[Development] What's the status of a moved-from object?
marc at kdab.com
Mon May 20 14:51:49 CEST 2019
On 2019-05-20 11:25, Giuseppe D'Angelo via Development wrote:
> Il 19/05/19 18:54, Thiago Macieira ha scritto:
>> But I think all Qt classes should go beyond that, unless they have
>> VERY good
>> reasons not to do so (and document so). The moved-from object should
>> also be
>> in a valid state so all the accessor and mutation API in the class can
>> in the object without ill effects. What they actually do, we can't
>> tell, since
>> the initial state is unknowable. So apply the principle of GIGO.
> So basically the same stance as the Standard Library? One should be
> able to invoke any function without preconditions on a moved-from
Except that the standard library has an easy way of implementing that,
since there're no PIMPLs. For a PIMPLed class, it means that the move
constructor either must allocate memory or that each and every PIMPLed
value class needs to have a static unsharable null instance. This is
relatively easy for some, but try that for QBrush. Or we litter all
member functions with nullptr checks.
I agree that a moved-from object should be in the same state as a
default-constructed one. I disagree with that that state must always be
a valid value of the class. I agree with Stepanov that the default
constructor should be establishing the partially-formed state, ie, only
destruction and assignment are valid. It _can_ do more, but only if it
Or maybe we don't disagree at all and Thiago would accept allocating
memory (or, by extension, anything that's noexcept(false)) as a very
good reason to have a nullptr d?
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