# deduction guides for `std::unordered_map`

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< cpp‎ | container‎ | unordered map

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 <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 <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 <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 <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 InputIt> 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 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 std::pair <const Key, T> | use std::pair <Key, T>

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