template< class Iter >
struct iterator_traits;
template< class T >
struct iterator_traits<T*>;
template< class T >
struct iterator_traits<const T*>;
std::iterator_traits is the type trait class that provides uniform interface to the properties of LegacyIterator types. This makes it possible to implement algorithms only in terms of iterators.
The template can be specialized for user-defined iterators so that the information about the iterator can be retrieved even if the type does not provide the usual typedefs.
User specializations may define the nested type iterator_concept to one of iterator category tags, to indicate conformance to the iterator concepts.
difference_type Iter::difference_type value_type Iter::value_type pointer Iter::pointer reference Iter::reference iterator_category Iter::iterator_category
If Iter does not have any of the five nested types above, then this template has no members by any of those names (std::iterator_traits is SFINAE-friendly).
If Iter does not have pointer, but has all four remaining nested types, then these four nested types are declared as follows:
difference_type Iter::difference_type value_type Iter::value_type pointer void reference Iter::reference iterator_category Iter::iterator_category
Otherwise, if Iter satisfies the exposition-only concept __LegacyInputIterator, the nested types are declared as follows:
Otherwise, if Iter satisfies the exposition-only concept __LegacyIterator, the nested types are declared as follows:
Otherwise, this template has no members by any of those names (std::iterator_traits is SFINAE-friendly).
This type trait may be specialized for user-provided types that may be used as iterators. The standard library provides partial specializations for pointer types T*, which makes it possible to use all iterator-based algorithms with raw pointers.
The standard library also provides partial specializations for some standard iterator adaptors.
(since C++20) [edit]T* specialization nested types const T* specialization nested types (until C++20) [edit] Specializations for library types [edit] Example
Shows a general-purpose std::reverse() implementation for bidirectional iterators.
#include <iostream>
#include <iterator>
#include <list>
#include <vector>
template<class BidirIt>
void my_reverse(BidirIt first, BidirIt last)
{
typename std::iterator_traits<BidirIt>::difference_type n = std::distance(first, last);
for (--n; n > 0; n -= 2)
{
typename std::iterator_traits<BidirIt>::value_type tmp = *first;
*first++ = *--last;
*last = tmp;
}
}
int main()
{
std::vector<int> v{1, 2, 3, 4, 5};
my_reverse(v.begin(), v.end());
for (int n : v)
std::cout << n << ' ';
std::cout << '\n';
std::list<int> l{1, 2, 3, 4, 5};
my_reverse(l.begin(), l.end());
for (int n : l)
std::cout << n << ' ';
std::cout << '\n';
int a[]{1, 2, 3, 4, 5};
my_reverse(a, a + std::size(a));
for (int n : a)
std::cout << n << ' ';
std::cout << '\n';
// std::istreambuf_iterator<char> i1(std::cin), i2;
// my_reverse(i1, i2); // compilation error: i1, i2 are input iterators
}
Output:
5 4 3 2 1 5 4 3 2 1 5 4 3 2 1[edit] See also
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