Why use std::back_inserter instead of end() during std::copy?

Question

I have seen std::copy() using std::back_inserter but I used std::end() and both works. My question is, why was std::back_inserter needed if std::end() does the works just fine?

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

int main() 
{ 
    // Declaring first container 
    vector<int> v1 = { 1, 2, 3 }; 

    // Declaring second container for 
    // copying values 
    vector<int> v2 = { 4, 5, 6 }; 

    // Using std::back_inserter inside std::copy 
    //std::copy(v1.begin(), v1.end(), std::back_inserter(v2));  // works
    std::copy(v1.begin(), v1.end(), v2.end());  // also works
    // v2 now contains 4 5 6 1 2 3 

    // Displaying v1 and v2 
    cout << "v1 = "; 

    int i; 
    for (i = 0; i < 3; ++i) { 
        cout << v1[i] << " "; 
    } 

    cout << "\nv2 = "; 
    for (i = 0; i < 6; ++i) { 
        cout << v2[i] << " "; 
    } 

    return 0; 
}

Answer 1

The first one inserts values into the vector, the other one is undefined behavior, it writes items into the spot just past the end of the vector.

Try printing the resulting vector:

std::copy(v1.begin(), v1.end(), std::back_inserter(v2));  // works
for (auto x : v2) cout << " " << x;
cout << endl;

Prints

 4 5 6 1 2 3

Whereas

std::copy(v1.begin(), v1.end(), v2.end());
for (auto x : v2) cout << " " << x;
cout << endl;

Prints

 4 5 6

(In Debug mode raises an assertion failure)

The fact that it works for you in your particular compiler doesn't make it correct. Appearing to work is a typical manifestation of UB.