[vlc-devel] [PATCH] vlc_common: Add C++ memory management helpers

Filip Roséen filip at atch.se
Fri Jul 13 07:28:07 CEST 2018

Hi again,

I now realize that the email I sent earlier suffers from a few
unfortunate typos, but not only that; what I wrote about your helpers
(accepting a custom deleter) is extremely vague.

Sorry about that. I hope the added comments in this email will make it
easier to understand what I meant and mean.

Best Regards,\

On 2018-07-12 23:17, Filip Roséen wrote:

> Helpers are always nice, but one immediate reaction to the patch is
> that these four, as a consequence will force modules written in `C++`
> to be compiled as at least `C++11`, something we formerly did not
> demand.

The above was meant to be read as *"Helpers are nice, but an
unfortunate consequence of putting them in `vlc_common.h` is that the
C++ compiler used to build a module then must support (and enable)
*c++11*, even if the implementation of said module does not mandate

> This could be circumvented by introducing `#if __cplusplus >= 201103`
> so that we do not force an error diagnostic down someones throat who
> are for whatever stuck with an older compiler. One could argue that we

*"Who are, for whatever **reason**, stuck..."*

> As far as I am aware, both of the above overloads with custom
> releasers are doing more than they are required in terms of
> `noexcept`. If `Releaser` in `std::unique_ptr<class T, class Releaser>`
> is not an *lvalue-reference*, it shall not throw an exception in its
> *move-* and/or *copy-constructor* (which one depends on the reference
> type of `r`).

The usage of *"shall"* in the above can be very misleading, at least I
fear it might. I should have explained that constructing a
`std::unique_ptr<T, U>( expr1, expr2 )` where `U` is not an *lvalue*
type is *UB* if;

 - the result of *expr2* is an *lvalue*, and the *copy-constructor* of
   `U` is not *nothrow copy-constructible*, or;
 - the result of *expr2* is an *rvalue*, and the *move-constructor* of
   `U` is not *nothrow move-constructible*.

What the above boils down to is that the expressions passed to the
`noexcept` specifier of the two functions are superfluous as we are in
*UB-land* if the requirements above are not met.

See `[unique.ptr.single.ctor]p9-12` for more information.

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