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Subject: Re: [std-proposals] Allowing access to object representations
From: Timur Doumler (cpp_at_[hidden])
Date: 2019-08-21 08:30:44


> On 21 Aug 2019, at 15:01, sdkrystian via Std-Proposals <std-proposals_at_[hidden]> wrote:
> Well the implementation of a standard library function by the implementation never invokes UB: they get to break all sorts of rule. It need not even be written in C++.

Sure, but then it would also mean that those functions are essentially unimplementable if you are NOT a compiler vendor. Which is kind of insane imho.

> As for the reason why the first code snippet is UB, it's because as of C++17, pointers point to objects, not addresses. So when we do that reinterpret_cast, there is no existing rule stating that it binds to any char object - it still points to the original object, but through a pointer to a different type. That means, when you perform pointer arithmetic, it violates [expr.add] p6 (type of the pointer is not similar to the type it points to).

Yep, that makes sense and is consistent with the current wording afaics. Thanks for the explanation!


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