deductionguidesforstd::unordered_map(3)

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>