[Development] What's the status of a moved-from object?

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Sat May 25 10:48:30 CEST 2019


Il 19/05/19 14:24, Giuseppe D'Angelo via Development ha scritto:
> Hence, I'll ask here: what should the status of a moved-from object be?
> I'm not really interested in_how_  to achieve such status (although of
> course it's very important, and should influence the decision); I'm
> interested in what's our contract with our users.

Trying to summarize the discussion so far. It seems to me that there's 
some agreement on these points:

1) We want move constructors for pimpled valued classes.

2) They must be noexcept, incl. not allocating memory.

3) At least for Qt 6's lifetime, the documented state of a moved-from 
object is "valid, but unspecified", à la stdlib, and NOT 
partially-formed. Any function without preconditions can be called on a 
moved-from instance; the results are unspecified. ("Unspecified" used in 
Standardese jargon here).

4) We will deliberately break source compatibility by adding move 
constructors to the classes currently lacking them (and thus copying 
instead of moving; leaving the source object in a well-defined state).

Hopefully, in the next years, static and runtime analyzers will be able 
to catch more and more usages of use-after-move. If we're really 
paranoid about this we could hide the move constructors behind a macro 
to avoid the SC break.


With these constraints, then we're left with "how do we do it"?

A) Allow for the d pointer to be nullptr; that's what gets set in the 
source object. To keep the object valid, this means checking the d 
pointer for nullptr in any function without preconditions (... read: in 
any member function, better safe than sorry). The cost of these extra 
checks is to be determined.

B) Introduce a "shared_null" for every pimpled class, under the form of 
a Q_GLOBAL_STATIC or similar. The move constructor resets the source 
object's d pointer to that shared_null.

As a by-product, both A) and B) can be used to make the default 
constructor not allocate and noexcept.

Am I forgetting anything?

Thank you,
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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