[cfe-dev] Dynamic memory allocation and deleted definitions

[rahul]

Thanks Richard.

> On 28-Nov-2013, at 2:37 am, Richard Smith <richard at metafoo.co.uk> wrote:
> 
>> On Wed, Nov 27, 2013 at 8:34 AM, Rahul Jain <1989.rahuljain at gmail.com> wrote:
>> Thanks David for your valuable inputs.
> 
> David provided you with an approximation of the answer a few messages ago:
> 
> > 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"
> 
> This wording was somewhat defective, because it didn't deal with the case of a deleted destructor (and also incorrectly suggests that a destructor might be ambiguous). In the latest draft, this wording has been fixed, and it now says:
> 
> 5.3.4p19: "If the new-expression creates an array of objects of class type, the destructor is potentially invoked (12.4)."
> 12.4p11: "A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation."
> 
>> Hi Richard,
>> 
>> Any comments on this behaviour??
>> 
>> Thanks,
>> Rahul
>> 
>> 
>>> On Wed, Nov 27, 2013 at 12:43 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>> On Tue, Nov 26, 2013 at 10:53 AM, Rahul Jain <1989.rahuljain at gmail.com> wrote:
>>> > I think I got something relevant: Do these words make any sense in our
>>> > context?
>>> 
>>> Not quite
>>> 
>>> > An allocation or deallocation function
>>> 
>>> Allocation/deallocation function does not refer to
>>> constructor/destructor. See 3.4p1:
>>> 
>>> "A C++ implementation provides access to, and management of, dynamic
>>> storage via the global allocation functions operator new and operator
>>> new[] and the global deallocation functions operator delete and
>>> operator delete[]."
>>> 
>>> These are actually not the ctor/dtor, in 3.4p2 their declarations are shown as:
>>> 
>>> void* operator new(std::size_t);
>>> void* operator new[](std::size_t);
>>> void operator delete(void*);
>>> void operator delete[](void*);
>>> 
>>> So, roughly, if you say "new T" first operator new(sizeof(T)) is
>>> called, then the ctor for T is called on the returned storage, if the
>>> T ctor throws, then operator delete is called on the storage before
>>> the exception is propagated (that's why even non-array "new T"
>>> odr-uses the deallocation function, operator delete - even though it
>>> doesn't/shouldn't odr-use T's dtor).
>>> 
>>> > 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?
>>> 
>>> The simplistic interpretation is that if something is odr-used it must
>>> be defined (ie: there must be (one) definition). For example:
>>> 
>>> void f1();
>>> void f2();
>>> 
>>> int main() {
>>>   f1(); // odr use of f1 - the program is ill-formed if 'f1' is not defined
>>>   size_t s = sizeof(f2); // non-odr-use of f2 - the program is valid
>>> even if 'f2' is never defined
>>> }
> 
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