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deduction guides for std::unordered_map

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 description */::size_type = /* see description */,
               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 description */::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 description */::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 InputIter >
using iter_val_t =
  typename std::iterator_traits<InputIter>::value_type;
 (exposition only)
(since C++17) 
template< class InputIter >
using iter_key_t =
  std::remove_const_t< std::tuple_element_t<0, iter_val_t<InputIter>>>;
 (exposition only)
(since C++17) 
template< class InputIter >
using iter_mapped_t =
  std::tuple_element_t<1, iter_val_t<InputIter>>;
 (exposition only)
(since C++17) 
template< class InputIter >
using iter_to_alloc_t =
  std::pair<std::add_const_t<tuple_element_t<0, iter_val_t<InputIter>>>,
            std::tuple_element_t<1, iter_val_t<InputIter>>>;
 (exposition only)
(since C++17) 
template< ranges::input_range Range >
using range_key_t =
  std::remove_const_t<typename ranges::range_value_t<Range>::first_type>;
 (exposition only)
(since C++23) 
template< ranges::input_range Range >
using range_mapped_t =
  typename ranges::range_value_t<Range>::second_type;
 (exposition only)
(since C++23) 
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>;
 (exposition only)
(since C++23)
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.

Notes

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

Example

#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
}

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 take std::pair<const Key, T> use std::pair<Key, T>

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