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

(1) 
unordered_map()
    : unordered_map(size_type(/* implementation-defined */)) {}
 (since C++11)
(until C++20)		 
unordered_map();
 (since C++20) 
explicit unordered_map( size_type bucket_count,
                        const Hash& hash = Hash(),
                        const key_equal& equal = key_equal(),
                        const Allocator& alloc = Allocator() );
 (2) (since C++11) 
unordered_map( size_type bucket_count,
               const Allocator& alloc )
    : unordered_map(bucket_count, Hash(), key_equal(), alloc) {}
 (3) (since C++14) 
unordered_map( size_type bucket_count,
               const Hash& hash,
               const Allocator& alloc )
    : unordered_map(bucket_count, hash, key_equal(), alloc) {}
 (4) (since C++14) 
explicit unordered_map( const Allocator& alloc );
 (5) (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() );
 (6) (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) {}
 (7) (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) {}
 (8) (since C++14) 
unordered_map( const unordered_map& other );
 (9) (since C++11) 
unordered_map( const unordered_map& other, const Allocator& alloc );
 (10) (since C++11) 
unordered_map( unordered_map&& other );
 (11) (since C++11) 
unordered_map( unordered_map&& other, const Allocator& alloc );
 (12) (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() );
 (13) (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) {}
 (14) (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) {}
 (15) (since C++14) 
template< container-compatible-range<value_type> R >
unordered_map( std::from_range_t, R&& rg,
               size_type bucket_count = /* see description */,
               const Hash& hash = Hash(),
               const key_equal& equal = key_equal(),
               const Allocator& alloc = Allocator() );
 (16) (since C++23) 
template< container-compatible-range<value_type> R >
unordered_map( std::from_range_t, R&& rg,
               size_type bucket_count,
               const Allocator& alloc )
    : unordered_map(std::from_range, std::forward<R>(rg),
                    bucket_count, Hash(), key_equal(), alloc) {}
 (17) (since C++23) 
template< container-compatible-range<value_type> R >
unordered_map( std::from_range_t, R&& rg,
               size_type bucket_count,
               const Hash& hash,
               const Alloc& alloc )
    : unordered_map(std::from_range, std::forward<R>(rg),
                    bucket_count, hash, key_equal(), alloc) {}
 (18) (since C++23)

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.

1-5) Constructs empty container. Sets max_load_factor() to 1.0. For the default constructor, the number of buckets is implementation-defined.
6-8) 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).
9,10) 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()).

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

 (since C++23)
11,12) 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.

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

 (since C++23)
13-15) Initializer-list constructor. Constructs the container with the contents of the initializer list init, same as unordered_map(init.begin(), init.end()).
16-18) Constructs the container with the contents of rg. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

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 [first, last) to copy the elements from
rg - a container compatible range, that is, an input_range whose elements are convertible to value_type
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

1-5) Constant.
6-8) Average case linear (i.e. O(N), where N is std::distance(first, last)), worst case quadratic, i.e. O(N2).
9,10) Linear in size of other.
11,12) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
13-15) Average case O(N) (N is std::size(init)), worst case O(N2).
16-18) Average case O(N) (N is ranges::distance(rg)), worst case O(N2).

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 [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321.

Although not formally required until C++23, some implementations have already put the template parameter Allocator into non-deduced contexts in earlier modes.

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overloads (16-18)

Example

#include <bitset>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
 
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; }
};
 
template<> struct std::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 (1) was explicit made non-explicit

See also

assigns values to the container
(public member function)

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