[cfe-dev] Dynamic memory allocation and deleted definitions

[Rahul Jain]

Hi David, Halfdan,


Thanks a lot for your valuable inputs. Got caught up in some personal work
so couldnt reply
before.

I tried tracking down the exact wordings to confirm the behaviour, but was
of no luck.

We are basically putting a restriction on dynamic allocation of an array of
objects by
marking the destructor of that class as deleted.

The standard says:
A program that refers to a deleted function implicitly or explicitly, other
than to declare it, is ill-formed.

Please if you could help nail down the exact behaviour with reference to
the standard??
Would be of great help!

Thanks,
Rahul


On Thu, Nov 14, 2013 at 2:45 AM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
>
> On Wed, Nov 13, 2013 at 12:22 PM, Rahul Jain <1989.rahuljain at gmail.com>wrote:
>
>> Thanks David, Halfdan for your valuable inputs.
>> Just need some more clarification so that I get the complete thing.
>>
>>
>> Yes I understood that when we new[] an array of objects and the
>> constructor of one of the objects throws, the already constructed objects
>> should be destructed as the complete array construction could not get
>> through successfully.
>>
>> But assume if the constructor is not throwing, and all the objects of the
>> array get constructed completely, than where does the need of the
>> destructor call arise from?
>>
>> Is it the possibility of a runtime throw which prevents the compiler from
>> compiling a deleted destructor definition in case of an array allocation?
>>
>
> Precisely - whether or not an exception occurs is a runtime property, so
> the compiler can't know whether it will happen and needs to emit the code
> to handle it regardless.
>
>
>>
>> Also what exactly is the difference in this context when we define a
>> default constructor in our class vs a synthesized default constructor in
>> our class?
>>
>
> It's possible that GCC is just being lazy, or that there's a requirement
> that the default ctor is 'nothrow' then no dtors need be called.
>
>
>> g++ seems to accept the synthesized version whereas errors when one
>> defines a default constructor(as Halfsan mentioned above), whereas clang
>> rejects both the versions. Who is at fault here?
>>
>
> I'd have to go track down the wording. I don't know off hand.
>
>
>>
>> Thanks,
>> Rahul
>>
>>
>>
>> On Thu, Nov 14, 2013 at 12:22 AM, Halfdan Ingvarsson <halfdan at sidefx.com>wrote:
>>
>>> On 13-11-13 01:39 PM, Rahul Jain wrote:
>>>
>>>> But why will the destructor get called if the array is not explicitly
>>>> deleted?
>>>>
>>>
>>> Because if you new[] an array of objects, and the constructor of one of
>>> the objects throws an exception, then the already constructed objects will
>>> get destructed (destroyed?).
>>>
>>> Example:
>>> #include <iostream>
>>>
>>>
>>> int i = 0;
>>> struct A
>>> {
>>>     A() { id = i++; if(id == 5) throw std::exception(); }
>>>     ~A() { std::cout << "Destroying " << id << "\n"; }
>>>
>>>     int id;
>>> };
>>>
>>> int main()
>>> {
>>>     try
>>>     {
>>>     A *a = new A[10];
>>>     }
>>>     catch(...) {}
>>>     return 0;
>>> }
>>>
>>>  - ½
>>>
>>>
>>
>
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