Hello,

> C++ could potentially allow the invocation of a method on a nullptr.

Why would this be desirable? This would just add more footguns to a
language that already has too many. If you don't need member access, why
not just make the methods static?

I think this email thread would benefit from a concrete example of
something you feel could uniquely be implemented with this sort of thing
that the language doesn't have an existing solution for.

> But if we were to mark the class as nullptr-friendly:
>
> class MyClass friend nullptr {
> string name;
> public:
> virtual string GetName(void)
> {
> if ( nullptr == this ) return {};
> return this->name;
> }
> };
>
> Then our 'Monkey' function would then know to check first whether 'p'
> is a nullptr, and if it is a nullptr, then instead of dereferencing
> 'p' to find a VTable pointer, it instead just uses the VTable of
> MyClass.

Imo this would be a very surprising reinterpretation of what "virtual" means.

Cheers,
Jeremy


On Oct 27 2024, at 3:16 pm, Frederick Virchanza Gotham via Std-Proposals
<std-proposals_at_[hidden]> wrote:

> This is a totally half-baked idea, but I just want to float it here
> sort of like a thought experiment to see if anyone is intrigued by the
> possibilities it could open up.
>
> C++ could potentially allow the invocation of a method on a nullptr.
> For example:
>
> class MyClass {
> string name;
> public:
> string GetName(void)
> {
> if ( nullptr == this ) return {};
> return this->name;
> }
> };
>
> It gets a little more complicated if the method is virtual, for example:
>
> class MyClass {
> string name;
> public:
> virtual string GetName(void)
> {
> if ( nullptr == this ) return {};
> return this->name;
> }
> };
>
> Because if we supply the following standalone function to an x86_64
> compiler on Linux:
>
> string Monkey(MyClass *const p)
> {
> return p->GetName();
> }
>
> It will give us back something like:
>
> mov r11, rdi # Put the object pointer in R11
> mov r11, [r11] # Put the address of the VTable in R11
> mov r11, [r11] # Put the address of the first method in R11
> jmp r11
>
> This of course will segfault the first time it dereferences R11
> because it's a nullptr.
>
> But if we were to mark the class as nullptr-friendly:
>
> class MyClass friend nullptr {
> string name;
> public:
> virtual string GetName(void)
> {
> if ( nullptr == this ) return {};
> return this->name;
> }
> };
>
> Then our 'Monkey' function would then know to check first whether 'p'
> is a nullptr, and if it is a nullptr, then instead of dereferencing
> 'p' to find a VTable pointer, it instead just uses the VTable of
> MyClass.
>
> So it could be possible in the future in C++ to accommodate the
> invocation of a method on a nullptr.
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>