C++ unordered_map - operator=() Function

The C++ unordered_map::operator= function is used to assign new content to the container by replacing the current content.

Syntax

C++11

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

//move version - moves elements of x
//into the container
unordered_map& operator= (unordered_map&& x);

//initializer list version - copies all 
//elements of il into the container
unordered_map& operator= (initializer_list<value_type> il);

Parameters

`x`	Specify a unordered_map object of same type.
`il`	Specify an initializer_list object.

Return Value

Returns *this.

Time Complexity

Linear i.e, Θ(n) for copy version and move version.
On average: Linear i.e, Θ(n). Worst case: quadratic i.e, Θ(n²).

Example: using copy version

In the example below, the unordered_map::operator= function is used to assign new content to the given unordered_map.

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

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

  //copying all content of uMap1 into uMap2
  unordered_map<int, string> uMap2;
  uMap2 = uMap1;

  cout<<"uMap1 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 Kim
102 Marry
101 John

uMap2 contains:
103 Kim
102 Marry
101 John

Example: using move version

Using the move version of operator=, 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 uMap1
  uMap1[101] = "John";
  uMap1[102] = "Marry";
  uMap1[103] = "Kim";

  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;
  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 Kim
102 Marry
101 John

uMap1 contains:

uMap2 contains:
103 Kim
102 Marry
101 John

Example: using initializer list version

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

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

  //creating initializer list
  initializer_list<pair<const int, string>> ilist = {{101, "John"}, {102, "Marry"}};

  //assigning values of uMap using ilist
  uMap = ilist;

  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