[Development] Qt 5 types under consideration for deprecation / removal in Qt 6
giuseppe.dangelo at kdab.com
Wed Jun 19 21:03:59 CEST 2019
On 19/06/2019 18:42, Matthew Woehlke wrote:
> On 18/06/2019 19.59, Giuseppe D'Angelo via Development wrote:
>> On 18/06/2019 23:27, Matthew Woehlke wrote:
>>> So... to be clear, your plan is to deprive Qt users of a (mostly)
>>> perfectly good wheel, that *is* being used by said users, and instead
>>> tell all of those users that they need to go (individually) reinvent the
>> Why depriving? They can stay around, as-is. Current (and future) users
>> can still use them...
> Mark wants to deprecate Q[E]SDP. That implies eventual removal, which
> absolutely *is* depriving users of something they were previously using.
> (Even just deprecating is requiring users to either a) stop using it,
> which again, is depriving them of it, or b) ignore deprecation warnings,
> which will not end well.)
I don't agree with the deprecation until we have a better alternative as
public APIs. There's too much code using them.
>>> Here's a thought... how about, instead, just [...] mark the
>>> conversions-to-pointer deprecated.
>> I don't see why people writing "if (d)" should now get deprecation
>> warnings and require const_casts to avoid them,
> ...because, as previously noted, that code *detaches*, which is almost
> certainly not what you want!
I know that; I'm saying that 1) it's too late to change it (if we want,
put it in QESDPV2); and 2) if (d) is supposed to be legitimate code to
write, not something to trigger a deprecation warning.
> And no cast is required to fix it; just write `if(d.constData())`
> instead. (Also as previously noted.)
"Just" is a huge understatement. This is a workaround for an API flaw.
Not something to put under "best practices" of any kind, nor an
acceptable solution for a deprecation warning.
> Besides, I question the usefulness of implicit casts to pointer. The STL
> smart pointer classes don't have those, probably for a reason...
For the same reason implicit conversions are nitroglycerin:
use(p + 42);
> Personally, I only ever use Q[E]SDP via a d-func and associated helper
> macros that make it clear if I'm detaching or not, and so (AFAIK) I've
> never had such "surprises".
> // n.b. decltype(d) == QSharedDataPointer<MyClassData>
> if (d->m_visible) // did this detach?
> d->m_visible = false; // what about this?
> - vs. -
> // n.b. decltype(d) == MyClassData const*
> if (d->m_visible) // definitely did not detach
> QTE_D_DETACH(); // definitely *did* detach
> // n.b. decltype(d) == MyClassData*
> d->m_visible = false;
This is interesting, but I'd like to find a solution which avoids
enforcing the scope/block structure of the program. Compare that with
QEDSP, which allows to be more "streamlined" (of course at the cost of
if (d->foo == foo)
d->foo = foo;
d->bar = calc();
Note that proposing such macros is a case for deprecating QSDP/QESDP in
favour of something else.
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts
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