NAME¶

std::unordered_map::unordered_map - std::unordered_map::unordered_map

Synopsis¶

unordered_map() (since C++11)
: unordered_map(size_type(/* implementation-defined */)) (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(), (2) (since C++11)

const Allocator& alloc = Allocator()
);
unordered_map( size_type bucket_count,

const Allocator& alloc ) (3) (since C++14)

: unordered_map(bucket_count, Hash(), key_equal(), alloc)
{}
unordered_map( size_type bucket_count,

const Hash& hash,
const Allocator& alloc ) (4) (since C++14)

: unordered_map(bucket_count, hash, key_equal(), alloc)
{}
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 */, (6) (since C++11)
const Hash& hash = Hash(),
const key_equal& equal = key_equal(),

const Allocator& alloc = Allocator() );
template< class InputIt >

unordered_map( InputIt first, InputIt last,
size_type bucket_count,
const Allocator& alloc ) (7) (since C++14)
: unordered_map(first, last,

bucket_count, Hash(), key_equal(), alloc)
{}
template< class InputIt >

unordered_map( InputIt first, InputIt last,
size_type bucket_count,
const Hash& hash, (8) (since C++14)
const Allocator& alloc )
: unordered_map(first, last,

bucket_count, hash, key_equal(), alloc)
{}
unordered_map( const unordered_map& other ); (9) (since C++11)
unordered_map( const unordered_map& other, const Allocator& (1) (10) (since C++11)
alloc );
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 */, (13) (since C++11)
const Hash& hash = Hash(),
const key_equal& equal = key_equal(),

const Allocator& alloc = Allocator() );
unordered_map( std::initializer_list<value_type> init,

size_type bucket_count,
const Allocator& alloc ) (14) (since C++14)
: unordered_map(init, bucket_count,

Hash(), key_equal(), alloc) {}
unordered_map( std::initializer_list<value_type> init,

size_type bucket_count,
const Hash& hash, (15) (since C++14)
const Allocator& alloc )
: unordered_map(init, bucket_count,

hash, key_equal(), alloc) {}
template< container-compatible-range<value_type> R >

unordered_map( std::from_range_t, R&& rg,
size_type bucket_count = /* see description
*/, (16) (since C++23)
const Hash& hash = Hash(),
const key_equal& equal = key_equal(),

const Allocator& alloc = Allocator() );
template< container-compatible-range<value_type> R >

unordered_map( std::from_range_t, R&& rg,
size_type bucket_count,
const Allocator& alloc ) (17) (since C++23)
: unordered_map(std::from_range, std::forward<R>(rg),

bucket_count, Hash(), key_equal(), alloc)
{}
template< container-compatible-range<value_type> R >

unordered_map( std::from_range_t, R&& rg,
size_type bucket_count,
const Hash& hash, (18) (since C++23)
const Alloc& alloc )
: unordered_map(std::from_range, std::forward<R>(rg),

bucket_count, hash, key_equal(), alloc)
{}

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 (since C++23)
argument while used in class template argument deduction.

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 (since C++23)
argument while used in class template argument deduction.

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(N^2).
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(N^2).
16-18) Average case O(N) (N is ranges::distance(rg)), worst case O(N^2).

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¶

// Run this code

#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¶

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