deduction guides for std::unordered_map
<unordered_map>
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>,
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( InputIt, InputIt, typename /* see below */::size_type, Alloc )
-> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>,
std::hash </*iter-key-t*/<InputIt>>,
unordered_map( InputIt, InputIt, Alloc )
-> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>,
std::hash </*iter-key-t*/<InputIt>>,
unordered_map( InputIt, InputIt, typename /* see below */::size_type, Hash,
Alloc )
-> unordered_map</*iter-key-t*/<InputIt>, /*iter-val-t*/<InputIt>, Hash,
unordered_map( std::initializer_list <std::pair <Key, T>>,
typename /* see below */::size_type, Alloc )
unordered_map( std::initializer_list <std::pair <Key, T>>, Alloc )
unordered_map( std::initializer_list <std::pair <Key, T>>,
typename /* see below */::size_type, Hash, Alloc )
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>,
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>>,
unordered_map( std::from_range_t, R&&, Alloc )
-> unordered_map</*range-key-t*/<R>, /*range-mapped-t*/<R>,
std::hash </*range-key-t*/<R>>,
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,
using /*iter-val-t*/ =
using /*iter-key-t*/ =
using /*iter-mapped-t*/ =
using /*iter-to-alloc-t*/ =
std::pair <std::add_const_t <tuple_element_t<0, /*iter-val-t*/<InputIt>>>,
using /*range-key-t*/ =
(exposition only*)
using /*range-mapped-t*/ =
(exposition only*)
using /*range-to-alloc-t*/ =
std::pair <std::add_const_t <typename ranges::range_value_t <Range>::first_type>,
(exposition only*)
unordered_map to allow deduction from an iterator range (overloads (1,3-5)) and std::initializer_list (overloads (2,6-8)).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.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_containers_ranges |
202202L |
(C++23) | Ranges-aware construction and insertion; overloads (9-12) |
[edit] 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 }
[edit] 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> |