std::back_inserter in C++

std::back_inserter() is a utility function in the C++ Standard Library that creates a back-insert iterator for a container. Instead of overwriting existing elements, it automatically inserts new elements at the end of the container using push_back().

• Creates an output iterator that inserts elements at the end of a container.
• Commonly used with STL algorithms such as std::copy() and std::transform().

Example: Using std::back_inserter()

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
using namespace std;

int main()
{
    vector<int> v1 = {1, 2, 3};
    vector<int> v2 = {4, 5, 6};

    copy(v1.begin(), v1.end(), back_inserter(v2));

    for (auto x : v2)
        cout << x << " ";

    return 0;
}

4 5 6 1 2 3 

Explanation: std::back_inserter(v2) creates an output iterator that appends elements to v2 using push_back().

Syntax

std::back_inserter(Container& x);

Parameters: x - Container in which elements will be inserted at the end.
Return Value: Returns a std::back_insert_iterator associated with the container.

Working of std::back_inserter()

std::back_inserter() creates a special output iterator that inserts elements at the end of a container using the container's push_back() function.

• Creates a back_insert_iterator associated with the specified container.
• Every assignment to the iterator automatically calls push_back().
• Allows STL algorithms to append elements without requiring preallocated space.

Internally, the following operation:

*iterator = value;
is equivalent to:
container.push_back(value);

Example 1: Copying Elements into an Existing Container

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

int main()
{
    vector<int> v1 = {1, 2, 3};
    vector<int> v2 = {4, 5, 6};

    copy(v1.begin(), v1.end(),
         back_inserter(v2));

    for (auto x : v2)
        cout << x << " ";

    return 0;
}

4 5 6 1 2 3 

Example 2: Copying into an Empty Container

One of the biggest advantages of std::back_inserter() is that the destination container does not need to have a predefined size.

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

int main()
{
    vector<int> source = {1, 2, 3};
    vector<int> destination;

    copy(source.begin(),
         source.end(),
         back_inserter(destination));

    for (auto x : destination)
        cout << x << " ";

    return 0;
}

1 2 3 

Explanation: Since destination is initially empty, std::back_inserter() automatically inserts elements using push_back().

Containers Supported by std::back_inserter()

std::back_inserter() works only with containers that support the push_back() member function. Examples include:

• std::vector
• std::deque
• std::list
• std::string

It cannot be used with containers such as:

• std::set
• std::map
• std::array
• std::forward_list

Advantages of std::back_inserter()

std::back_inserter() provides a convenient way to insert elements into a container while using STL algorithms.

• Automatically appends elements to the end of a container.
• Eliminates the need to preallocate the container size.
• Allows algorithms such as std::copy() to work with initially empty containers.
• Improves code readability by avoiding explicit push_back() calls.

push_back() vs std::back_inserter()

Note: std::back_inserter() internally returns a std::back_insert_iterator object.