# deduction guides for `std::unordered_set`

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< cpp‎ | container‎ | 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

Run this code

    #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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