NAME¶

deductionguidesforstd::unordered_map - deductionguidesforstd::unordered_map

Synopsis¶

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 (1) (since C++17)
below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc()
)
-> unordered_map<iter_key_t<InputIt>,
iter_val_t<InputIt>,

Hash, Pred, Alloc>;
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>>, (2) (since C++17)
typename /*see below*/::size_type = /*see
below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc()
)

-> unordered_map<Key, T, Hash, Pred, Alloc>;
template< class InputIt, class Alloc >

unordered_map( InputIt, InputIt, typename /*see
below*/::size_type, Alloc )
-> unordered_map<iter_key_t<InputIt>, (3) (since C++17)
iter_val_t<InputIt>,
std::hash<iter_key_t<InputIt>>,

std::equal_to<iter_key_t<InputIt>>,
Alloc>;
template< class InputIt, class Alloc >

unordered_map( InputIt, InputIt, Alloc )
-> unordered_map<iter_key_t<InputIt>,
iter_val_t<InputIt>, (4) (since C++17)
std::hash<iter_key_t<InputIt>>,

std::equal_to<iter_key_t<InputIt>>,
Alloc>;
template< class InputIt, class Hash, class Alloc >

unordered_map( InputIt, InputIt, typename /*see
below*/::size_type, Hash, Alloc )
-> unordered_map<iter_key_t<InputIt>, (5) (since C++17)
iter_val_t<InputIt>, Hash,

std::equal_to<iter_key_t<InputIt>>,
Alloc>;
template< class Key, class T, typename Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Alloc ) (6) (since C++17)

-> unordered_map<Key, T, std::hash<Key>,
std::equal_to<Key>, Alloc>;
template< class Key, class T, typename Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>,
Alloc ) (7) (since C++17)

-> unordered_map<Key, T, std::hash<Key>,
std::equal_to<Key>, Alloc>;
template< class Key, class T, class Hash, class Alloc >

unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Hash, (8) (since C++17)
Alloc )

-> unordered_map<Key, T, Hash, std::equal_to<Key>,
Alloc>;
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&&, (9) (since C++23)
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>;
template< ranges::input_range R, class Alloc >

unordered_map( std::from_range_t, R&&,
typename /* see description */::size_type,
Alloc ) (10) (since C++23)
-> unordered_map<range_key_t<R>, range_mapped_t<R>,
std::hash<range_key_t<R>>,

std::equal_to<range_key_t<R>>, Alloc>;
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>, (11) (since C++23)
std::hash<range_key_t<R>>,

std::equal_to<range_key_t<R>>, Alloc>;
template< ranges::input_range R, class Hash, class Alloc >

unordered_map( std::from_range_t, R&&, typename /* see
description */::size_type,
Hash, Alloc ) (12) (since C++23)
-> unordered_map<range_key_t<R>, range_mapped_t<R>,
Hash,

std::equal_to<range_key_t<R>>, Alloc>;
Exposition-only helper type aliases
template< class InputIter >

using iter_val_t = (exposition only*)

typename std::iterator_traits<InputIter>::value_type;
template< class InputIter >

using iter_key_t = (exposition only*)

std::remove_const_t< std::tuple_element_t<0,
iter_val_t<InputIter>>>;
template< class InputIter >

using iter_mapped_t = (exposition only*)

std::tuple_element_t<1, iter_val_t<InputIter>>;
template< class InputIter >

using iter_to_alloc_t =
std::pair<std::add_const_t<tuple_element_t<0, (exposition only*)
iter_val_t<InputIter>>>,

std::tuple_element_t<1,
iter_val_t<InputIter>>>;
template< ranges::input_range Range >

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

typename ranges::range_value_t<Range>::second_type;
template< ranges::input_range Range >

using range_to_alloc_t =
std::pair<std::add_const_t<typename (since C++23)
ranges::range_value_t<Range>::first_type>, (exposition only*)

typename
ranges::range_value_t<Range>::second_type>;

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