C++ Unordered_multimap::emplace_hint() Function

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The C++ unordered_multimap::emplace_hint() function is used to insert a new element in unordered_multimap using a hint or position. The position only acts as a hint; it does not decide the position at which the insertion is to be done, and it extends the container size by one by inserting the new element. This function is similar to the emplace() function.

Syntax

Following is the syntax of unordered_map::emplace_hint() function.

iterator emplace_hint ( const_iterator position, Args&&... args );

Parameters

• position − Hint for the position to insert element.
• args − It specifies the arguments forwarded to construct the new element.

Return value

This function returns an iterator to the newly inserted element.

Example 1

In the following example, let's see the usage of the emplace_hint() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm = {
      {'b', 2},
      {'c', 3},
      {'d', 4},
   };
   umm.emplace_hint(umm.begin(), 'b', 2);
   umm.emplace_hint(umm.end(), 'e', 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
e = 3
b = 2
b = 2
c = 3
d = 4

Example 2

Consider the following example, where we are going to add key/values into the container at the starting and ending position.

#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 << "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_hint() function \n";
   umm.emplace_hint(umm.begin(), "Vivek", 440);
   umm.emplace_hint(umm.end(), "Aman", 460);
   umm.emplace_hint(umm.end(), "Akash", 460);
   cout << "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 −

multimap contains following elements before
Akash = 500
Vivek = 485
Sonam = 450
Aman = 490
after use of the emplace_hint() function 
multimap contains following elements
Aman = 460
Aman = 490
Sonam = 450
Vivek = 440
Vivek = 485
Akash = 460
Akash = 500

Example 3

Let's look at the following example, where we are going to use the emplace_hint() and storing the key-value pairs in random order.

#include <iostream>
#include <unordered_map>
using namespace std;
int main() { 
   unordered_multimap<int, string> Ummap;
   auto it = Ummap.emplace_hint(Ummap.begin(), 1, "Jun");
   it = Ummap.emplace_hint(it, 1, "January");
   it = Ummap.emplace_hint(it, 2, "February");
   it = Ummap.emplace_hint(it, 3, "March");
   Ummap.emplace_hint(it, 3, "April");
   Ummap.emplace_hint(it, 2, "May");
 
   cout << "The unordered_multimap contains following element : \n";
   cout << "KEY\tELEMENT"<<endl;
   for (auto itr = Ummap.begin(); itr != Ummap.end(); itr++)
      cout << itr->first << "\t"<< itr->second << endl;
   return 0;
}

Output

Output of the above code is as follows −

The unordered_multimap contains following element : 
KEY	ELEMENT
3	March
3	April
2	May
2	February
1	Jun
1	January