niedz., 21 sie 2022 o 07:11 Walt Karas via Std-Proposals
<std-proposals_at_[hidden]> napisał(a):
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> In C++, there are two ways to use subobjects to build up an object, base classes and data members. A significant distinction is that, in a Standard-blessed way, you can derive (using static_cast) the address of the object from the address of a base class. This proposal implicitly asserts that this distinction is undesirable and unnecessary.
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> I propose adding these two templates to the Standard Library:
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> template <class Mbr, typename Obj>
> Obj & obj_from_dmbr(Mbr Obj::*mbr_ptr, Mbr &mbr);
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> template <class Mbr, typename Obj>
> Obj * obj_from_dmbr(Mbr Obj::*mbr_ptr, Mbr *mbr);
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> For mbr a reference/pointer to the indicated (by Mbr_ptr) data member of an object (of tclass Obj), these functions return a reference/pointer to the object. Their behavior is undefined if mbr does not actually refer/point to the indicated data member of an instance of Obj.
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> Here is a model implementation: https://godbolt.org/z/1oM5rGPK8
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> The effectiveness of this proposal is a bit hamstrung by unfortunate limitations with member pointers. For example, if array[] is an array within class C, I can't write &(C::array[i42]). But it would be possible to implement this proposal with a macro, where an invocation like:
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> C *cp = OBJ_FROM_DMBR(C, array[i42], pointer_to_element_42_in_array);
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I had some time ago a similar idea but not by using special functions
but by adding arithmetic operations to member pointers.

```
Obj* obj = mbr - &Obj::Mbr;
mbr == obj + &Obj::Mbr;
```

One benefit over function is that it better compose like:
```
mbr - &Obj::Mbr + &Obj::Foo;
```
It would also handle some use cases of `addressof` usage.

In case of array it could be done by:
```
(p - 42) - &Obj::array
```

>
> would work.
> --
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