How to know which body collide? - xcode

Question

i am using cocos2d and box2d, with contact listener, and lets say i have a body that can hit a number of other bodies, BUT each one of them is turn on the contact listener.
so how can i know who hit who ?
i have this in my tick :
for(pos = _contactListener->_contacts.begin(); pos != _contactListener->_contacts.end(); ++pos)
{
MyContact contact = *pos;
b2Body *bodyA=contact.fixtureA->GetBody();
b2Body *bodyB=contact.fixtureB->GetBody();
//check if collision between to bodies
if( bodyA->GetUserData() !=NULL && bodyB->GetUserData() !=NULL) //if ((contact.fixtureA == _bottomFixture && contact.fixtureB == _ballFixture) ||(contact.fixtureA == _ballFixture && contact.fixtureB == _bottomFixture))
{
NSLog(#"Ball hit bottom!");
}
thanks a lot .

Answer 1

while creating the body set userdata like this
CCSprite *red=[CCSprite spriteWithFile:#"red.png"];
red.tag=3;
[self addChild:red];
b2BodyDef bd;
bd.type=b2_dynamicBody;
bd.position.Set(w/PTM_RATIO,h/PTM_RATIO);
bd.userData=red;
for(pos = _contactListener->_contacts.begin(); pos != _contactListener->_contacts.end(); ++pos)
{
MyContact contact = *pos;
b2Body *bodyA=contact.fixtureA->GetBody();
b2Body *bodyB=contact.fixtureB->GetBody();
//check if collision between to bodies
if( bodyA->GetUserData() !=NULL && bodyB->GetUserData() !=NULL) //if ((contact.fixtureA == _bottomFixture && contact.fixtureB == _ballFixture) ||(contact.fixtureA == _ballFixture && contact.fixtureB == _bottomFixture))
{
so here
CCSprite *actor = (CCSprite*)bodyA->GetUserData();
if ([actor tag] == 3) {
//red box
}
}

Answer 2

Put some identifier into user data. For example:
struct MyUserData
{
int myUniqueId;
};
When creating bodies attach some unique number to each and then you will be able to understand which body was colliding.