Crystal clear. Thanks for the explanation. I tried the same program
with scalars instead of arrays, writing
Base *a = new Final;
delete a;
and it works as expected.
I just wonder why the array version does not even trigger a warning or
a remark from the compiler.
Best
On Tue, Feb 7, 2012 at 6:59 PM, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
> On 7 February 2012 17:21, Mattia Jona-Lasinio wrote:
>>
>> The program wants to test the C++ feature that pointers to a derived
>> class can be assigned consistently to a pointer variable to the base
>> class. This does not seem to be the case with g++. Is it the intended
>> behavior?
>
> The program has undefined behaviour.
>
> "In the first alternative (delete object), the value of the operand of
> delete may be a null pointer
> value, a pointer to a non-array object created by a previous
> new-expression, or a pointer to a subobject (1.8) representing a base
> class of such an object (Clause 10). If not, the behavior is
> undefined. In the second alternative (delete array), the value of the
> operand of delete may be a null pointer value or a pointer value that
> resulted from a previous array new-expression. If not, the behavior is
> undefined."
>
> N.B. 'delete array' cannot be used for pointers to base class.
>
> "In the first alternative (delete object), if the static type of the
> object to be deleted is different from its dynamic type, the static
> type shall be a base class of the dynamic type of the object to be
> deleted and the static type shall have a virtual destructor or the
> behavior is undefined. In the second alternative (delete array) if the
> dynamic type of the object to be deleted differs from its static type,
> the behavior is undefined."
>
> N.B. A virtual destructor does not help for the delete array case.
>
> You should not upcast a pointer to an array - you can't access array
> elements and you can't delete the array - don't do it.
>
> If the derived type has a different size to the base type then pointer
> arithmetic won't work and you can't access the array elements:
>
> #include <iostream>
> struct B { int i; B() : i(1) { } };
> struct D : B { int j; D() : j(2) { }};
>
> int main()
> {
> B* b = new D[2];
> std::cout << b[1].i << std::endl;
> }
>
> This prints 2 even though B::i always has the value 1.
