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deduction guides for std::unordered_map

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C++
Containers library
std::unordered_map
Member types
Member functions
Non-member functions
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.

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 ( 2 ) and ( 6-8 ) take std:: pair < const Key, T > use std:: pair < Key, T >