Vector emplace_back in C++ STL (original) (raw)

Last Updated : 26 Nov, 2024

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:

C++ `

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

int main() { vector v;

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

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

}

`

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

Table of Content

• Syntax of Vector emplace_back()
• Examples of vector emplace_back()
  • Inserting Elements in Vector of Strings
  • Inserting Elements in a Vector of Custom Type
• Vector emplace_back() vs push_back()

Syntax of Vector emplace_back()

The vector emplace_back() is the member method of std::vector class defined inside ****** 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

C++ `

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

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

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

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

}

`

Inserting Elements in a Vector of Custom Type

C++ `

#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 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;

}

`

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++: