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std::iterator_traits

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
Iterator concepts
Iterator primitives
(deprecated in C++17)
iterator_traits


Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)  
(C++11)(C++14)
(C++14)(C++14)  
(C++17)(C++20)
(C++17)
(C++17)
 
Defined in header <iterator>
template< class Iter >
struct iterator_traits;
template< class T >
struct iterator_traits<T*>;
template< class T >
struct iterator_traits<const T*>;
(removed in C++20)

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.

(since C++20)

Contents

[edit] Template parameters

Iter - the iterator type to retrieve properties for

[edit] Nested types

Nested type Definition
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).

(since C++17)
(until C++20)

If Iter does not have pointer, but has all four remaining nested types, then these four nested types are declared as follows:

Nested type Definition
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:

Nested type Definition
difference_type std::incrementable_traits<Iter>::difference_type
value_type std::indirectly_readable_traits<Iter>::value_type
pointer
  • Iter::pointer if valid.
  • Otherwise decltype(std::declval<Iter&>().operator->()) if valid.
  • Otherwise void.
reference
iterator_category


Otherwise, if Iter satisfies the exposition-only concept __LegacyIterator, the nested types are declared as follows:

Nested type Definition
difference_type
value_type void
pointer void
reference void
iterator_category std::output_iterator_tag

Otherwise, this template has no members by any of those names (std::iterator_traits is SFINAE-friendly).

(since C++20)

[edit] Specializations

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

Only specialized if std::is_object_v<T> is true.

(since C++20)


Nested type Definition
difference_type std::ptrdiff_t
value_type T(until C++20)std::remove_cv_t<T>(since C++20)
pointer T*
reference T&
iterator_category std::random_access_iterator_tag
iterator_concept (since C++20) std::contiguous_iterator_tag


const T* specialization nested types

Nested type Definition
difference_type std::ptrdiff_t
value_type T
pointer const T*
reference const T&
iterator_category std::random_access_iterator_tag
(until C++20)

[edit] Specializations for library types

provides uniform interface to the properties of the std::common_iterator type
(class template specialization) [edit]
provides uniform interface to the properties of the std::counted_iterator type
(class template specialization) [edit]

[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

(deprecated in C++17)
base class to ease the definition of required types for simple iterators
(class template) [edit]
empty class types used to indicate iterator categories
(class) [edit]
computes the associated types of an iterator
(alias template)[edit]