deduction guides for std::unordered_map

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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 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*)
template < class InputIter >

using iter_key_t =

std::remove_const_t < std::tuple_element_t < 0, iter_val_t<InputIter>>> ;
(exposition only*)
template < class InputIter >

using iter_mapped_t =

std::tuple_element_t < 1, iter_val_t<InputIter>> ;
(exposition only*)
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*)
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

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 { } )

The size_type parameter type in these guides in an refers to the size_type

Notes

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

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
}

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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