C++ unordered_multimap::emplace() Function

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The C++ std::unordered_multimap::emplace() function is used to insert the new element and extends the container size by one.

If the same key is emplaced multiple times, then the unordered_multimap stores every key or value because the unordered_multimap is a container that stores duplicate or multiple keys of the same value.

Syntax

Following is the syntax of std::unordered_multimap::emplace() function.

iterator emplace(Args&&... args );

Parameters

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

Return value

This function returns an iterator to newly inserted element.

Example 1

Let's look at the following example, where we are going to demonstrate the usage of unordered_multimap::emplace() function.

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

Output

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

unordered multimap contains following elements
a = 1
b = 2
b = 2
c = 3
c = 3
d = 4
e = 5

Example 2

In the following example, we are going to the add more keys and values to the unordered_multimap.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void)  {
   unordered_multimap<string, int> umm = {{"Aman", 490},{"Vivek", 485},{"Akash", 500},{"Sonam", 450}};
   cout << "Unordered multimap contains following elements before" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   cout<<"after use of the emplace function \n";
   umm.emplace("Sarika", 440);
   umm.emplace("Sonam", 480);
   cout << "Unordered multimap contains following elements" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Following is the output of the above code −

unordered multimap contains following elements before
Akash = 500
Vivek = 485
Sonam = 450
Aman = 490
after use of the emplace function 
Unordered multimap contains following elements
Aman = 490
Sonam = 480
Sonam = 450
Vivek = 485
Akash = 500
Sarika = 440

Example 3

Consider the following example, wehere we are trying to change the value of an already present key using the emplace() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<int, int> umm = {{10, 100},{20, 200},{30, 300},{40, 400}};
   cout << "Unordered map contains following elements before usages of emplace" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   umm.emplace(20, 440);
   umm.emplace(30, 460);
   cout << "Unordered map contains same elements after the usages of emplace() function" << endl;
   for (auto it = umm.begin(); it != umm.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Output of the above code is as follows −

Unordered map contains following elements before usages of emplace
40 = 400
30 = 300
20 = 200
10 = 100
Unordered map contains same elements after the usages of emplace() function
10 = 100
20 = 440
20 = 200
30 = 460
30 = 300
40 = 400