Different methods to copy in C++ STL | std::copy(), copy_n(), copy_if(), copy_backward()

Various varieties of copy() exist in C++ STL that allows to perform the copy operations in different manners, all of them having their own use. These all are defined in header <algorithm>. This article introduces everyone to these functions for usage in day-to-day programming. 
1. copy(strt_iter1, end_iter1, strt_iter2) : The generic copy function used to copy a range of elements from one container to another. It takes 3 arguments: 

• strt_iter1 : The pointer to the beginning of the source container, from where elements have to be started copying.
• end_iter1 : The pointer to the end of source container, till where elements have to be copied.
• strt_iter2 : The pointer to the beginning of destination container, to where elements have to be started copying.

2. copy_n(strt_iter1, num, strt_iter2) : This version of copy gives the freedom to choose how many elements have to be copied in the destination container. IT also takes 3 arguments: 

• strt_iter1 : The pointer to the beginning of the source container, from where elements have to be started copying.
• num : Integer specifying how many numbers would be copied to destination container starting from strt_iter1. If a negative number is entered, no operation is performed.
• strt_iter2 : The pointer to the beginning of destination container, to where elements have to be started copying.

// C++ code to demonstrate the working of copy()
// and copy_n()

#include<iostream>
#include<algorithm> // for copy() and copy_n()
#include<vector>
using namespace std;

int main()
{
    
   // initializing source vector 
   vector<int> v1 = { 1, 5, 7, 3, 8, 3 };
   
   // declaring destination vectors
   vector<int> v2(6);
   vector<int> v3(6);
   
   // using copy() to copy 1st 3 elements
   copy(v1.begin(), v1.begin()+3, v2.begin());
   
   // printing new vector
   cout << "The new vector elements entered using copy() : ";
   for(int i=0; i<v2.size(); i++)
   cout << v2[i] << " ";
   
   cout << endl;
   
   // using copy_n() to copy 1st 4 elements
   copy_n(v1.begin(), 4, v3.begin());
   
   // printing new vector
   cout << "The new vector elements entered using copy_n() : ";
   for(int i=0; i<v3.size(); i++)
   cout << v3[i] << " ";
  
}

Output: 

The new vector elements entered using copy() : 1 5 7 0 0 0 
The new vector elements entered using copy_n() : 1 5 7 3 0 0 

3. copy_if(): As the name suggests, this function copies according to the result of a "condition".This is provided with the help of a 4th argument, a function returning a boolean value. 
This function takes 4 arguments, 3 of them similar to copy() and an additional function, which when returns true, a number is copied, else number is not copied.
4. copy_backward(): This function starts copying elements into the destination container from backward and keeps on copying till all numbers are not copied. The copying starts from the "strt_iter2" but in the backward direction. It also takes similar arguments as copy().

// C++ code to demonstrate the working of copy_if()
// and copy_backward()

#include<iostream>
#include<algorithm> // for copy_if() and copy_backward()
#include<vector>
using namespace std;

int main()
{
    
    // initializing source vector 
    vector<int> v1 = { 1, 5, 6, 3, 8, 3 };
        
    // declaring destination vectors
    vector<int> v2(6);
    vector<int> v3(6);
        
    // using copy_if() to copy odd elements
    copy_if(v1.begin(), v1.end(), v2.begin(), [](int i){return i%2!=0;});
        
    // printing new vector
    cout << "The new vector elements entered using copy_if() : ";
    for(int i=0; i<v2.size(); i++)
    cout << v2[i] << " ";
        
    cout << endl;
        
    // using copy_backward() to copy 1st 4 elements
    // ending at second last position
    copy_backward(v1.begin(), v1.begin() + 4, v3.begin()+ 5);
        
    // printing new vector
    cout << "The new vector elements entered using copy_backward() : ";
    for(int i=0; i<v3.size(); i++)
    cout << v3[i] << " ";

}

Output: 

The new vector elements entered using copy_if() : 1 5 3 3 0 0 
The new vector elements entered using copy_backward() : 0 1 5 6 3 0 

5. Copy using inserter():

Before copy() operation let us understand the syntax of inserter(). 

inserter() is used as a destination that where we want to copy the elements of the container.

inserter() takes two parameters. The first is a container of arbitrary type and the second is an iterator into the container.

It returns an instance of insert_iterator working on a container of arbitrary type. This wrapper function helps in creating insert_iterator instances. Typing the name of the %iterator requires knowing the precise full type of the container, which can be tedious and impedes generic programming. Using this function lets you take advantage of automatic template parameter deduction, making the compiler match the correct types for you.

The syntax for inserter():

std::inserter(Container& x, typename Container::iterator it);

x: Destination container where the new elements will 
be inserted.
it: Iterator pointing to the insertion point.

Returns: An insert_iterator that inserts elements into 
x at the position indicated by it.

The syntax for copy using inserter():

copy(strt_iter1, end_iter1, inserter(Container& x, typename Container::iterator it));

// C++ code to demonstrate the working of copy() using inserter()

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

int main()
{

    vector<int> v1 = {1, 5, 7, 3, 8, 3};
    vector<int>::iterator itr;
    vector<int> v2;

    //using inserter()
    copy(v1.begin(), v1.end(), inserter(v2, itr));

    cout << "\nThe new vector elements entered using inserter: ";
    for (int i = 0; i < v2.size(); i++)
        cout << v2[i] << " ";

}

Output:

The new vector elements entered using inserter: 1 5 7 3 8 3