Re: Is the design of operator new/delete flawed?

Old Wolf <>
Thu, 7 Jun 2007 06:09:24 CST
On Jun 5, 1:59 am, Kristof Zelechovski <> wrote:

"Seungbeom Kim" <> napisal:

I came across an article that discusses the design of operator
new/delete, which the author argues is "badly wrong".

I'm not quite sure what the guy is getting at; for
example in this code (quote from above link):

  typedef struct intlist { int size; int i[1]; } intlist;

  intlist *makeintlist (int size)
    intlist *ilp = malloc (offsetof (intlist, i[size])); /* not C++ */
    ilp->size = size;
    return ilp;

he bemoans the lack of a C++ equivalent; but surely
the whole of the above can be replaced with:
  std::vector<int> il(size);

It looks like he is complaining that new/delete cannot
do some particular task; however in C++ there are many
ways to skin the cat, and the task can be accomplished
by other means.

There is no match for new(args) T; delete(args) p is ill-formed.
It is a serious inconsistency.

I don't see why you would want args to delete; but if
you did then you could include in the definition of T:

  void delete_me(args....) { foo(args); delete this; }

Or you could do the same thing as a non-member function.

I am not sure if the following problem was mentioned:
Pick such a number N that size_t(4 * N) + 4 == 0.
What do you get from new char[N + 2][4]?
And you can fall into this trap quite inadventently.
The whole construct is shaky and it only happens to work sometimes.

C has the same problem; what is the result of:
  calloc(N+2, 4)

The C standard says that it should either return a
pointer to a valid allocation of the intended size,
or NULL, but there have been popular implementations
that returned a pointer to a smaller block.

I'm not sure exactly what the C++ standard says,
but a reasonable behaviour would be to throw the
allocation failure exception if it cannot allocate
a block of the intended size.

(Is there anything in C++ preventing this allocation
from succeeding? In C it does not seem to be
explicitly forbidden for calloc to allocate an
object larger than SIZE_MAX).

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