Writing to a class pointer gives me an access violation error

Question

Please view this image of the crash after I choose to debug it in MVS2010: http://i48.tinypic.com/dr8q9u.jpg

Here's the Game.h header that shows the Game class structure, and in the picture you will see the offending method that's causing the access violation (setBot(botInfo * b)).

class botInfo; // Forward declaration

class Game {
 int gameState;
 int flagDropTime;
 botInfo * bot;
public:
 Game();
 ~Game(void);
 void startGame();
 void gameOver(int victoriousTeam);
 void resetBall();
 void hideBall();
 int getState();

 void setBot(botInfo * bot);
 botInfo * getBot();
};

From an instance of botInfo (another class) I'm calling a function with this code, (Game _dsbTrench is a member variable of the botInfo instance).

botInfo * botPointer = this;
_dsbTrench->setBot(botPointer);

Problem is, whenever I call this it causes an exception: Unhandled exception at 0x72332569 (PubBot.dll) in MERVBot.exe: 0xC0000005: Access violation writing location 0xcdcdcdd5.

So whats the cause of this error? And how can I fix it?

Thanks.

Answer 1

I don't think there is enough information for me to help. But I will try.

You are writing to memory that is not assigned to your program by the operating system -- you need to allocate the memory before you write to it.

1. You should not be passing the this parameter to other functions -- your offset calculations are probably off -- can't you use a reference ? .
2. Have you allocated memory for dsbtrench ?

-- edit --

As mentioned by the other answers"0xCDCDCDCD" is a sentinel variable used for uninitialized words.

Answer 2

did you debug that code? it seems that _dsbTrench is null when you call setBot method for it. passing "this" pointer is ok as long as you know what you are doing with it :)

Answer 3

0xCDCDCDCD is a flag value that the MS C runtime uses to fill newly allocated memory. In your "_dsbTrench->setBot(botPointer);", _dsbTrench is 0xCDCDCDCD - which is obviously a bogus pointer. Chances are you forgot to initialize that variable.

Answer 4

For more details on the various memory states 0xCDCDCDCD, 0xDDDDDDDD and so on this is a great reference:

http://www.nobugs.org/developer/win32/debug_crt_heap.html