How to remove an object from a std::vector

Question

So I have a vector of bullets that show up on the screen and I do not want these bullets to live forever. I want them to "die as soon as they go off screen or collide with an enemy. How can I make sure they are removed from the array and no longer take up memory.

code:

std::vector<Bullet> bulletArray;

for(int i=0; i<bulletArray.size(); i++)
{
    bulletArray[i].MoveBullet();
    bulletArray[i].Draw();
    if(bulletArray[i].PosY<0)
    {
        //Delete object forever 
    }
}

Answer 1

There are two ways to do it:

1. You can simply remove object i from the vector using bulletArray.erase(bulletArray.begin() + i) and then i-- to process everything for the new bullet in i'th position. But this way vector will move all the object after the that specific bullet, meaning this kind of removal is O(bulletArray.size()).

2. But usually order of the bullets is not important, so you can simply put the last bullet in the i'th cell, and only remove it's spot. This way it's much more faster since these operations are O(1). And it's implemented as bulletArray[i] = bulletArray.back(); bulletArray.pop_back(); i--; again you need to decrement i by one to check this new object in this i'th position in the next loop.

Answer 2

In general, I find that it's a good idea to add a flag (normally I call this "dead") to the objects.

Then you can do this (in pseudo-pythonic code):

// Processing
for obj in my_list:
  if obj has hit enemy:
    obj.dead = true
  if obj outside play area:
    obj.dead = true
// Rendering (at some different point)
for obj in my_list:
  render somehow

Then you can use the std::remove_if higher-order function (in algorithms) to remove the ones with the "dead" flag set. Remember to use std::erase to truncate the vector to the right point.

The main advantage of using this remove_if/erase is that it avoids problems when trying to remove elements while iterating the list.

Additionally, sometimes you may want to remove object X while processing object Y (in collision detection, such situations are normal) - we can do this easily without breaking any iterator objects.

(Note that iterator objects are normally only valid as long as no new elements are added or removed. Especially for std::vector)