C++ Standard Library C++ STL Library
C++ Standard Library C++ STL Library

C++ multimap - multimap() Function

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

Syntax

//default version - construct an empty 
//container with no elements
explicit multimap (const key_compare& comp = key_compare(),
                   const allocator_type& alloc = allocator_type());

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
  multimap (InputIterator first, InputIterator last,
            const key_compare& comp = key_compare(),
            const allocator_type& alloc = allocator_type());

//copy version - copies all elements
//of x into the container	
multimap (const multimap& x);

//default version - construct an empty 
//container with no elements
explicit multimap (const key_compare& comp = key_compare(),
                   const allocator_type& alloc = allocator_type());
explicit multimap (const allocator_type& alloc);

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
  multimap (InputIterator first, InputIterator last,
       const key_compare& comp = key_compare(),
       const allocator_type& = allocator_type());

//copy version - copies all elements
//of x into the container	
multimap (const multimap& x);
multimap (const multimap& x, const allocator_type& alloc);

//move version - moves elements of x
//into the container	
multimap (multimap&& x);
multimap (multimap&& x, const allocator_type& alloc);

//initializer list version - copies all 
//elements of il into the container	
multimap (initializer_list<value_type> il,
     const key_compare& comp = key_compare(),
     const allocator_type& alloc = allocator_type());

//default version - construct an empty 
//container with no elements
multimap();
explicit multimap (const key_compare& comp,
                   const allocator_type& alloc = allocator_type());
explicit multimap (const allocator_type& alloc);

//range version - Constructs a container with 
//elements as the range [first,last)
template <class InputIterator>
  multimap (InputIterator first, InputIterator last,
       const key_compare& comp = key_compare(),
       const allocator_type& = allocator_type());
template <class InputIterator>
  multimap (InputIterator first, InputIterator last,
       const allocator_type& = allocator_type());

//copy version - copies all elements
//of x into the container	
multimap (const multimap& x);
multimap (const multimap& x, const allocator_type& alloc);

//move version - moves elements of x
//into the container	
multimap (multimap&& x);
multimap (multimap&& x, const allocator_type& alloc);

//initializer list version - copies all 
//elements of il into the container		
multimap (initializer_list<value_type> il,
     const key_compare& comp = key_compare(),
     const allocator_type& alloc = allocator_type());
multimap (initializer_list<value_type> il,
     const allocator_type& alloc = allocator_type());

Parameters

alloc Specify the Allocator object. The container keeps and uses an internal copy of this allocator.
comp A binary predicate that takes two keys of map as arguments and returns a bool. Keys are sorted by using this function.
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).
x Specify a 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, Linear i.e, Θ(n).

Example: using default version

In the example below, the multimap::multimap function is used to construct a multimap object.

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

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

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

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

  return 0;
}

The output of the above code will be: 

MMap contains:

MMap contains:
CAN  Montreal
IND  Delhi

Example: using range and copy version

A multimap can also be constructed using range or copy version. Consider the following example: 

#include <iostream>
#include <map>
using namespace std;
 
int main (){
  multimap<string, string> MMap1;
  multimap<string, string>::iterator it;

  //populating map
  MMap1.insert(pair<string, string>("CAN", "Montreal"));
  MMap1.insert(pair<string, string>("IND", "Delhi"));

  //range version - construct MMap2 using range
  multimap<string, string> MMap2(MMap1.begin(), MMap1.end());

  //copy version - construct MMap3 from MMap1
  multimap<string, string> MMap3(MMap1); 

  cout<<"MMap2 contains:\n";
  for(it = MMap2.begin(); it != MMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nMMap3 contains:\n";
  for(it = MMap3.begin(); it != MMap3.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

MMap2 contains:
CAN  Montreal
IND  Delhi

MMap3 contains:
CAN  Montreal
IND  Delhi

Example: using move version

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

#include <iostream>
#include <map>
using namespace std;
 
int main (){
  multimap<string, string> MMap1;
  multimap<string, string>::iterator it;

  //populating map
  MMap1.insert(pair<string, string>("CAN", "Montreal"));
  MMap1.insert(pair<string, string>("IND", "Delhi"));

  cout<<"MMap1 contains:\n";
  for(it = MMap1.begin(); it != MMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  //moving all content of MMap1 into MMap2
  multimap<string, string> MMap2(move(MMap1));

  cout<<"\nMMap1 contains:\n";
  for(it = MMap1.begin(); it != MMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nMMap2 contains:\n";
  for(it = MMap2.begin(); it != MMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be: 

MMap1 contains:
CAN  Montreal
IND  Delhi

MMap1 contains:

MMap2 contains:
CAN  Montreal
IND  Delhi

Example: using initializer list version

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

#include <iostream>
#include <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	
  multimap<string, string> MMap(ilist);
  multimap<string, string>::iterator it;

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

  return 0;
}

The output of the above code will be: 

MMap contains:
CAN  Montreal
IND  Delhi

❮ C++ <map> Library
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