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std::unordered_map::unordered_map

(1)
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explicit unordered_map( size_type bucket_count = /*implementation-defined*/,
                        const Hash& hash = Hash(),
                        const KeyEqual& equal = KeyEqual(),
                        const Allocator& alloc = Allocator() );
 (since C++11)
(until C++14)
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unordered_map() : unordered_map( size_type(/*implementation-defined*/) {}
explicit unordered_map( size_type bucket_count,
                        const Hash& hash = Hash(),
                        const KeyEqual& equal = KeyEqual(),
                        const Allocator& alloc = Allocator() );
 (since C++14)
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unordered_map( size_type bucket_count,
               const Allocator& alloc )
              : unordered_map(bucket_count, Hash(), KeyEqual(), alloc) {}
unordered_map( size_type bucket_count,
               const Hash& hash,
               const Allocator& alloc )
              : unordered_map(bucket_count, hash, KeyEqual(), alloc) {}
 (1) (since C++14)
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explicit unordered_map( const Allocator& alloc );
 (1) (since C++11)
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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() );
 (2) (since C++11)
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template< class InputIt >
unordered_map( InputIt first, InputIt last,
               size_type bucket_count,
               const Allocator& alloc )
              : unordered_map(first, last,
                  bucket_count, Hash(), KeyEqual(), alloc) {}
 (2) (since C++14)
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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, KeyEqual(), alloc) {}
 (2) (since C++14)
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unordered_map( const unordered_map& other );
 (3) (since C++11)
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unordered_map( const unordered_map& other, const Allocator& alloc );
 (3) (since C++11)
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unordered_map( unordered_map&& other );
 (4) (since C++11)
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unordered_map( unordered_map&& other, const Allocator& alloc );
 (4) (since C++11)
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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() );
 (5) (since C++11)
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unordered_map( std::initializer_list<value_type> init,
               size_type bucket_count,
               const Allocator& alloc )
              : unordered_map(init, bucket_count,
                  Hash(), KeyEqual(), alloc) {}
 (5) (since C++14)
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unordered_map( std::initializer_list<value_type> init,
               size_type bucket_count,
               const Hash& hash,
               const Allocator& alloc )
              : unordered_map(init, bucket_count,
                  hash, KeyEqual(), alloc) {}
 (5) (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) Constructs empty container. Sets max_load_factor() to 1.0. For the default constructor, the number of buckets is implementation-defined.
2) constructs the container with the contents of the range [first, last). Sets max_load_factor() to 1.0.
3) 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()).
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, same as unordered_map(init.begin(), init.end()).

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

1) constant
2) average case linear worst case quadratic 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) average case linear worst case quadratic in size of init

Example

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#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)
doc_CPP
doc_CPP
2025-01-10 15:47:30
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