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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Wed, 22 Sep 2021 18:44:24 +0100
Message-ID: <CAH6eHdQ94_SrmNM2=hQivf3_wax32aPO+N=K2d9_2ve=eq9-0w@mail.gmail.com>
Subject: Re: [PATCH] assert that deleting by pointer to base in unique_ptr
 does not cause UB
To: Antony Polukhin <antoshkka@gmail.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>,
 gcc-patches List <gcc-patches@gcc.gnu.org>
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On Wed, 22 Sept 2021 at 18:09, Antony Polukhin wrote:
>
> std::unique_ptr allows construction from std::unique_ptr of derived
> type as per [unique.ptr.single.asgn] and [unique.ptr.single.ctor]. If
> std::default_delete is used with std::unique_ptr, then after such
> construction a delete is called on a pointer to base. According to
> [expr.delete] calling a delete on a non similar object without a
> virtual destructor is an undefined behavior.
>
> This patch turns that undefined behavior into static assertions inside
> std::unique_ptr.

The undefined behaviour only happens if the destructor is actually
reached at runtime, but won't these static assertions make it
ill-formed to instantiate these members, even if the UB never happens?

For example, if you ensure that release() is called before
destruction, the undefined delete never happens.