Weird behaviour in my collision Detection

I'm trying to detect a collision between multiple Objects. But my list iteration seems to work wrong and I can't figure out why. It has to be a simple problem.

I got a "Collision Manager" which have a method "checkCollision"

The Collision Manager got a pointer of the GameObject Manager, which has a list with every collideable Object.

I feel like im doing something wrong with my list Iteration.

Maybe you are able to see my problem?

void collisionManager::checkCollision()
{
    this->aIT = this->goManager->GBList.begin();
    this->bIT = this->goManager->GBList.begin();

    SDL_Rect * result = new SDL_Rect();

    while (this->aIT != this->goManager->GBList.end())
    {
        GameObject * a = *this->aIT;

        while (this->bIT != this->goManager->GBList.end())
        {
            GameObject * b = *this->bIT;

            if (a != b)
            {               
                SDL_Rect * result = new SDL_Rect();
                if (SDL_IntersectRect(a->collider->collisionBox, b->collider->collisionBox, result))
                {
                    a->onCollide(b);
                    b->onCollide(a);
                }
                else {

                }
            }
            else
            {
                a->onCollide(b);
            }

            this->bIT++;
        }
        this->aIT++;
    }

Solution

Your problem comes from the fact that the iterator bIt never gets reset when the outer loop (the one with aIt) is iterated. Let's decompose your code in steps :

you initialize the iterators to the beginning of the list before the two nested loops.
You enter the outer loop (the one with aIt) and select a as the first object of the list
You enter the inner loop and your iterator bIt goes through the list of all objects.
At the end of the inner loop, your iterator bItdoes not get reset to the beginning of the list of game objects and stay at its last value : the end of the list.

Therefore you never enter the inner loop a second time.

The solution is to move the line:

this->bIT = this->goManager->GBList.begin();

inside the outer loop, so after the first while.