Question

I am new to Objective-C. I am currently working on a game using Cocos2D and Box2D. My problem is that when 3 objects collide together, the game crashes. Now let me describe my game in details:

In my game I have a main character standing on top of a building. Below the building there’s this the road. Enemies pass by the road at various random speeds entering the screen from right and exiting from the left. I have created the enemies as b2_kinematicBodies and set a random velocity for each of them using SetLinearVelocity(). The main character shoots the enemies. The projectile (the object being shot) is a b2_dynamicBody. When the projectile hits the enemies, both the projectile and the enemy are destroyed. During gameplay sometimes an enemy moving at a slow speed is crossed by one which is moving at a higher speed. If a projectile hits the two enemies just at the point when they are overlapping and one is about to pass the other one, the game crashes! Please help me with this.

I have detected collision using b2contactListener class.

One thing I didn’t mention before is that I am not creating the enemies as individual distinct bodies. Instead, I am creating it once and making it move and I am calling this method (which creates the enemies and makes them move) inside init as below:

[self schedule:@selector(addRightTarget) interval:2.0];

Answer 1

I believe the issue is that the collisions are calculated before your handler gets any calls. Meaning that when your handler gets called, the bullet has hit 2 objects. So you get 2 call-backs as shown below.

Collision Detected: Bullet + Enemy1

• Destroy Enemy1
• Destroy Bullet

Collision Detected: Bullet + Enemy2

• Destroy Enemy2
• Destroy Bullet [CRAAAASH!!! You just tried to delete a non-existent object]

1st: You should not be removing anything except in your step function (as someone mentioned in another answer)

2nd: Pick one of these:

• Make your list/array of objects-to-delete be a ‘set’ or implemented in such a way that duplicates are avoided.
• Check for existence of your object in the world