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[std-proposals] New type 'dvoid' -- a pointer to a destructible object

From: Frederick Virchanza Gotham <cauldwell.thomas_at_[hidden]>
Date: Tue, 23 Aug 2022 12:28:33 +0100
At first I was going to suggest that classes such as "unique_ptr",
"shared_ptr" and "weak_ptr" should inherit from a common interface
which has a virtual destructor, something like:

        class ISmartPtr {
           virtual ~ISmartPtr(void);

        class unique_ptr : public ISmartPtr { . . . };
        class shared_ptr : public ISmartPtr { . . . };
        class weak_ptr : public ISmartPtr { . . . };

So this would mean that we could have a container that automatically
destroys any kind of smart pointer as follows:

        vector<ISmartPtr> my_vector;

But then I thought it would be useful if *every* class inherited from
a class with a virtual destructor. As if doing this:

        struct MyClass { int a; }

would be the same as:

       struct BaseClassWithVirtualDestructor { virtual
~BaseClassWithVirtualDestructor(){} };

        struct MyClass : public BaseClassWithVirtualDestructor {
            int a;

But of course that would mean that all simple POD's would have the
overhead of a pointer to a v-table (and also the address of the first
member would not be equal to the address of the object -- on most

But then I thought of another idea, how about we have a new type
called "dvoid", and a new cast called 'virtual_destructor_cast':

    Some_POD_Class a;
    Some_Complicated_Class_With_Virtual_Destructor b;

    dvoid *p = virtual_destructor_cast<dvoid*>(&a); // This returns a nullptr
    dvoid *p = virtual_destructor_cast<dvoid*>(&b); // This returns a
valid pointer

And then you can use this pointer as follows:


or perhaps an alternative syntax would be:


So then you could have a container of pointers to anything that can be

    std::vector<dvoid*> objects;

In order to implement the 'dvoid*' type on a system where a pointer is
8 bytes, the 'dvoid' type could be 16 bytes, as internally it would be
two pointers:
(1) The pointer to the object
(2) The pointer to the destructor of the object (possibly obtained
from the v-table)

Received on 2022-08-23 11:28:44