# std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::unordered_map

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< cpp‎ | container‎ | unordered map

unordered_map() : unordered_map( size_type(/*implementation-defined*/) ) {} explicit unordered_map( size_type bucket_count, const Hash& hash = Hash(), const key_equal& equal = key_equal(), const Allocator& alloc = Allocator() ); | (1) | (since C++11)
unordered_map( size_type bucket_count, const Allocator& alloc ) : unordered_map(bucket_count, Hash(), key_equal(), alloc) {} unordered_map( size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(bucket_count, hash, key_equal(), alloc) {} | (1) | (since C++14)
explicit unordered_map( const Allocator& alloc ); | (1) | (since C++11)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count = /*implementation-defined*/, const Hash& hash = Hash(), const key_equal& equal = key_equal(), const Allocator& alloc = Allocator() ); | (2) | (since C++11)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Allocator& alloc ) : unordered_map(first, last, bucket_count, Hash(), key_equal(), alloc) {} | (2) | (since C++14)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(first, last, bucket_count, hash, key_equal(), alloc) {} | (2) | (since C++14)
unordered_map( const unordered_map& other ); | (3) | (since C++11)
unordered_map( const unordered_map& other, const Allocator& alloc ); | (3) | (since C++11)
unordered_map( unordered_map&& other ); | (4) | (since C++11)
unordered_map( unordered_map&& other, const Allocator& alloc ); | (4) | (since C++11)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count = /*implementation-defined*/, const Hash& hash = Hash(), const key_equal& equal = key_equal(), const Allocator& alloc = Allocator() ); | (5) | (since C++11)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Allocator& alloc ) : unordered_map(init, bucket_count, Hash(), key_equal(), alloc) {} | (5) | (since C++14)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(init, bucket_count, hash, key_equal(), alloc) {} | (5) | (since C++14)

Constructs new container from a variety of data sources. Optionally uses user supplied `bucket_count` as a minimal number of buckets to create, `hash` as the hash function, `equal` as the function to compare keys and `alloc` as the allocator.

1) Constructs empty container. Sets `max_load_factor()` to 1.0. For the default constructor, the number of buckets is implementation-defined.
2) constructs the container with the contents of the range `[first, last)`. Sets `max_load_factor()` to 1.0. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
3) copy constructor. Constructs the container with the copy of the contents of `other`, copies the load factor, the predicate, and the hash function as well. If `alloc` is not provided, allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()).
4) move constructor. Constructs the container with the contents of `other` using move semantics. If `alloc` is not provided, allocator is obtained by move-construction from the allocator belonging to `other`.
5) constructs the container with the contents of the initializer list `init`, same as unordered_map(init.begin(), init.end()).

### Parameters

alloc | - | allocator to use for all memory allocations of this container
bucket_count | - | minimal number of buckets to use on initialization. If it is not specified, implementation-defined default value is used
hash | - | hash function to use
equal | - | comparison function to use for all key comparisons of this container
first, last | - | the range to copy the elements from
other | - | another container to be used as source to initialize the elements of the container with
init | - | initializer list to initialize the elements of the container with
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.

### Complexity

1) constant
2) average case linear worst case quadratic in distance between `first` and `last`
3) linear in size of `other`
4) constant. If `alloc` is given and alloc != other.get_allocator(), then linear.
5) average case linear worst case quadratic in size of `init`

### Exceptions

Calls to `Allocator::allocate` may throw.

### Notes

After container move construction (overload (4)), references, pointers, and iterators (other than the end iterator) to `other` remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in §23.2.1[container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

### Example

Run this code

    #include <unordered_map>
    #include <vector>
    #include <bitset>
    #include <string>
    #include <utility>

    struct Key {
        std::string first;
        std::string second;
    };

    struct KeyHash {
     std::size_t operator()(const Key& k) const
     {
         return std::hash<std::string>()(k.first) ^
                (std::hash<std::string>()(k.second) << 1);
     }
    };

    struct KeyEqual {
     bool operator()(const Key& lhs, const Key& rhs) const
     {
        return lhs.first == rhs.first && lhs.second == rhs.second;
     }
    };

    struct Foo {
        Foo(int val_) : val(val_) {}
        int val;
        bool operator==(const Foo &rhs) const { return val == rhs.val; }
    };

    namespace std {
        template<> struct hash<Foo> {
            std::size_t operator()(const Foo &f) const {
                return std::hash<int>{}(f.val);
            }
        };
    }

    int main()
    {
        // default constructor: empty map
        std::unordered_map<std::string, std::string> m1;

        // list constructor
        std::unordered_map<int, std::string> m2 =
        {
            {1, "foo"},
            {3, "bar"},
            {2, "baz"},
        };

        // copy constructor
        std::unordered_map<int, std::string> m3 = m2;

        // move constructor
        std::unordered_map<int, std::string> m4 = std::move(m2);

        // range constructor
        std::vector<std::pair<std::bitset<8>, int>> v = { {0x12, 1}, {0x01,-1} };
        std::unordered_map<std::bitset<8>, double> m5(v.begin(), v.end());

        //Option 1 for a constructor with a custom Key type
        // Define the KeyHash and KeyEqual structs and use them in the template
        std::unordered_map<Key, std::string, KeyHash, KeyEqual> m6 = {
                { {"John", "Doe"}, "example"},
                { {"Mary", "Sue"}, "another"}
        };

        //Option 2 for a constructor with a custom Key type
        // Define a const == operator for the class/struct and specialize std::hash
        // structure in the std namespace
        std::unordered_map<Foo, std::string> m7 = {
            { Foo(1), "One"}, { 2, "Two"}, { 3, "Three"}
        };

        //Option 3: Use lambdas
        // Note that the initial bucket count has to be passed to the constructor
        struct Goo {int val; };
        auto hash = [](const Goo &g){ return std::hash<int>{}(g.val); };
        auto comp = [](const Goo &l, const Goo &r){ return l.val == r.val; };
        std::unordered_map<Goo, double, decltype(hash), decltype(comp)> m8(10, hash, comp);
    }

### 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 2193 | C++11 | the default constructor is explicit | made non-explicit

### See also

operator= | assigns values to the container (public member function)

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