Question

I try to program a top-down-shooter in SFML at the moment, but ran into a problem. I’m quite new to C++ and programming in general, so please excuse messy code and/or overly complicated solutions.

I have two std::lists, one containing the randomly spawning enemies, the other containing the bullets I fired. When a bullet hits a enemy, both of them should get erased, but it doesn’t work.

Here’s the problem-part of my code:

for(MonsterIt = MonsterList.begin(); MonsterIt != MonsterList.end(); MonsterIt++)
{

    //Here would be Monster-Movement

    //Collision Monster-Player (MonsterIt = iterator of MonsterList)
    if ((MonsterIt -> getPosition().x + 25) >= PlayerX - 25 && 
        (MonsterIt -> getPosition().x - 25) <= PlayerX + 25 &&
        (MonsterIt -> getPosition().y + 25) >= PlayerY - 25 && 
        (MonsterIt -> getPosition().y - 25) <= PlayerY + 25   )
    {
        MonsterList.erase(MonsterIt);
        break;              
    }


        window.draw(*MonsterIt);
}

That’s the way I’ve done collision between Monster and Player. That worked fine, so I tried the same with Monsters and Lasers:

for(LaserIt = LaserList.begin(); LaserIt != LaserList.end(); LaserIt++)
{

    //Here would be "Laser-Movement"

    //Collision-Laser                               // Doesn't work
    if ((MonsterIt -> getPosition().x + 25) >=      //
        (LaserIt   -> getPosition().x - 7) &&       //
        (MonsterIt -> getPosition().x - 25) <=      //
        (LaserIt   -> getPosition().x + 7) &&       //
        (MonsterIt -> getPosition().y + 25) >=      //
        (LaserIt   -> getPosition().y - 7) &&       //
        (MonsterIt -> getPosition().y - 25) <=      //
        (LaserIt   -> getPosition().x + 7))         //
    {                                               //
        MonsterList.erase(MonsterIt);               //
                                                    //
        LaserList.erase(LaserIt);                   //
                                                    //
        break;                                      //
    }                                               //  

    window.draw(*LaserIt);
}

When I put in the part of code I marked (with // on right side) I get a “list iterator not dereferencable”-error while debugging as soon as I shoot. When I cut out said code it runs fine (I can shoot, walk into monsters and they disappear, etc.). Because of that I guess the rest of my code is working.

So, is collision between iterators of different lists even possible?
And if so, how do I do it?

If you need more information or code, please ask. I’d be glad for your help…

Answer 1

The iterator you use is not valid.

There are several solutions to your problems. First one:

for(MonsterIt = MonsterList.begin(); MonsterIt != MonsterList.end(); MonsterIt++)
{
    //Here would be Monster-Movement

    //Collision Monster-Player (MonsterIt = iterator of MonsterList)

    // Collision with laser, inside the loop for monsters
    for(LaserIt = LaserList.begin(); LaserIt != LaserList.end(); LaserIt++)
    {
    }

    window.draw(*MonsterIt);
}

This solution is not really C++ish. With OOP, you can have a much clearer code:

class Player
{
    public:
        int X, Y; // for code simplicity on SO.
};
// ...

Player player;
for(MonsterIt = MonsterList.begin(); MonsterIt != MonsterList.end(); MonsterIt++)
{
    MonsterIt->update(player,LaserList);
    if(!MonsterIt->isAlive())
    {
        // Remove monster from list *without* break
    }
}

Then, Monster is a class like the following:
class Monster
{
bool alive;
public:
bool isAlive() const { return alive; }

        bool collideWithPlayer(Player p) const
        {
            // Returns whether it collide with player
            // ...
        }

        bool collideWithLaser(Laser l) const
        {
            // Returns whether it collide with one laser
            // ...
        }

        bool collideWithLasers(LaserList l) const
        {
            for(LaserIt = LaserList.begin(); LaserIt != LaserList.end(); LaserIt++)
                if(collideWithLaser(*LaserIt))
                    return true;
            return false;
        }


        void update(Player p, LaserList l)
        {
            if(collideWithPlayer(p) || collideWithLasers(l))
                alive = false;
            else
                window.draw(this);
        }
};