Vector emplace_back in C++ STL

In C++, the vector emplace_back() is a built-in method used to insert an element at the end of the vector by constructing it in-place. It means that the element is created directly in the memory allocated to the vector avoiding unnecessary copies or moves.

Let’s take a quick look at a simple example that illustrates vector emplace_back() method:

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v;

    // Inserting elements
    v.emplace_back(1);
    v.emplace_back(9);
    v.emplace_back(5);

    for (auto i : v)
        cout << i << " ";
    return 0;
}

1 9 5 

This article covers the syntax, usage, and common queries of vector emplace_back() method in C++:

Syntax of Vector emplace_back()

The vector emplace_back() is the member method of std::vector class defined inside <vector> header file.

v.emplace_back(val);

Parameters:

• val: Value to be added. It is forwarded to the constructor of the type of vector.

Return Value:

• Until C++ 17, this function didn't used to return any value, but now, it returns the reference to the inserted value.

Examples of vector emplace_back()

The following examples demonstrates the use of vector emplace_back() function for different scenarios:

Inserting Elements in Vector of Strings

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<string> v = {"Hi", "Geeks!"};

    // Inserting element at the end
    v.emplace_back("Welcome");

    for (auto i : v)
        cout << i << " ";
    return 0;
}

Hi Geeks! Welcome 

Inserting Elements in a Vector of Custom Type

#include <bits/stdc++.h>
using namespace std;

// Class that notifies when copied
class A {
  public:
  int a;
  A(int x = 0) {
      a = x;
  }
  A(const A& other) {
    	a = other.a;
    	cout << a << "'s CC called\n";
  }
};

int main() {
    vector<A> v;
  
  	v.reserve(5);
    
  	// Inserting element using emplace_back()
    v.emplace_back(1);
  	v.emplace_back(9);
  	
  	// Inserting element using push_back()
  	v.push_back(5);
  
    return 0;
}

5's CC called

From the above example, we can confirm that vector emplace_back() doesn't create extra copies but vector push_back() does.

Vector emplace_back() vs push_back()

Both functions are used to insert elements at the back of vector but they differs in the way of insertion. Following table lists the primary differences of the vector emplace_back() and vector push_back() in C++: