std::unordered_map::unordered_map

转自：http://en.cppreference.com/w/cpp/container/unordered_map/unordered_map

explicit unordered_map( size_type bucket_count = /*implementation-defined*/, const Hash& hash = Hash(), const KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() ); unordered_map( size_type bucket_count, const Allocator& alloc = Allocator() ); unordered_map( size_type bucket_count, const Hash& hash, const Allocator& alloc = Allocator() );	(1)	(since C++11) (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 KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() ); template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Allocator& alloc = Allocator() ); template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Hash& hash, const Allocator& alloc = Allocator() );	(2)	(since C++11) (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 KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() ); unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Allocator& alloc = Allocator() ); unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Hash& hash, const Allocator& alloc = Allocator() );	(5)	(since C++11) (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) default constructor. Constructs empty container.

2) constructs the container with the contents of the range

[first, last)

.

3) copy constructor. Constructs the container with the copy of the contents of

other

. If

alloc

is
not provided, allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_copy_construction(other).

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

.

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 InputIterator .

Complexity

This section is incomplete

1) constant

2) linear 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) linear in size of

init

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;
}
};

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());

// constructor for a custom type
std::unordered_map<Key, std::string, KeyHash, KeyEqual> m6 = {
{ {"John", "Doe"}, "example"},
{ {"Mary", "Sue"}, "another"}
};
}

See also

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