C++ unordered_map - unordered_map() Function

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

Syntax

C++11
C++14

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

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

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

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

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

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

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

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

  //populating unordered_map
  uMap[101] = "John";
  uMap[102] = "Marry";

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

  return 0;
}

The output of the above code will be:

uMap contains:

uMap contains:
102  Marry
101  John

Example: using range and copy version

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

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap1;
  unordered_map<int, string>::iterator it;

  //populating unordered_map
  uMap1[101] = "John";
  uMap1[102] = "Marry";
  uMap1[103] = "Sam";

  //range version - construct uMap2 using range
  unordered_map<int, string> uMap2(uMap1.begin(), uMap1.end());

  //copy version - construct uMap3 from uMap1
  unordered_map<int, string> uMap3(uMap1); 

  cout<<"uMap2 contains:\n";
  for(it = uMap2.begin(); it != uMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nuMap3 contains:\n";
  for(it = uMap3.begin(); it != uMap3.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be:

uMap2 contains:
101  John
102  Marry
103  Sam

uMap3 contains:
103  Sam
102  Marry
101  John

Example: using move version

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

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap1;
  unordered_map<int, string>::iterator it;

  //populating unordered_map
  uMap1[101] = "John";
  uMap1[102] = "Marry";
  uMap1[103] = "Sam";

  cout<<"uMap1 contains:\n";
  for(it = uMap1.begin(); it != uMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  //moving all content of uMap1 into uMap2
  unordered_map<int, string> uMap2(move(uMap1));

  cout<<"\nuMap1 contains:\n";
  for(it = uMap1.begin(); it != uMap1.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  cout<<"\nuMap2 contains:\n";
  for(it = uMap2.begin(); it != uMap2.end(); ++it)
    cout<<it->first<<"  "<<it->second<<"\n";

  return 0;
}

The output of the above code will be:

uMap1 contains:
103  Sam
102  Marry
101  John

uMap1 contains:

uMap2 contains:
103  Sam
102  Marry
101  John

Example: using initializer list version

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

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  //creating initializer list
  initializer_list<pair<const int, string>> ilist = 
     {{101, "John"}, {102, "Marry"}};

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

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

  return 0;
}

The output of the above code will be:

uMap contains:
102  Marry
101  John

❮ C++ <unordered_map> Library