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deduction guides for std::unordered_set

Defined in header <unordered_set> 
template<class InputIt,
         class Hash = std::hash<typename std::iterator_traits<InputIt>::value_type>,
         class Pred = std::equal_to<typename std::iterator_traits<InputIt>::value_type>,
         class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type>>
unordered_set(InputIt, InputIt,
         typename /*see below*/::size_type = /*see below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc())
  -> unordered_set<typename std::iterator_traits<InputIt>::value_type, Hash, Pred, Alloc>;
 (1) (since C++17) 
template<class T,
         class Hash = std::hash<T>,
         class Pred = std::equal_to<T>,
         class Alloc = std::allocator<T>>
unordered_set(std::initializer_list<T>,
         typename /*see below*/::size_type = /*see below*/,
         Hash = Hash(), Pred = Pred(), Alloc = Alloc())
  -> unordered_set<T, Hash, Pred, Alloc>;
 (2) (since C++17) 
template<class InputIt, class Alloc>
unordered_set(InputIt, InputIt, typename /*see below*/::size_type, Alloc)
  -> unordered_set<typename std::iterator_traits<InputIt>::value_type,
              std::hash<typename std::iterator_traits<InputIt>::value_type>,
              std::equal_to<typename std::iterator_traits<InputIt>::value_type>,
              Alloc>;
 (3) (since C++17) 
template<class InputIt, class Hash, class Alloc>
unordered_set(InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc)
  -> unordered_set<typename std::iterator_traits<InputIt>::value_type, Hash,
             std::equal_to<typename std::iterator_traits<InputIt>::value_type>, Allocator>;
 (4) (since C++17) 
template<class T, class Allocator>
unordered_set(std::initializer_list<T>, typename /*see below*/::size_type, Allocator)
  -> unordered_set<T, std::hash<T>, std::equal_to<T>, Alloc>;
 (5) (since C++17) 
template<class T, class Hash, class Alloc>
unordered_set(std::initializer_list<T>, typename /*see below*/::size_type, Hash, Alloc)
  -> unordered_set<T, Hash, std::equal_to<T>, Alloc>;
 (6) (since C++17)

These deduction guides are provided for unordered_set to allow deduction from an iterator range (overloads (1,3,4)) and std::initializer_list (overloads (2,5.6)). This overload only participates in overload resolution if InputIt satisfies LegacyInputIterator, Alloc satisfies Allocator, neither Hash nor Pred satisfy Allocator, 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 refers to the size_type member type of the type deduced by the deduction guide.

Example

#include <unordered_set>
int main() {
   std::unordered_set s = {1,2,3,4};            // guide #2 deduces std::unordered_set<int>
   std::unordered_set s2(s.begin(), s.end());   // guide #1 deduces std::unordered_set<int>
}

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