C++ Unordered_map::emplace() Function

Previous
Next

The C++ function unordered_map::emplace() is used to insert new element into the container and increases the container size by one. The insertion will only takes place if the key doesn't exists in the container making it unique.

If the same key is emplaced multiple times, the map stores the first element only because the map is a container that does not store multiple keys of the same value.

Syntax

Following is the syntax of unordered_map::emplace() function

pair<iterator, bool>  emplace( Args&&... args);

Parameters

  • args − It indicates the arguments to forward to the constructor of the element.

Return value

Returns a pair consisting of a bool to indicate whether insertion has happened or not and returns an iterator to the newly inserted element.

Example 1

Let's look at the basic usage of unordered_map::emplace() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<char, int> um;
   um.emplace('a', 1);
   um.emplace('b', 2);
   um.emplace('c', 3);
   um.emplace('d', 4);
   um.emplace('e', 5);
   cout << "Unordered map contains following elements" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Let us compile and run the above program, this will produce the following result −

Unordered map contains following elements
e = 5
d = 4
c = 3
b = 2
a = 1

Example 2

In the following example, we created an unordered_map, then we are going to add two more keys and values using the emplace() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<string, int> um = {{"Aman", 490},{"Vivek", 485},{"Akash", 500},{"Sonam", 450}};
   cout << "Unordered map contains following elements before" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   cout<<"after use of the emplace function \n";
   um.emplace("Sarika", 440);
   um.emplace("Satya", 460);
   cout << "Unordered map contains following elements" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

If we run the above code it will generate the following output −

Unordered map contains following elements before
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490
after use of the emplace function 
Unordered map contains following elements
Sarika = 440
Satya = 460
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490

Example 3

Consider the following example, where we are trying to change the value of an present key using the emplace() function as follows:

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_map<string, int> um = {{"Aman", 490},{"Vivek", 485},{"Akash", 500},{"Sonam", 450}};
   cout << "Unordered map contains following elements before usages of emplace" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   um.emplace("Aman", 440);
   um.emplace("Sonam", 460);
   cout << "Unordered map contains same elements after the usages of emplace() function" << endl;
   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Following is the output of the above code −

Unordered map contains following elements before
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490
Unordered map contains same elements after the usages of emplace() function
Sonam = 450
Akash = 500
Vivek = 485
Aman = 490
Print Page
Previous
Next
Advertisements