Question

I have a pointer that is set to 0, then later on in the same function, inside some loops/conditions I try to re assign it.. (please read the comments)

for(Entity* tile : _originalFloorTiles)
        {
            for(Turns turn : pointsUpLeftDownRight)
            {
                if(tile->GetCollisionRect().ContainsPoint(turn.first.x, turn.first.y)){
                    turn.second = tile; //everything looks fine here, turn.second is still null and tile is a valid pointer
                    assert(turn.second); //turn.second is definitely assigned the value of tile here.
                }
               HAPI->DebugText("check pointsUpLeftDownRight now");//!Here's where it gets weird,
            // If i hover over turn and inspect it in visual studio now, turn.second is still completely valid 
            // (with the assigned value of tile).
            // But hovering over pointsUpLeftDownRight shows its contents for each turn..
            // and inside there the current turn is a NULL pointer for turn.second!
            }
        }

So one moment i have assignd my pointer no problem, and the next moment the pointer doesn’t seem to have changed at all.

To clarify, Turns is a lazy typedef for std::pair<Vect, Entity*> , apologies if that makes my code harder to read, it’s some quickly thrown together enemy ai. I’ll post the complete function below.

I’m really stumped here and not sure if i’m being an idiot or something weird is going on, would really appreciate anyone taking the time to look.

//looks for turns that the ghost can take.

void IceGhostNPC::RespondToTimePassed()
{   
    //Entity* t = _originalFloorTiles[0];

    //test if enough time has passed since ghost last decided to look for turns
    if(_lastTimeTurned < timeGetTime() - _timeBeforeSearchingForTurns)
    {
        //store points surrounding ghost in a way that we can associate them with a valid floor tile to move onto 
        std::vector<Turns> pointsUpLeftDownRight;
        pointsUpLeftDownRight.push_back(
            Turns(Vect(GetCenterXPos(), GetCenterYPos() - floorTileHeight), 0)); //point above
        pointsUpLeftDownRight.push_back(
            Turns(Vect(GetCenterXPos() - floorTileWidth, GetCenterYPos()), 0)); //point left
        pointsUpLeftDownRight.push_back(
            Turns(Vect(GetCenterXPos(), GetCenterYPos() + floorTileHeight), 0)); //point down
        pointsUpLeftDownRight.push_back(
            Turns(Vect(GetCenterXPos() + floorTileWidth, GetCenterYPos()), 0)); //point right

        //look through original floor tiles, 
        for(Entity* tile : _originalFloorTiles)
        {
            //see if its possible to take a turn 
            for(Turns turn : pointsUpLeftDownRight)
            {
                if(tile->GetCollisionRect().ContainsPoint(turn.first.x, turn.first.y)){
                    turn.second = tile;
                    assert(turn.second);
                }
                HAPI->DebugText("check pointsUpLeftDownRight now");
            }
        }

        //Now to make the behaviour more interesting we have the ghost randomly take one of the turns,
        // ( we use associated tile to check the turn is possible, and we can also change that tile to an icy patch )
        bool turnTaken = false;
        do{
            int turnChoice = rand() % 4;
            if(pointsUpLeftDownRight[turnChoice].second == 0)
                continue; //go back to top of loop if that turn had a null tile
            else
            {
                switch(turnChoice){
                    case 0: //turn upwards
                        _moveable->SetYDirection(Controller::UP);
                        _moveable->SetXDirection(Controller::NONE);
                        break;
                    case 1: //turn leftwards
                        _moveable->SetYDirection(Controller::NONE);
                        _moveable->SetXDirection(Controller::LEFT);
                        break;
                    case 2: //turn downwards
                        _moveable->SetYDirection(Controller::DOWN);
                        _moveable->SetXDirection(Controller::NONE);
                        break;
                    case 3: //turn right
                        _moveable->SetYDirection(Controller::NONE);
                        _moveable->SetXDirection(Controller::RIGHT);
                        break;
                }
                turnTaken = true;
                _lastTimeTurned = timeGetTime();
                //ice tile up baby
            }

        }while(turnTaken = false);
    }

    FinishResponding(timeGetTime());
}

Answer 1

Check this line:

for(Turns turn : pointsUpLeftDownRight)

You are iterating over copies of the elements in pointsUpLeftDownRight. Whatever value you assign to that copy will be lost when the copy is destroyed (at the end of the for body). Your assignment changes a temporary.

Try with this instead:

for(Turns& turn : pointsUpLeftDownRight)