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Re: C++ PATCH for c++/88136, -Wdeprecated-copy too noisy


On Wed, 12 Dec 2018 at 16:52, Jason Merrill <jason@redhat.com> wrote:

> > For what it's worth, I find it unfortunate that this deprecation and its resulting warnings end up
> > making the decision on whether a "rule of 5" must be followed; correct code needs to be adjusted
> > to cope with a fairly stylistic matter, with false positives and all.
>
> I don't see it as a stylistic matter.  If you need a user-provided
> copy constructor to get proper copy semantics for a class, you almost
> certainly need the same thing for copy assignment.  This was too noisy
> for destructors, for which it's fairly common to define a virtual
> destructor just to make a class polymorphic, not because there are
> significant destruction semantics.  But I don't see a similar argument
> for copy constructors: in your example, there was no need for
> QVariant::Private to define a copy constructor, and that seems like a
> situation where a warning is reasonable, even if the code is in fact
> correct.

I have other cases where the compiler warns besides QVariant::Private.
I don't have a good grasp of what
those cases are like, yet, because I'll look at those warnings some
time later, possibly next year. :)
Thus I don't have any idea whether there are practical cases where the
copy constructor allocates but the assignment
can just blast bits, although I find that rather plausible. (Side
note: disabling warnings in Qt is painful; it's
not really more or less painful than doing code changes and regtesting
those on all platforms; that's not your
problem, though.)


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