[Discuss] Casting the return value of malloc bad?

[Adam Parkin]

Andrew Resch wrote:
>> Paul Nienaber wrote:
>>> Do it this way:
>>>
>>> foo = malloc(n * sizeof(*foo));
 >>
>> Good point, but if the line immediately before the malloc looked like:
>>
>> SomeType * foo = NULL;
>>
>> then wouldn't sizeof (*foo) choke (NULL pointer dereference)?
> 
> OH sorry about the last email..  I should read before replying. 
> 
> The sizeof(*foo) will return the size of SomeType, not the length of
> data that it is pointing to.

Exactly, and that's why Paul's tip is good (so long as foo is pointing 
to a valid instance of SomeType).

I actually experienced this week in my labs as I was showing the class 
why casting the return value of malloc isn't so good.  I wrote on the board:

/* alloc memory for array of x ints */
int * p = (int *) malloc (sizeof (int) * x);	

And then said, "well what if you decide that you want an array of longs 
instead of ints, but forget to change the cast":

long * p = (int *) malloc (sizeof(int) * x);

But then of course a smart student pointed out that the code is borked 
anyways because I forgot to change the sizeof(int) to sizeof(long).  So 
then I changed it to

long * p = (int *) malloc (sizeof(long) * x);

But the odds you'll remember to change the sizeof, but not change the 
cast are pretty slim so at this point the exercise became a bit 
artificial and contrived. =8-p

I think in the end the best approach is something like:

typedef int ArrayDataType;
ArrayDataType * p = malloc (sizeof(ArrayDataType) * numElements);

Now if you decide on an array of longs instead of ints, you only need 
change the typedef line and the malloc line remains the same.

Side rant: typedef is one of the biggest things I miss going from C/C++ 
to Java, as often I want to change the type of data contained in a 
container class, but in order to do so there's usually multiple places 
where that information appears.  Usually I end up creating an artificial 
"DataElement" class which is just a wrapper for the type to be contained 
which often feels like swatting a fly with a bazooka. =8-p  Generics 
help with this, but if you have multiple containers then you still have 
multiple points where the data type appears and is still a bit of a 
maintenance problem.
-- 
--
Adam Parkin
E-mail: pzelnip at telus.net
----------------------
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