🚀 go-pugleaf

marek@iiasa.ac.at (Marek  MAKOWSKI) writes:

>I would like to ask for comments on three easy questions illustrated 
>by the following piece of code:
> 
>template <class I, class T>
>void mVect<I,T>::resize(I new_size) {
>   T *old = v;
>   v = new T[new_size];    // T *v is a private member of mVect  
>   int size_of_elem = sizeof(T);    // <--- question 3 
>   // 
>   // do something 
>   // 
>   delete[] old;     // <--- questions 1 & 2 
>} 
> 
>I have the following questions:
>1. Is it absolutely robust and portable to delete[] old, even
>   if a previous call was for new_size == 0 (or if v was allocated
>   by the ctor for the size == 0) ?

The code

	T *old = new T[0];
	delete [] old;

is definitely perfectly legal (strictly conforming).

Whether or not it is in practice portable is another question -- that
is something that can only be determined empirically.

>	In other words: is it guaranteed that:
>   (v = new T[0]) == 0;

No, after 'v = new T[0]', it is guaranteed that `v' is *not* a null pointer.

>2. Is it correct to assume that no destructor is called by this statement
>	if old was set as: old = new T[0]  ??

Yes.

>3. Is there any risk involved in using sizeof(T) in this statement ?

Basically no, but of course that would depend on what you used it for.

>The Watcom code blows-up on the delete[] statement (if a previous
>call was for new_size == 0)

If what you have described is correct, then it sounds like a Watcom bug to me.
(However, I suspect there is a good chance that there is a bug somewhere
in the parts of your code that you haven't shown us.)

>and gives a warning whenever it sees the sizeof(T).

Compilers are allowed to warn about anything they like;
however I don't see why warning about `sizeof(T)' would be useful.

>If the answer for quaestions 1 and 2 is negative then I would like to
>know the reason why the standard does not require new to return 0
>for zero_size array of objects.

Well, an array of size zero is conceptually different to no array at all.

>If there is a good reason for allowing new to return "anything" in such
>situations then one should add to every ctor setting a ptr to 0
>(which indeed made the Watcom version of my application running).

--
Fergus Henderson             	WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au              	PGP: finger fjh@128.250.37.3

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