# 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>, Alloc>; | (4) | (since C++17)
template< class T, class Alloc > unordered_set( std::initializer_list <T>, typename /* see below */::size_type, Alloc ) -> 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)

template< ranges::input_range R,

class Hash = std::hash < ranges::range_value_t <R>>,
class Pred = std::equal_to < ranges::range_value_t <R>>,
class Alloc = std::allocator < ranges::range_value_t <R>> >
unordered_set( std::from_range_t, R&&,
typename /* see below */::size_type = /* see below */,
Hash = Hash(), Pred = Pred(), Alloc = Alloc() )

-> unordered_set< ranges::range_value_t <R>, Hash, Pred, Alloc>;

(7)

(since C++23)

template< ranges::input_range R, class Alloc > unordered_set( std::from_range_t, R&&, typename /* see below */::size_type, Alloc ) -> unordered_set< ranges::range_value_t <R>, hash< ranges::range_value_t <R>>, std::equal_to < ranges::range_value_t <R>>, Alloc>; | (8) | (since C++23)
template< ranges::input_range R, class Alloc > unordered_set( std::from_range_t, R&&, Alloc ) -> unordered_set< ranges::range_value_t <R>, hash< ranges::range_value_t <R>>, std::equal_to < ranges::range_value_t <R>>, Alloc>; | (9) | (since C++23)
template< ranges::input_range R, class Hash, class Alloc > unordered_set( std::from_range_t, R&&, typename /* see below */::size_type, Hash, Alloc ) -> unordered_set< ranges::range_value_t <R>, Hash, std::equal_to < ranges::range_value_t <R>>, Alloc>; | (10) | (since C++23)

1-6) 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 participates in overload resolution only if `InputIt` satisfies LegacyInputIterator, `Alloc` satisfies Allocator, neither `Hash` nor `Pred` satisfy Allocator, `Hash` is not an integral type.
7-10) These deduction guides are provided for `unordered_set` to allow deduction from a `std::from_range_t` tag and an `input_range`.

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.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion; overloads (7-10)

### Example

Run this code

    #include <unordered_set>

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

        // guide #1 deduces std::unordered_set<int>
        std::unordered_set s2(s.begin(), s.end());
    }

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