Intermediate Lesson 10 question

[Nurlan]

When I put a _CrtDumpMemoryLeaks() function in ~Game,and tested it, it shows that we have a memoryleaks. Why we have it?. The second question we have a destructor in SurfaceSequence and destructor in Surface ,so what is destoryed in Surface?

[Nurlan]

I mean how destructor works in situation with inheritance?

[chili]

It could be memory that has not yet been destructed (destruction does not end at the end of ~Game() !) or it could be an actual leak somewhere. Best bet is to clean your solution and upload it here. I'll take a look at it and answer your question about the destructor too.

[Nurlan]

Here is the solution

[Pindrought]

Is it just me or should you be using delete[] instead of delete when you use new[]?


Example:

In D3dGraphics constructor, you have

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pSysBuffer = new D3DCOLOR[ SCREENWIDTH * SCREENHEIGHT ];

In D3DGraphics destructor, you have

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delete pSysBuffer;

Shouldn't it be this?

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delete[] pSysBuffer;

It appears that in the rest of the code you are using delete[] instead of delete so i'm guessing this was just in this specific part that this occurred.

[albinopapa]

Yes, allocations with new/new[] should be deallocated with delete/delete[].

When you call delete, it calls the destructor of the pointer that you pass it. When you call delete[] it calls the destructor for each item in the array that you pass it. That is why it's so important to use the correct new/delete or new[]/delete[] pair.

[Nurlan]

So you mean that delete doesnt actually delete an object, it just call destructor of the object?

[cameron]

If you delete a ptr created with new[] then it will try to free the WRONG ptr. Calling new[] actually allocates a extra size_t before the data so it can store the number of objects allocated so it knows to what extent to loop and call the destructor of the objects. When delete[] is called it does something similar to:

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char* basePtr = (char*)ptr - sizeof(size_t);
const size_t size = *basePtr;
for(size_t i  = 0; i < size; i++)
(ptr + i)->~T();

free(basePtr);

T is a templated type determined by args passed and ptr is the ptr passed.

[Nurlan]

Thank you

[chili]

Yes, if you have a pointer

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Surface* ptr

the code

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delete ptr

will attempt to deallocate memory for one Surface object, whereas

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delete [] ptr

will attempt to deallocate memory for an array of Surface objects.

Now for your questions about the destructors. A Surface object owns a surface, which is basically an array of pixels. A SurfaceSequence object owns an array of surfaces. So the destructor in the SurfaceSequence deallocates the array of Surface objects it owns, which in turn calls the destructor for each Surface object in the array, and for each of these calls to Surface's destructor, Surface's destructor frees the memory for the array of pixels that that Surface object owns.

Calling new[] actually allocates a extra size_t before the data so it can store the number of objects allocated so it knows to what extent to loop and call the destructor of the objects.

Interesting. I never thought about where the CRT stores that meta information about an allocated block.