Creating stl vector from pair of iterator c++

Question

I am trying to create a stl vector from iterator pair and I am not sure how many elements the vector might have. It could have only one element.

#include <iostream>
#include <vector>

int main()
{
    using namespace std;

    vector<int> vect;
    for (int nCount=0; nCount < 6; nCount++)
        vect.push_back(nCount);

    vector<int> second(vect.begin(), vect.begin() );

    vector<int>::iterator it; // declare an read-only iterator
    it = second.begin(); // assign it to the start of the vector

    while (it != second.end()) // while it hasn't reach the end
    {
        cout << *it << " "; // print the value of the element it points to
        it++; // and iterate to the next element
    }

    cout << endl;
}

I thought that vector 'second' would have one element pointed by vect.begin(). Is it not the case?

Thanks

Answer 1

vector<int> second(vect.begin(), vect.begin() + 1);

The vector constructor uses an open interval so the end is not included ie. [first, last)

As lip pointed out in his comment it's more generic to next :

second(vect.begin(), next(vect.begin()));

Answer 2

No, this is not the case. The documentation is pretty clear:

template< class InputIt > 
vector( InputIt first, InputIt last, const Allocator& alloc = Allocator() ); (4)    

4) Constructs the container with the contents of the range [first, last).

The notation "[first, last)" indicates that all the elements between first and last but not including last are copied. Since first == last , no elements are copied.

Reading the documentation further, it would appear that you can use another constructor:

explicit vector( size_type count, 
                 const T& value = T(),
                 const Allocator& alloc = Allocator());  (until C++11)
         vector( size_type count, 
                 const T& value,
                 const Allocator& alloc = Allocator());  (since C++11)

...in this way:

vector<int> second(1, vect.front());

Answer 3

No. In the constructor vector<int> second(vect.begin(), vect.begin()); the second iterator should point past the end, so you get exactly empty array.

Example: vect.end() points exactly past the end of vector vect , so vector<int> second(vect.begin(), vect.end()); would copy the whole vect into second .