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 lt int gt    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

User · Answer

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);
}

User · Answer

void do something int el  std  vector lt int gt    arr    should be  void do something int el  std  vector lt int gt  amp  arr        arr push back el       Pass by reference has been simplified to use the  amp  in C

User · Answer

If you define your function to take argument of  std  vector lt int gt  amp  arr and integer value  then you can use  push back inside that function   void do something int el  std  vector lt int gt  amp  arr        arr push back el                  usage   std  vector lt int gt  arr  do something 1  arr

User · Answer

You can pass vector by reference just like this   void do something int el  std  vector lt int gt   amp 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 lt int gt   amp 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

User · Answer

You don t need to use   arr  you can either use   void do something int el  std  vector lt int gt   arr       arr- gt push back el       or    void do something int el  std  vector lt int gt   amp 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 lt int gt    arr         arr - gt push back el       but again there is no reason to do so

[c++] Passing vector by reference

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