Strange behaviour : OpenGL + C + Pointers

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AmiDARK

Posted on: 2009/9/22 13:52 #1

Quite a regular

Hi,

This post follow the last one I've made concerning pointers.
Just to tell that I encounter a strange behaviour. Something that appear not logic for me.

To define an object, I created a structure called : Object3DType:


struct Object3DStruct{

  int ...;

  float ...;

  ...

 };

typedef struct Object3DStruct Object3DType;

I've also defined a type for a pointer to this kind of structures :


typedef Object3DType *Object3DPointer;

Up to here, everything should be ok.

I create a blank memory for storing a list of 256 objects pointers :


Object3DPointer *ObjectList;

ObjectList = IExec->AllocMem( 512 * (int*), MEMF_CLEAR );

ObjectList is then the pointer to the freshly created memory location to store 512 objects pointers.
( pointer to an array of 512 Object3DPointer )

Now, when I create a new 3D Object, I do this :


Object3DPointer NewObject;

NewObject = IExec -> AllocMem( sizeof( Object3DType ), MEMF_CLEAR );

and to store this pointer in the list, I do this :


AddObjectToList( ObjectID, NewObject);

Here is the AddObjectToList function:


void AddObjectToList( int ObjectID, Object3DPointer ObjectPTR ){

  if ( ObjectID >> 0 ){

    if (Basic3D.ObjectListAmount < ObjectID ){

      ResizeObjectList( ObjectID );

     }

    ObjectList[ ObjectID ] = ObjectPTR;

    Basic.ObjectAmount++;

   }

 }

The first thing that is wrong for me is that I should logically do this :


*ObjectList[ ObjectID ] = ObjectPTR;

To write in the memory location at offset ( ObjectID * 4 ) but if I do this, I get an error telling that types are incompatibles ...

The second thing is now when I want to get the pointer of the object. I do this :


Object3DPointer MyObject;

MyObject = GetObjectPTR( ObjectID );

I should then, following the AddObjectToList quotes, do this :


Object3DPointer GetObjectPTR( int ObjectID )[

  Object3DPointer NewObjectPTR = NULL;

  if ( ObjectID > 0 ){

    if ( ObjectID <= Basic3D.ObjectListAmount ){

      NewObjectPTR = ObjectList[ ObjectID ];

     }

   }

  return NewObjectPTR;

 }

If I do all of these, my program crash... strange.
if I simply change 1 line with :


NewObjectPTR = &ObjectList[ ObjectID ];

The program run fine ...
Why ???

Can someone light me about these odd things ?
I'm sure I'm doing something wrong with pointer of pointer array but I don't find any solution.

Thank you

Regards,
Freddix/AmiDARK.

All we have to decide is what to do with the time that is given to us.

DaveAE

Re: Strange behaviour : OpenGL + C + Pointers

Posted on: 2009/9/22 14:18 #2

Just popping in

@freddix

if ( ObjectID >> 0 ){

should be >= 0 I think.

abalaban

Re: Strange behaviour : OpenGL + C + Pointers

Posted on: 2009/9/22 14:59 #3

Quite a regular

@freddix


ObjectList[ ObjectID ]

is the same as writing


*(ObjectList + (ObjectID*sizeof(Object3DPointer))

If you prefer we can take an example with an array of char :


char * charArray;

to modify character at position <n> you would either write


*(charArray+n*sizeof(char)) = 'a';

either


charArray[n]='a';

If you can understand this, just replace char by Object3DPointer, charArray by ObjectList and <n> by ObjectID.

Once this is clear, you'll understand the warning about line


*ObjectList[ ObjectID ]=NewObjectPTR;

(i.e. you are trying to put an Object3DPointer variable into something having type *Object3DPointer in other word into an Object3D. Then if you remove the '*' in front of this line the second line should not need the '&'

Back to a quiet home... At last

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