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>>> (since
unordered_map(InputIt, InputIt, (1) C++17)
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>;
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>>> (since
unordered_map(std::initializer_list<std::pair<Key, T>>, (2) 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) (3) (since
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, C++17)
std::hash<iter_key_t<InputIt>>,

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

unordered_map(InputIt, InputIt, Alloc) (since
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, (4) 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, (since
Alloc) (5) C++17)
-> unordered_map<iter_key_t<InputIt>, 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>>, (6) (since
typename /*see below*/::size_type, Alloc) C++17)

-> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
template<class Key, class T, typename Alloc>
(since
unordered_map(std::initializer_list<std::pair<Key, T>>, Alloc) (7) 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>>, (8) (since
typename /*see below*/::size_type, Hash, Alloc) C++17)

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

where the type aliases iter_key_t, iter_val_t, iter_to_alloc_t are defined as if as
follows

template< class InputIt >
(exposition
using iter_key_t = std::remove_const_t< only)

typename std::iterator_traits<InputIt>::value_type::first_type>;
template< class InputIt > (exposition
using iter_val_t = typename only)
std::iterator_traits<InputIt>::value_type::second_type;
template< class InputIt >

using iter_to_alloc_t = std::pair<
std::add_const_t<typename (exposition
std::iterator_traits<InputIt>::value_type::first_type>, only)
typename std::iterator_traits<InputIt>::value_type::second_type

>

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.

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>