EXC_BAD_ACCESS error when box2d joint is destroyed

Question

When I destroy the weldJoint in the update method (see below) I get an EXC_BAD_ACCESS error pointing to the line

world->DestroyJoint(weldJoint);

in the update method below:

    -(void) update: (ccTime) dt
{

int32 velocityIterations = 8;
int32 positionIterations = 1;

// Instruct the world to perform a single step of simulation. It is
// generally best to keep the time step and iterations fixed.
world->Step(dt, velocityIterations, positionIterations);

// using the iterator pos over the set
std::set<BodyPair *>::iterator pos;

for(pos = bodiesForJoints.begin(); pos != bodiesForJoints.end(); ++pos)
{

    b2WeldJointDef weldJointDef;

    BodyPair *bodyPair = *pos;
    b2Body *bodyA = bodyPair->bodyA;
    b2Body *bodyB = bodyPair->bodyB;

    weldJointDef.Initialize(bodyA, bodyB, bodyA->GetWorldCenter());

    weldJointDef.collideConnected = false;
    weldJoint = (b2WeldJoint*) world->CreateJoint(&weldJointDef);

    // Free the structure we allocated earlier.
    free(bodyPair);

    // Remove the entry from the set.
    bodiesForJoints.erase(pos);
}

for(b2Body *b = world->GetBodyList(); b; b=b->GetNext())    {
    if (b->GetUserData() != NULL)
    {
        CCSprite *mainSprite = (CCSprite*)b->GetUserData();
        if (mainSprite.tag == 1) {
            mainSprite.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
            CGPoint mainSpritePosition = mainSprite.position;
            if (mainSprite.isMoved) {

                        world->DestroyJoint(weldJoint);

                    }
            }
        }
    }

}  

In the HelloWorldLayer.h I set the weldJoint with the assign property. Am I destroying the joint in the wrong way? I would really appreciate any help. Thanks

Answer 1

Are you sure you are not calling DestroyJoint(weldJoint) more than once?

You have a for next and as far as I can tell, weldJoint is never set to a different value during this loop. Should the if condition matches more than once in this loop you will certainly have a problem if memory is being deleted each call to DestroyJoint().

Either add a break to exit the loop once DestroyJoint is called or set the value of weldJoint during the loop.

Also, make sure the DestroyJoint() function won't mess up the for next loop either if you don't intend to exit the loop on first match.

for(b2Body *b = world->GetBodyList(); b; b=b->GetNext())    {

If you call DestroyJoint() will it effectively delete what b is pointing at? If the answer is yes, then b = b->GetNext() is unsafe and unpredictable.