Namespaces
Variants
Views
Actions

deduction guides for std::unordered_map

From cppreference.com

[edit template]
 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Non-member functions
(until C++20)
Deduction guides(C++17)
 
Defined in header <unordered_map>
template< class InputIt,

          class Hash = std::hash</*iter-key-t*/<InputIt>>,
          class Pred = std::equal_to</*iter-key-t*/<InputIt>>,
          class Alloc = std::allocator</*iter-to-alloc-t*/<InputIt>> >
unordered_map( InputIt, InputIt,
               typename /* see below */::size_type = /* see below */,
               Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
    -> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>,

                     Hash, Pred, Alloc>;
 (1) (since C++17)
template< class Key, class T, class Hash = std::hash<Key>,

          class Pred = std::equal_to<Key>,
          class Alloc = std::allocator<std::pair<const Key, T>> >
unordered_map( std::initializer_list<std::pair<Key, T>>,
               typename /* see below */::size_type = /* see below */,
               Hash = Hash(), Pred = Pred(), Alloc = Alloc() )

    -> unordered_map<Key, T, Hash, Pred, Alloc>;
 (2) (since C++17)
template< class InputIt, class Alloc >

unordered_map( InputIt, InputIt, typename /* see below */::size_type, Alloc )
    -> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>,
                     std::hash</*iter-key-t*/<InputIt>>,

                     std::equal_to</*iter-key-t*/<InputIt>>, Alloc>;
 (3) (since C++17)
template< class InputIt, class Alloc >

unordered_map( InputIt, InputIt, Alloc )
    -> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>,
                     std::hash</*iter-key-t*/<InputIt>>,

                     std::equal_to</*iter-key-t*/<InputIt>>, Alloc>;
 (4) (since C++17)
template< class InputIt, class Hash, class Alloc >

unordered_map( InputIt, InputIt, typename /* see below */::size_type, Hash,
               Alloc )
    -> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>, Hash,

                     std::equal_to</*iter-key-t*/<InputIt>>, Alloc>;
 (5) (since C++17)
template< class Key, class T, typename Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>,
               typename /* see below */::size_type, Alloc )

    -> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
 (6) (since C++17)
template< class Key, class T, typename Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>, Alloc )

    -> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
 (7) (since C++17)
template< class Key, class T, class Hash, class Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>,
               typename /* see below */::size_type, Hash, Alloc )

    -> unordered_map<Key, T, Hash, std::equal_to<Key>, Alloc>;
 (8) (since C++17)
template< ranges::input_range R,

          class Hash = std::hash</*range-key-t*/<R>>,
          class Pred = std::equal_to</*range-key-t*/<R>>,
          class Alloc = std::allocator</*range-to-alloc-t*/<R>> >
unordered_map( std::from_range_t, R&&,
               typename /* see below */::size_type = /* see below */,
               Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
    -> unordered_map</*range-key-t*/<R>, /*range-mapped-t*/<R>,

                     Hash, Pred, Alloc>;
 (9) (since C++23)
template< ranges::input_range R, class Alloc >

unordered_map( std::from_range_t, R&&,
               typename /* see below */::size_type, Alloc )
    -> unordered_map</*range-key-t*/<R>, /*range-mapped-t*/<R>,
                     std::hash</*range-key-t*/<R>>,

                     std::equal_to</*range-key-t*/<R>>, Alloc>;
 (10) (since C++23)
template< ranges::input_range R, class Alloc >

unordered_map( std::from_range_t, R&&, Alloc )
    -> unordered_map</*range-key-t*/<R>, /*range-mapped-t*/<R>,
                     std::hash</*range-key-t*/<R>>,

                     std::equal_to</*range-key-t*/<R>>, Alloc>;
 (11) (since C++23)
template< ranges::input_range R, class Hash, class Alloc >

unordered_map( std::from_range_t, R&&, typename /* see below */::size_type,
               Hash, Alloc )
    -> unordered_map</*range-key-t*/<R>, /*range-mapped-t*/<R>, Hash,

                     std::equal_to</*range-key-t*/<R>>, Alloc>;
 (12) (since C++23)
Exposition-only helper type aliases
template< class InputIt >

using /*iter-val-t*/ =

    typename std::iterator_traits<InputIt>::value_type;
 (exposition only*)
template< class InputIt >

using /*iter-key-t*/ =

    std::remove_const_t< std::tuple_element_t<0, /*iter-val-t*/<InputIt>>>;
 (exposition only*)
template< class InputIt >

using /*iter-mapped-t*/ =

    std::tuple_element_t<1, /*iter-val-t*/<InputIt>>;
 (exposition only*)
template< class InputIt >

using /*iter-to-alloc-t*/ =
    std::pair<std::add_const_t<tuple_element_t<0, /*iter-val-t*/<InputIt>>>,

              std::tuple_element_t<1, /*iter-val-t*/<InputIt>>>;
 (exposition only*)
template< ranges::input_range Range >

using /*range-key-t*/ =

    std::remove_const_t<typename ranges::range_value_t<Range>::first_type>;
 (since C++23)
(exposition only*)
template< ranges::input_range Range >

using /*range-mapped-t*/ =

    typename ranges::range_value_t<Range>::second_type;
 (since C++23)
(exposition only*)
template< ranges::input_range Range >

using /*range-to-alloc-t*/ =
    std::pair<std::add_const_t<typename ranges::range_value_t<Range>::first_type>,

              typename ranges::range_value_t<Range>::second_type>;
 (since C++23)
(exposition only*)
1-8) These deduction guide are provided for unordered_map to allow deduction from an iterator range (overloads (1,3-5)) and std::initializer_list (overloads (2,6-8)).
9-12) These deduction guides are provided for unordered_map to allow deduction from a std::from_range_t tag and an input_range.

These overloads participate in overload resolution only if InputIt satisfies LegacyInputIterator, Alloc satisfies Allocator, neither Hash nor Pred satisfy Allocator, and Hash is not an integral type.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type Alloc::value_type must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

The size_type parameter type in these guides in an refers to the size_type member type of the type deduced by the deduction guide.

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overloads (9-12)

[edit] Example

Run this code
#include <unordered_map>
 
int main()
{
    // std::unordered_map m1 = {{"foo", 1}, {"bar", 2}};
        // Error: braced-init-list has no type: cannot
        // deduce pair<Key, T> from {"foo", 1} or {"bar", 2}
 
    std::unordered_map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
    std::unordered_map m2(m1.begin(), m1.end()); // guide #1
}

[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
LWG 3025 C++17 initializer-list guides (2) and (6-8) take std::pair<const Key, T> use std::pair<Key, T>
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/unordered_map/deduction_guides&oldid=135907"