public member function
<unordered_set>

std::unordered_set::unordered_set

empty (default) (1)
explicit unordered_set ( size_type n = /* see below */,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& alloc = allocator_type() );
copy (2)
unordered_set ( const unordered_set& ust );
move (3)
unordered_set ( unordered_set&& ust );
empty+allocator (4)
explicit unordered_set ( const allocator_type& alloc );
copy+allocator (5)
unordered_set ( const unordered_set& ust, const allocator_type& alloc );
move+allocator (6)
unordered_set ( unordered_set&& ust, const allocator_type& alloc );
range (7)
template <class InputIterator>
         unordered_set ( InputIterator first, InputIterator last,
                         size_type n = /* see below */,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& alloc = allocator_type() );
initializer list (8)
unordered_set ( initializer_list<value_type> il,
                size_type n = /* see below */,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& alloc = allocator_type() );
Construct unordered_set
Constructs an unordered_set container object, initializing its contents depending on the constructor version used:

(1) empty container constructor (default constructor)
Constructs an empty unordered_set object, containing no elements and with a size of zero.
It can construct the container with specific hasher, key_equal and allocator objects, along with a minimum number of hash buckets.
(2) copy constructor
The object is initialized to have the same contents and properties as the ust unordered_set object.
(3) move constructor
The object acquires the contents of the rvalue ust.
(4),(5),(6) allocator constructors
Same as (1), (2), (3), but taking an allocator object as additional argument.
(7) range constructor
Constructs an unordered_set object containing copies of each of the elements in the range [first,last).
(8) initializer list
Initializes the container with the contents of the list.

Parameters

n
Minimum number of initial buckets.
This is not the number of elements in the container, but the minimum number of slots desired for the internal hash table on construction.
Each library has some specific default value for this parameter if none is specified.
Member type size_type is an unsigned integral type.
hf
Hasher function object. A hasher is a function that returns an integral value based on the container object key passed to it as argument.
Member type hasher is defined in unordered_set as an alias of its second template parameter (Hash).
eql
Comparison function object, that returns true if the two container object keys passed as arguments are to be considered equal.
Member type key_equal is defined in unordered_set as an alias of its third template parameter (Pred).
alloc
Allocator object to be used instead of constructing a new one.
For class instantiations using their version of the default allocator class template, this parameter is not relevant.
Member type allocator_type is defined in unordered_set as an alias of its fourth template parameter (Alloc).
ust
Another unordered_set object of the same type (with the same class template arguments), whose contents are either copied or moved.
first, last
Input iterators to the initial and final positions in a range. The range used is [first,last), which includes all the elements between first and last, including the element pointed by first but not the element pointed by last.
The function template type can be any type of input iterator.
il
An initializer_list object.
These objects are automatically constructed from initializer list declarators.
Member type value_type is the type of the elements in the container, defined in unordered_set as an alias of the first template parameter (Key).

Example

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// constructing unordered_sets
#include <iostream>
#include <string>
#include <unordered_set>

template<class T>
T cmerge (T a, T b) { T t(a); t.insert(b.begin(),b.end()); return t; }

int main ()
{
  std::unordered_set<std::string> first;                                // empty
  std::unordered_set<std::string> second ( {"red","green","blue"} );    // init list
  std::unordered_set<std::string> third ( {"orange","pink","yellow"} ); // init list
  std::unordered_set<std::string> fourth ( second );                    // copy
  std::unordered_set<std::string> fifth ( cmerge(third,fourth) );       // move
  std::unordered_set<std::string> sixth ( fifth.begin(), fifth.end() ); // range

  std::cout << "sixth contains:";
  for (const std::string& x: sixth) std::cout << " " << x;
  std::cout << std::endl;

  return 0;
}


Possible output:
sixth contains: pink yellow red green orange blue

Complexity

For the empty (1,4) and move (3,6) constructors: constant time.
For the copy (2,5), range (7), and initializer list (8) constructors: average case linear, worst case quadratic.

See also