vector emplace() function in C++ STL

The vector::emplace() is an STL in C++ which extends the container by inserting a new element at the position. Reallocation happens only if there is a need for more space. Here the container size increases by one.
Syntax:

template 
iterator vector_name.emplace (const_iterator position, element);

Parameter:
The function accepts two mandatory parameters which are specified as below:

• position – It specifies the iterator pointing to the position in the container where the new element is to be inserted. 
  
• element- It specifies the element to be inserted in the vector container. 
  

Return value: The function returns an iterator that points to the newly inserted element.

Complexity: Linear 
Below programs illustrates the above function:
Program 1:

CPP

// C++ program to illustrate the
// vector::emplace() function
// insertion at thefront
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> vec = { 10, 20, 30 };

    // insert element by emplace function
    // at front
    auto it = vec.emplace(vec.begin(), 15);

    // print the elements of the vector
cout << "The vector elements are: ";
    for (auto it = vec.begin(); it != vec.end(); ++it)
        cout << *it << " ";

    return 0;
}

Output:

The vector elements are: 15 10 20 30

Program 2:

CPP

// C++ program to illustrate the
// vector::emplace() function
// insertion at the end
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> vec = { 10, 20, 30 };

    // insert element by emplace function
    // at the end
    auto it = vec.emplace(vec.end(), 16);

    // print the elements of the vector
cout << "The vector elements are: ";
    for (auto it = vec.begin(); it != vec.end(); ++it)
        cout << *it << " ";

    return 0;
}

Output:

The vector elements are: 10 20 30 16

Program 3:

CPP

// C++ program to illustrate the
// vector::emplace() function
// insertion at the middle
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> vec = { 10, 20, 30 };

    // insert element by emplace function
    // in the middle
    auto it = vec.emplace(vec.begin() + 2, 16);

    // print the elements of the vector
cout << "The vector elements are: ";
    for (auto it = vec.begin(); it != vec.end(); ++it)
        cout << *it << " ";

    return 0;
}

Output:

The vector elements are: 10 20 16 30

Additional Related Links: emplace_front( ) and emplace_back( )

Regular insertion using shift operation in arrays V/s emplace( ) function :

a) If want to insert an element at some specific position between the first and last index, we have to shift all the elements next to that specific index. So, if want to keep our code precise so emplace( ) would be a good choice. In terms of Time Complexity both take the same linear time which is directly dependent on the number of shift operations.

Ex: 

C++

#include <iostream>
#include <vector>
#include <array>
using namespace std;

int main() {
    array<int,6> a={1,2,4,5};
    vector<int>  v={1,2,4,5};

    // Insert 3 in the arr at index 2

    for(int i=3;i>=0;i--)
    {
      if(i!=1)
        a[i+1]=a[i];
      else {
        a[i+1]=3; break;
        }
    }   // Time complexity is high

    cout<<"Content of a: ";
    for(int i=0;i<5;i++)
       cout<<a[i]<<" ";

    v.emplace( v.begin() + 2 , 3);

    cout<<"\nContent of v: ";
    for(int i=0;i<v.size();i++)
       cout<<v[i]<<" ";

    return 0;
}

Output

Content of a: 1 2 3 4 5 
Content of v: 1 2 3 4 5