[cfe-dev] operator delete[] does not call derived class destructor

John McCall rjmccall at apple.com
Sun May 20 23:15:54 PDT 2012

On May 20, 2012, at 10:25 PM, Florian Weimer wrote:
> * M. E. O'Neill:
>> In contrast, for arrays, what it needs to do is:
>> - Call the destructors for all the objects in the array (starting at
>>  the *end* of the array and working backwards)
>> - call operator delete [] (void* ptr), where ptr start of the memory
>>  block previously allocated with operator new[]
>> It's the first of these things that is problematic.  How can the
>> compiler know where that last element is.  It does know that the
>> array has three things in it, but it can't use n*sizeof(Base) to
>> calculate the value it needs, which is n*sizeof(Derived).
> The element size could be written next to the place where the number
> of elements is stored.  Or it could be derived from the vtable pointer
> (which is the same for all the elements).  Probably not without ABI
> changes, though.

Yes, it would require ABI changes.  Also, you'd presumably want this
"polymorphic array" handling to work with subscripting and other kinds
of pointer arithmetic, which would be a substantial unrecoverable
overhead when dealing with arrays whose elements happen to have
polymorphic type.

Anyway, the fundamental language brokenness here is that new[]
just returns a pointer instead of some sort of slice type.  The confusion
of pointers and arrays is a C legacy that works out well enough there
but is quite problematic in C++ for many reasons, of which this is one.


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