🚀 go-pugleaf

fjh@munta.cs.mu.OZ.AU (Fergus Henderson) writes:

>clamage@Eng.Sun.COM (Steve Clamage) writes:

>>The expression "new T[0]" is invalid, but if you
>>use a non-const expression for the count, it is OK if the value is zero.

>What makes you think that the expression `new T[0]' is invalid?
>As I read the working paper, it seems perfectly legal, and the
>three compilers I tried it on were all quite happy to accept it.

I now believe I was wrong, and "new T[0]" is valid. Here's the
background, if anyone is interested.

A while back this question arose in another context and I studied
the then-current draft very carefully. I came to the conclusion 
that "new T[0]" violated some explicit requirements. (I don't
remember which version of the draft that was, but that is
moot anyway.) When the question came up this time, I checked the
"New" section of the current draft (5.3.4) and found a sentence that
says that the value of any constant-expression must be strictly
positive.  In addition, the "Declarators" chapter of the draft has
always said that an array type may be declared only if the number of
elements is greater than zero.

So I assumed that my earlier analysis was correct.

Fergus posted an opinion that "new T[0]" was allowed, and I also
received some private email on the subject, so I thought I should
check more carefully. I re-read that paragraph and discovered
the "strictly-positive" requirement doesn't apply to the first
dimension of the array, but only to the second and subsequent
dimensions. The first dimension must be non-negative.

Then I checked with two people who deal with these sorts of issues
in the C++ Committee. One at first made the same argument I did,
but agreed when I showed how it didn't apply. Both believed this
subject had never been explicitly discussed.

We agreed that it was inconsistent to allow "new T[n]" when n==0
(which has always been allowed) but not "new T[0]". There also don't
seem to be any technical difficulties in allowing the expression.
(The new-expression yields type T*, not T[0].)

--
Steve Clamage, stephen.clamage@eng.sun.com
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