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); } 

Answer 2

If you define your function to take argument of std::vector<int>& arr and integer value, then you can use push_back inside that function:

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

usage:

std::vector<int> arr; do_something(1, arr);