C++ Logo


Advanced search

Re: [std-proposals] Relocation in C++

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Thu, 19 May 2022 12:58:10 +0100
On Wed, 18 May 2022 at 07:54, Sebastian Wittmeier <
wittmeier_at_[hidden]> wrote:

> *I would suggest that in those situations - 'reloc a' without using the
> result - the compiler may just call the destructor and end the scope
> instead of relocating to a new instance.*
Yes, that sounds a very good idea, however there is something to consider:
what happens if `a` cannot be destroyed immediately, if it is a function
parameter without trivial or user relocation operation? Since you want to
release resources, it will be necessary to move-construct a temporary
instance of `T` from `a`, then destroy the temporary. On the other hand, if
`T` has trivial or user relocation operation, then even if it is a function
parameter the function is in full control of its lifetime so it can be
destroyed immediately.

This could possibly be accomplished by an elision rule - eliding relocate +
destroy to just relocate. However, that would make it optional and a matter
of optimization with user-visible effects. It would probably be better to
adjust the rules on *discarded-value expression* such that temporary
materialization conversion is not applied if the discarded-value expression
is a relocation expression. It would be important to ensure that this has
the desired effect of move-constructing and destroying a temporary if

Received on 2022-05-19 11:58:22