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deduction guides for std::unordered_map

From cppreference.com
 
 
 
 
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 below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc())

  -> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, Pred, Alloc>;
(1) (since C++17)
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<const Key, T>>,
         typename /*see below*/::size_type = /*see below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc())

-> unordered_map<Key, T, Hash, Pred, Alloc>;
(2) (since C++17)
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>,
           std::hash<iter_key_t<InputIt>>,

           std::equal_to<iter_key_t<InputIt>>, Alloc>;
(3) (since C++17)
template<class InputIt, class Alloc>

unordered_map(InputIt, InputIt, Alloc)
  -> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>,
            std::hash<iter_key_t<InputIt>>,

            std::equal_to<iter_key_t<InputIt>>, Alloc>;
(4) (since C++17)
template<class InputIt, class Hash, class Alloc>

unordered_map(InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc)
  -> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash,

              std::equal_to<iter_key_t<InputIt>>, Alloc>;
(5) (since C++17)
template<class Key, class T, typename Alloc>

unordered_map(std::initializer_list<std::pair<const Key, T>>,
     typename /*see below*/::size_type, Alloc)

  -> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
(6) (since C++17)
template<class Key, class T, typename Alloc>

unordered_map(std::initializer_list<std::pair<const Key, T>>, Alloc)

  -> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
(7) (since C++17)
template<class Key, class T, class Hash, class Alloc>

unordered_map(std::initializer_list<std::pair<const Key, T>>,
       typename /*see below*/::size_type, Hash, Alloc)

  -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;
(8) (since C++17)

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<

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

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

>
(exposition only)

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 InputIterator, 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 InputIterator 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] Example

#include <unordered_map>
int main() {
// std::unordered_map m1 = {{"foo", 1}, {"bar", 2}}; // Error: braced-init-list has no type
                                                     // cannot deduce pair<const Key, T> from
                                                     // {"foo", 1} or {"bar", 2}
   std::unordered_map m1 = std::initializer_list<
                        std::pair<char const* const, int>>({{"foo", 2}, {"bar", 3}}); // guide #2
   std::unordered_map m2(m1.begin(), m1.end()); // guide #1
}