[cfe-dev] Dynamic memory allocation and deleted definitions

[Rahul Jain]

I think I got something relevant: Do these words make any sense in our
context?

An allocation or deallocation function for a class is odr-used by a
new expression appearing in a potentially-evaluated expression as specified
in 5.3.4(New) and 12.5(Free store). A deallocation function for a class is
odr-used by a delete expression appearing in a potentially-evaluated
expression as specified in 5.3.5(Delete) and 12.5(Free store).

What exactly does one mean by saying that something is odr-used?


On Tue, Nov 26, 2013 at 11:10 PM, David Blaikie <dblaikie at gmail.com> wrote:

> I'm not sure this is quite the right wording (I was expecting something
> that used the term "odr-used"), but:
>
> 5.3.4p17 states "... If the new expression creates an array of objects of
> class type, access and ambiguity control are done for the destructor"
>
>
> On Tue, Nov 26, 2013 at 6:42 AM, Rahul Jain <1989.rahuljain at gmail.com>wrote:
>
>> 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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