[c++] Passing vector by reference

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?

You can pass vector by reference just like this:

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

However, note that this function would always add a new element at the back of the vector, whereas your array function actually modifies the first element (or initializes it value).

In order to achieve exactly the same result you should write:

void do_something(int el, std::vector<int> &arr){
    if (arr.size() == 0) { // can't modify value of non-existent element
        arr.push_back(el);
    } else {
        arr[0] = el;
    }
}

In this way you either add the first element (if the vector is empty) or modify its value (if there first element already exists).

void do_something(int el, std::vector<int> **arr)

should be

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

Pass by reference has been simplified to use the & in C++.

You don't need to use **arr, you can either use:

void do_something(int el, std::vector<int> *arr){
    arr->push_back(el);
}

or:

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

**arr makes no sense but if you insist using it, do it this way:

void do_something(int el, std::vector<int> **arr){
    (*arr)->push_back(el);
}

but again there is no reason to do so...

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); 

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);
}