C++ unordered_multiset - unordered_multiset() Function
The C++ unordered_multiset::unordered_multiset function is used to construct a unordered_multiset object, initializing its contents depending on the version of constructor used:
Syntax
//default version - construct an empty
//container with no elements
explicit unordered_multiset ( size_type n = /* see below */,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& alloc = allocator_type() );
explicit unordered_multiset ( const allocator_type& alloc );
//range version - Constructs a container with
//elements as the range [first,last)
template <class InputIterator>
unordered_multiset ( 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() );
//copy version - copies all elements
//of ust into the container
unordered_multiset ( const unordered_multiset& ums );
unordered_multiset ( const unordered_multiset& ums, const allocator_type& alloc );
//move version - moves elements of ust
//into the container
unordered_multiset ( unordered_multiset&& ums );
unordered_multiset ( unordered_multiset&& ums, const allocator_type& alloc );
//initializer list version - copies all
//elements of il into the container
unordered_multiset (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() );
//default version - construct an empty
//container with no elements
unordered_multiset();
explicit unordered_multiset ( size_type n,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& alloc = allocator_type() );
explicit unordered_multiset ( const allocator_type& alloc );
unordered_multiset ( size_type n, const allocator_type& alloc );
unordered_multiset ( size_type n, const hasher& hf, const allocator_type& alloc );
//range version - Constructs a container with
//elements as the range [first,last)
template <class InputIterator>
unordered_multiset ( 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() );
template <class InputIterator>
unordered_multiset ( InputIterator first, InputIterator last,
size_type n, const allocator_type& alloc );
template <class InputIterator>
unordered_multiset ( InputIterator first, InputIterator last,
size_type n, const hasher& hf, const allocator_type& alloc );
//copy version - copies all elements
//of ust into the container
unordered_multiset ( const unordered_multiset& ums );
unordered_multiset ( const unordered_multiset& ums, const allocator_type& alloc );
//move version - moves elements of ust
//into the container
unordered_multiset ( unordered_multiset&& ums );
unordered_multiset ( unordered_multiset&& ums, const allocator_type& alloc );
//initializer list version - copies all
//elements of il into the container
unordered_multiset (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() );
unordered_multiset ( initializer_list<value_type> il, size_type n,
const allocator_type& alloc );
unordered_multiset ( initializer_list<value_type> il, size_type n, const hasher& hf,
const allocator_type& alloc );
Parameters
alloc |
Specify the Allocator object. The container keeps and uses an internal copy of this allocator. |
n |
It contains information about minimum number of initial buckets. |
hf |
It is a hasher function object. |
eql |
The comparison function object, that returns true if the two container object keys passed as arguments are equal. |
first |
Specify initial position of the input iterator of the range. The range used is [first,last). |
last |
Specify final position of the input iterator of the range. The range used is [first,last). |
ums |
Specify an unordered_multiset object of same type. |
il |
Specify an initializer_list object. |
Return Value
Constructor never returns value.
Time Complexity
Constant i.e, Θ(1), for default version and move version.
For all the other cases, Average case: Linear i.e, Θ(n), Worst case: Quadratic i.e, Θ(n2).
Example: using default version
In the example below, the unordered_multiset::unordered_multiset function is used to construct an unordered_multiset object.
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
//default version - construct an empty unordered_multiset
unordered_multiset<int> uMSet;
unordered_multiset<int>::iterator it;
cout<<"uMSet contains: ";
for(it = uMSet.begin(); it != uMSet.end(); ++it)
cout<<*it<<" ";
//insert elements in uMSet
uMSet.insert(10);
uMSet.insert(20);
cout<<"\nuMSet contains: ";
for(it = uMSet.begin(); it != uMSet.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uMSet contains: uMSet contains: 20 10
Example: using range and copy version
An unordered_multiset can also be constructed using range or copy version. Consider the following example:
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
unordered_multiset<int> uMSet1{100, 200, 300};
unordered_multiset<int>::iterator it;
//range version - construct uMSet2 using range
unordered_multiset<int> uMSet2(uMSet1.begin(), uMSet1.end());
//copy version - construct uMSet3 from uMSet1
unordered_multiset<int> uMSet3(uMSet1);
cout<<"uMSet2 contains: ";
for(it = uMSet2.begin(); it != uMSet2.end(); ++it)
cout<<*it<<" ";
cout<<"\nuMSet3 contains: ";
for(it = uMSet3.begin(); it != uMSet3.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uMSet2 contains: 100 200 300 uMSet3 contains: 300 200 100
Example: using move version
Using the move version of unordered_multiset, the content of one unordered_multiset can be moved to another unordered_multiset. Consider the following example:
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
unordered_multiset<int> uMSet1{10, 20, 30, 40, 50};
unordered_multiset<int>::iterator it;
cout<<"uMSet1 contains: ";
for(it = uMSet1.begin(); it != uMSet1.end(); ++it)
cout<<*it<<" ";
//moving all content of uMSet1 into uMSet2
unordered_multiset<int> uMSet2(move(uMSet1));
cout<<"\nuMSet1 contains: ";
for(it = uMSet1.begin(); it != uMSet1.end(); ++it)
cout<<*it<<" ";
cout<<"\nuMSet2 contains: ";
for(it = uMSet2.begin(); it != uMSet2.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uMSet1 contains: 50 40 30 20 10 uMSet1 contains: uMSet2 contains: 50 40 30 20 10
Example: using initializer list version
The initializer list can also be used to assign values into an unordered_multiset container. Consider the example below:
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
//creating initializer list
initializer_list<int> ilist = {15, 30, 45, 60, 75};
//initializer list version - copies all
//elements of ilist into the container
unordered_multiset<int> uMSet(ilist);
unordered_multiset<int>::iterator it;
cout<<"uMSet contains: ";
for(it = uMSet.begin(); it != uMSet.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uMSet contains: 75 60 45 30 15
❮ C++ <unordered_set> Library
