erasing elements from a boost::ptr_vector (boost n00b)

I'm having trouble ereasing elements from the ptr_vector, sometimes it crashes my program, here is the method where it happens

Code:

---

void Juego::checkState()
{
    Bullet *b;
    boost::ptr_vector<Entity>::iterator iter;
   
    for (int i = 0; i < entities.size(); i++)
    {
        if (entities[i].getInst() == inst_bullet)
        {
            b = (Bullet*)&entities[i];
            if (b->getDestroy())
            {
                iter = entities.begin();
                for (int j = 0; j < i; j++)
                {
                    iter++;
                }
                entities.erase(iter);
            }
        }
    }
}

---

Here is what I'm trying to do:

Go through the ptr_vector of Entities and if one is a Bullet (which is derived from entity) I call getDestroy() (which will tell me if the bullet must be destroyed, and is not inherited from Entity) if the bullet must be destroyed I erase that bullet from the vector.

As you can see I'm not familiar at all with iterators and boost, how should I do what I want?

I'm not sure why it would crash. One problem I see is thet you are skipping over some values in your loop. If i is 3 and you erase that value, then all the elements after it will be moved over, and a new element will be put in the 3 spot. In the next iteration of the loop i will be 4 and will never check the element that got moved into the 3 spot.

Also, the entire block of code inside the if (b->getDestroy()) can be changed to entities.erase(entities.begin() + i);

Another thing that doesn't look right is b = (Bullet*)&entities[i]; This is generally an indication of bad design. Your Entity class should have the virtual getDestroy() function that you can call without casting directly to a Bullet.

Normally, you would just use iterators to iterate over the whole thing. Assuming thet getDestroy is virtual in Entity:

Code:

---

    boost::ptr_vector<Entity>::iterator iter = entities.begin();
   
    while (iter != entities.end())
    {
        if (iter->getInst() == inst_bullet && iter->getDestroy())
        {
            iter = entities.erase(iter);
        }
        else
        {
            ++iter;
        }
    }

---

I can't think of why any of these things would cause a crash, but they are worth looking into. Also make sure Entity has a virtual destructor.

Thanks a bunch Daved, as always you are the man.

Especially thanks for the last comment about making sure the destructor was virtual, it got me checking and I discovered that under certain circumstances de destructor would try to delete a pointer which didn't point to anything causing the program to crash.