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Re: [ub] memcpy blessing a new object of statically unknown type

From: David Krauss <david_work_at_[hidden]>
Date: Sat, 09 Jan 2016 10:41:26 +0800
> On 2016–01–09, at 1:54 AM, Hubert Tong <hubert.reinterpretcast_at_[hidden]> wrote:
> My two cents: even when memcpy becomes capable of initiating the lifetime of (and initializing) an object in an "aligned_storage blob", the kosher way of accessing that object would require std::launder.

Then the question becomes, does std::launder require a static type? I was under the impression that it does. In type erasure, memcpy needs to support polymorphism.

> Now, I am not sure if it is a copy/paste error, my misunderstanding of the code, or what, but it seems that (2) crashes because "&erasure()" and "&storage" overlap (and if the memcpy UB was not enough, it probably does not do what you intended).

Oops, copy-paste error. It should say &o.storage. (I just double-checked, the post does differ from the defective test.)

It’s a good thing you mentioned it, because I ran some more tests and now it looks like an inlining issue, not cleverness in alias analysis. Apparently it works whenever both memcpy arguments are function calls; it doesn’t matter what type they cast to.

So, my bugfix didn’t clarify types at all; it merely obfuscated the object identities. The port works, but it’s still as UB as ever.

Received on 2016-01-09 03:41:39