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deduction guides for std::unordered_map - cppreference.com

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)).

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. 

#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
}

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.