c++ - Passing vector by reference

Question

Using normal C arrays I'd do something like that:

void do_something(int el, int **arr)
{
   *arr[0] = el;
   // do something else
}

Now, I want to replace standard array with vector, and achieve the same results here:

void do_something(int el, std::vector<int> **arr)
{
   *arr.push_front(el); // this is what the function above does
}

But it displays "expression must have class type". How to do this properly?

Answer 1

You can pass the container by reference in order to modify it in the function. What other answers haven’t addressed is that std::vector does not have a push_front member function. You can use the insert() member function on vector for O(n) insertion:

void do_something(int el, std::vector<int> &arr){
    arr.insert(arr.begin(), el);
}

Or use std::deque instead for amortised O(1) insertion:

void do_something(int el, std::deque<int> &arr){
    arr.push_front(el);
}