deductionguidesforstd::unordered_map (3) - Linux Manuals

deductionguidesforstd::unordered_map: deductionguidesforstd::unordered_map

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>>> (1) (since C++17)
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>;
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>, iter_val_t<InputIt>, (3) (since 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)
-> 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) (5) (since 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 C++17)
typename /*see below*/::size_type, Alloc)
-> 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>>, (8) (since C++17)
typename /*see below*/::size_type, Hash, Alloc)
-> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;

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

template<class InputIt>
using iter_key_t = std::remove_const_t< (exposition only)
typename std::iterator_traits<InputIt>::value_type::first_type>;
template<class InputIt> (exposition only)
using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;
template<class InputIt>
using iter_to_alloc_t = std::pair<
std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>, (exposition 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 only participate in overload resolution 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 std::pair<const Key, T> use std::pair<Key, T>