[unord.map.cnstr]

# 23 Containers library [[containers]](./#containers)

## 23.5 Unordered associative containers [[unord]](unord#map.cnstr)

### 23.5.3 Class template unordered_map [[unord.map]](unord.map#cnstr)

#### 23.5.3.2 Constructors [unord.map.cnstr]

[ð](#lib:unordered_map,constructor)

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

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13745)

*Effects*: Constructs an empty unordered_map using the
specified hash function, key equality predicate, and allocator, and
using at least n buckets[.](#1.sentence-1)

For the default constructor,
the number of buckets is implementation-defined[.](#1.sentence-2)

max_load_factor() returns 1.0[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13754)

*Complexity*: Constant[.](#2.sentence-1)

[ð](#lib:unordered_map,constructor_)

`template<class InputIterator>
  constexpr 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](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<value_type> R>
  constexpr 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());
constexpr 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());
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13778)

*Effects*: Constructs an empty unordered_map using the
specified hash function, key equality predicate, and allocator, and
using at least n buckets[.](#3.sentence-1)

If n is not
provided, the number of buckets is implementation-defined[.](#3.sentence-2)

Then
inserts elements from the range [f, l), rg, or il,
respectively[.](#3.sentence-3)

max_load_factor() returns 1.0[.](#3.sentence-4)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13789)

*Complexity*: Average case linear, worst case quadratic[.](#4.sentence-1)
