23 Containers library [containers]

23.5 Unordered associative containers [unord]

23.5.3 Class template unordered_map [unord.map]

23.5.3.2 Constructors [unord.map.cnstr]

unordered_map() : unordered_map(size_type(see below)) { }
explicit unordered_map(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

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template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
template<container-compatible-range<value_type> R>
  unordered_map(from_range_t, R&& rg,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
unordered_map(initializer_list<value_type> il,
              size_type n = see below,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.