C++ Standard Library C++ STL Library
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C++ unordered_multimap - unordered_multimap() Function

The C++ unordered_multimap::unordered_multimap function is used to construct a unordered_multimap object, initializing its contents depending on the version of constructor used:

Syntax

//default version - construct an empty 
//container with no elements
explicit unordered_multimap ( size_type n = /* see below */,
                              const hasher& hf = hasher(),
                              const key_equal& eql = key_equal(),
                              const allocator_type& alloc = allocator_type() );
explicit unordered_multimap ( const allocator_type& alloc );

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
     unordered_multimap ( 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_multimap ( const unordered_multimap& umm );
unordered_multimap ( const unordered_multimap& umm, const allocator_type& alloc );

//move version - moves elements of ust
//into the container	
unordered_multimap ( unordered_multimap&& umm );
unordered_multimap ( unordered_multimap&& umm, const allocator_type& alloc );

//initializer list version - copies all 
//elements of il into the container	
unordered_multimap (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_multimap();
explicit unordered_multimap ( size_type n,
                              const hasher& hf = hasher(),
                              const key_equal& eql = key_equal(),
                              const allocator_type& alloc = allocator_type() );
explicit unordered_multimap ( const allocator_type& alloc );
         unordered_multimap ( size_type n, const allocator_type& alloc );
         unordered_multimap ( 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_multimap ( 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_multimap ( InputIterator first, InputIterator last,
                       size_type n, const allocator_type& alloc );
template <class InputIterator>
  unordered_multimap ( 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_multimap ( const unordered_multimap& umm );
unordered_multimap ( const unordered_multimap& umm, const allocator_type& alloc );

//move version - moves elements of ust
//into the container	
unordered_multimap ( unordered_multimap&& umm );
unordered_multimap ( unordered_multimap&& umm, const allocator_type& alloc );

//initializer list version - copies all 
//elements of il into the container		
unordered_multimap (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_multimap ( initializer_list<value_type> il, 
                     size_type n, const allocator_type& alloc );
unordered_multimap ( 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).
ump Specify an unordered_multimap 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_multimap::unordered_multimap function is used to construct an unordered_multimap object.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  //default version - construct an empty unordered_multimap
  unordered_multimap<string, string> uMMap;
  unordered_multimap<string, string>::iterator it;

  cout<<"uMMap contains:\n";
  for(it = uMMap.begin(); it != uMMap.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  //populating unordered_multimap
  uMMap.insert(pair<string, string>("CAN", "Montreal"));
  uMMap.insert(pair<string, string>("IND", "Delhi"));

  cout<<"\nuMMap contains:\n";
  for(it = uMMap.begin(); it != uMMap.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

uMMap contains:

uMMap contains:
IND  Delhi
CAN  Montreal

Example: using range and copy version

An unordered_multimap can also be constructed using range or copy version. Consider the following example: 

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_multimap<string, string> uMMap1;
  unordered_multimap<string, string>::iterator it;

  //populating unordered_multimap
  uMMap1.insert(pair<string, string>("CAN", "Montreal"));
  uMMap1.insert(pair<string, string>("IND", "Delhi"));

  //range version - construct uMMap2 using range
  unordered_multimap<string, string> uMMap2(uMMap1.begin(), uMMap1.end());

  //copy version - construct uMMap3 from uMMap1
  unordered_multimap<string, string> uMMap3(uMMap1); 

  cout<<"uMMap2 contains:\n";
  for(it = uMMap2.begin(); it != uMMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nuMMap3 contains:\n";
  for(it = uMMap3.begin(); it != uMMap3.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

uMMap2 contains:
CAN  Montreal
IND  Delhi

uMMap3 contains:
IND  Delhi
CAN  Montreal

Example: using move version

Using the move version of unordered_multimap, the content of one unordered_multimap can be moved to another unordered_multimap. Consider the following example: 

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_multimap<string, string> uMMap1;
  unordered_multimap<string, string>::iterator it;

  //populating unordered_multimap
  uMMap1.insert(pair<string, string>("CAN", "Montreal"));
  uMMap1.insert(pair<string, string>("IND", "Delhi"));

  cout<<"uMMap1 contains:\n";
  for(it = uMMap1.begin(); it != uMMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  //moving all content of uMMap1 into uMMap2
  unordered_multimap<string, string> uMMap2(move(uMMap1));

  cout<<"\nuMMap1 contains:\n";
  for(it = uMMap1.begin(); it != uMMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nuMMap2 contains:\n";
  for(it = uMMap2.begin(); it != uMMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

uMMap1 contains:
IND  Delhi
CAN  Montreal

uMMap1 contains:

uMMap2 contains:
IND  Delhi
CAN  Montreal

Example: using initializer list version

The initializer list can also be used to assign values into an unordered_multimap container. Consider the example below: 

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  //creating initializer list
  initializer_list<pair<const string, string>> ilist = 
     {{"CAN", "Montreal"}, {"IND", "Delhi"}};

  //initializer list version - copies all 
  //elements of ilist into the container  
  unordered_multimap<string, string> uMMap(ilist);
  unordered_multimap<string, string>::iterator it;

  cout<<"uMMap contains:\n";
  for(it = uMMap.begin(); it != uMMap.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

uMMap contains:
IND  Delhi
CAN  Montreal

❮ C++ <unordered_map> Library
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