赋值后不久指针值重置为null

Question

我有一个设置为的指针0,然后在同一个函数中,在一些循环/条件中,我尝试重新分配它..(请阅读评论)

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!
            }
        }

所以有一刻我指责我的指针没问题,下一刻指针似乎根本没有改变.

澄清,Turns是一个懒惰的typedef std::pair<Vect, Entity*>,道歉,如果这使我的代码更难阅读,它是一些快速抛出敌人ai.我将在下面发布完整的功能.

我真的很难过这里,不确定我是不是一个白痴或者一些奇怪的东西,真的很感激任何人花时间去看.

//寻找幽灵可以采取的转弯.

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

检查这一行:

for(Turns turn : pointsUpLeftDownRight)

您正在迭代元素的副本pointsUpLeftDownRight.当副本被销毁时(在for正文的末尾),您分配给该副本的任何值都将丢失.您的作业会暂时更改.

试试这个:

for(Turns& turn : pointsUpLeftDownRight)