tag corresponds toLegacyRandomAccessIterator.Iterator category tags carry information that can be used to select the most efficient algorithms for the specific requirement set that is implied by the category.
Iterator conceptFor every
If In any case, each concept is not satisfied if the required operations are not supported, regardless of the tag. | (since C++20) |
There is no separate tag forLegacyContiguousIterator. That is, it is not possible to tell aLegacyContiguousIterator based on itsiterator_category.To define specialized algorithm for contiguous iterators, use thecontiguous_iterator concept.(since C++20)
There are no correspondences betweenoutput_iterator_tag and theoutput_iterator concept. Settingiterator_concept tooutput_iterator_tag only indicates that the type does not modelinput_iterator.
Common technique for algorithm selection based on iterator category tags is to use a dispatcher function (the alternative isstd::enable_if).The iterator tag classes are also used in the corresponding concepts definitions to denote the requirements, which can't be expressed in terms of usage patterns alone.(since C++20)
#include <iostream>#include <iterator>#include <list>#include <vector> // Using concepts (tag checking is part of the concepts themselves) template<std::bidirectional_iterator BDIter>void alg(BDIter, BDIter){std::cout<<"1. alg()\t called for bidirectional iterator\n";} template<std::random_access_iterator RAIter>void alg(RAIter, RAIter){std::cout<<"2. alg()\t called for random-access iterator\n";} // Legacy, using tag dispatch namespace legacy{// Quite often implementation details are hidden in a dedicated namespacenamespace implementation_details{template<class BDIter>void alg(BDIter, BDIter, std::bidirectional_iterator_tag){std::cout<<"3. legacy::alg() called for bidirectional iterator\n";} template<class RAIter>void alg(RAIter, RAIter, std::random_access_iterator_tag){std::cout<<"4. legacy::alg() called for random-access iterator\n";}}// namespace implementation_details template<class Iter>void alg(Iter first, Iter last){ implementation_details::alg(first, last,typenamestd::iterator_traits<Iter>::iterator_category());}}// namespace legacy int main(){std::list<int> l; alg(l.begin(), l.end());// 1. legacy::alg(l.begin(), l.end());// 3. std::vector<int> v; alg(v.begin(), v.end());// 2. legacy::alg(v.begin(), v.end());// 4. // std::istreambuf_iterator<char> i1(std::cin), i2;// alg(i1, i2); // compile error: no matching function for call// legacy::alg(i1, i2); // compile error: no matching function for call}
Output:
1. alg() called for bidirectional iterator3. legacy::alg() called for bidirectional iterator2. alg() called for random-access iterator4. legacy::alg() called for random-access iterator
(deprecated in C++17) | base class to ease the definition of required types for simple iterators (class template)[edit] |
| provides uniform interface to the properties of an iterator (class template)[edit] |
