Pointer assignment in C++. (Pointer to a pointer to pointer is on the LHS)

Question

IDirect3DSurface9 *var = NULL;

IDirect3DSurface9 *** ret;

I want to assign the value dereferenced by var into the variable pointed by ret.

I did the foll:

(*(*(ret[0]))) = var;

I feel this is correct C++ syntax. But why is that I am getting compilation error as follows:

error C2679: binary '=' : no operator found which takes right hand operand of type "IDirect3DSurface9 *" (or these is no acceptable conversion).

What is the correct syntax?

Can you edit your question to remove all the bold face all caps shouting. You don't need to shout. We can here you fine. — David Heffernan
– David Heffernan, Commented Mar 22, 2012 at 10:41
I edited a bit, but I'm not actually sure about the intended number of asterisks now :-S — Kerrek SB
– Kerrek SB, Commented Mar 22, 2012 at 10:43
@engineerMaster: Whenever you want to say "it's a bug with [someone who is not myself]", you're probably wrong. Do try to find fault with yourself first and foremost; it's a simple numbers game to work out that that's a more productive approach. — Kerrek SB
– Kerrek SB, Commented Mar 22, 2012 at 10:44

David Heffernan · Accepted Answer · 2012-03-22 10:55:50Z

4

You have de-referenced the pointer 3 times. Once when you treated it as an array and used the index [0], and then twice more with the * operator. In order to be compatible with var you should de-reference only twice.

To be more explicit, let's break this down:

ret has type IDirect3DSurface9***.
ret[0] has type IDirect3DSurface9**.
*(ret[0]) has type IDirect3DSurface9*.
*(*(ret[0])) has type IDirect3DSurface9.

And clearly it follows that *(*(ret[0])) is not assignment compatible with var which has type IDirect3DSurface9*.

As to what your code really should be, I could not say for sure, but you will need to remove one level of indirection.

edited Mar 22, 2012 at 10:55

answered Mar 22, 2012 at 10:39

David Heffernan

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5 Comments

codeLover Over a year ago

Incredible!!!! Thank u so much for such a speedy answer!! I resolved it. :) I did *(*(ret[0])) = *var; Now both types LHS & RHS are same. So no compilation error.

David Heffernan Over a year ago

That removes the compilation error but I'm not convinced it is correct. Surely your assignment should be at the IDirect3DSurface9* level rather than the IDirect3DSurface9 level.

codeLover Over a year ago

Yes I can change it like (ret[0]) = var; But I have the doubt that how does it matter wheather we do the assignment at IDirectX3DSurface9 level or at IDirectX3DSurface9 level. Bcoz finally we are copying the values stored in the variable of tyoe IDirectX3DSurface9 right ? Sorry if I am wrong. Kindly correct me & clarify. Thanks.

David Heffernan Over a year ago

I strongly suspect that your right hand side be var rather than *var but I can't see the rest of your code.

molbdnilo Over a year ago

@engineerMaster: Don't do that - IDirect3DSurface9 is a COM interface and has no data members. Your assignment is going to compile but it will also destroy any data that the interface depends on. Only work with pointers to COM interfaces.

Attila · Accepted Answer · 2012-03-22 10:56:12Z

2

Every * and [] takes away one level of indirection. So in your code

(*(*(ret[0]))) = var;

the left-hand-side has type IDirect3DSurface9, while the right-hand-side has type IDirect3DSurface9 *. The two types are different, that is why you get the compiler error. You need to fix this by either removing (at least one) * from the LHS or add an & (address-of operator) to the RHS (to have &var).

answered Mar 22, 2012 at 10:56

Attila

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Comments

Luchian Grigore · Accepted Answer · 2012-03-22 10:46:54Z

0

You probably want:

(*(*(ret[0]))) = &var;

or

*(*(*(ret[0]))) = var;

answered Mar 22, 2012 at 10:46

Luchian Grigore

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