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Re: Object constructors/destructors [message #185129 is a reply to message #185124] Thu, 27 February 2014 21:09 Go to previous messageGo to previous message
Jerry Stuckle is currently offline  Jerry Stuckle
Messages: 2598
Registered: September 2010
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On 2/27/2014 12:28 PM, Christoph Michael Becker wrote:
> Jerry Stuckle wrote:
>
>> On 2/26/2014 5:39 PM, Christoph Michael Becker wrote:
>>> Jerry Stuckle wrote:
>>>
>>>> On 2/26/2014 4:59 PM, Christoph Michael Becker wrote:
>>>> > Jerry Stuckle wrote:
>>>> >
>>>> >> On 2/26/2014 3:10 PM, Christoph Michael Becker wrote:
>>>> >>> Jerry Stuckle wrote:
>>>> >>>
>>>> >>>> On 2/25/2014 6:22 PM, Adam Harvey wrote:
>>>> >>>>> On Tue, 25 Feb 2014 17:01:52 -0500, Jerry Stuckle wrote:
>>>> >>>>>> On 2/25/2014 4:55 PM, Christoph Michael Becker wrote:
>>>> >>>>>>> I firmly believe that it wouldn't make sense to call a
>>>> >>>>>>> (user-defined)
>>>> >>>>>>> constructor when unserializing or cloning an object. A
>>>> >>>>>>> constructor
>>>> >>>>>>> usually serves to initialize an object -- what already had
>>>> >>>>>>> happened in
>>>> >>>>>>> both cases.
>>>> >>>>>>>
>>>> >>>>>>>
>>>> >>>>>> It makes perfect sense. Not everything is necessarily valid in
>>>> >>>>>> the
>>>> >>>>>> new
>>>> >>>>>> object. For instance, a logging object may require opening the
>>>> >>>>>> log
>>>> >>>>>> file. There are many instances where a resource is no longer
>>>> >>>>>> available
>>>> >>>>>> and needs to be recreated.
>>>> >>>>>
>>>> >>>>> Indeed, which is why PHP provides the Serializable interface (and,
>>>> >>>>> for BC
>>>> >>>>> reasons, also __sleep and __wakeup) to allow those sorts of
>>>> >>>>> reinitialisation tasks.
>>>> >>>>>
>>>> >>>>> I agree with Christoph: since the object is already
>>>> >>>>> instantiated, it
>>>> >>>>> doesn't logically make sense to call the constructor once again.
>>>> >>>>>
>>>> >>>>
>>>> >>>> But the object is NOT instantiated. It was at one time, then
>>>> >>>> destroyed.
>>>> >>>> When the new script starts, there is no object in existence.
>>>> >>>>
>>>> >>>>>>> Jerry Stuckle wrote:
>>>> >>>>>>>> I know of no other OO language which would allow this.
>>>> >>>>>>>
>>>> >>>>>>> Others may.
>>>> >>>>>>>
>>>> >>>>>>>
>>>> >>>>>> None that I know of. Please name one.
>>>> >>>>>
>>>> >>>>> Python's pickle operates the same way:
>>>> >>>>> http://docs.python.org/2/library/pickle.html#object.__getinitargs__
>>>> >>>>>
>>>> >>>>> Providing a way to instantiate objects without calling the
>>>> >>>>> constructor
>>>> >>>>> does have valid uses (mostly for testing), which is why PHP 5.4 and
>>>> >>>>> later
>>>> >>>>> versions also provide a way to do so via reflection (avoiding the
>>>> >>>>> unserialize() hack):
>>>> >>>>> http://php.net/reflectionclass.newinstancewithoutconstructor
>>>> >>>>>
>>>> >>>>> Adam
>>>> >>>>>
>>>> >>>>
>>>> >>>> Not knowing Python, I can't say. But if it is a true object, then
>>>> >>>> they
>>>> >>>> are also violating OO principles.
>>>> >>>>
>>>> >>>> OO demands creation of a new object requires a constructor call.
>>>> >>>> Period.
>>>> >>>
>>>> >>> Do you understand, what's the purpose of a constructor? It is
>>>> >>> there to
>>>> >>> *initialize* an instance. If there is nothing to initialize, it
>>>> >>> is not
>>>> >>> strictly necessary to call any constructor.
>>>> >>>
>>>> >>
>>>> >> Yes, I understand the purpose of a constructor - it looks like much
>>>> >> more
>>>> >> than you do.
>>>> >>
>>>> >>> But anyway, unserializing an object is *not* creating a new object.
>>>> >>> Consider a class man. When a new object is instantiated the
>>>> >>> constructor
>>>> >>> is called to initialize the person with respective properties
>>>> >>> (e.g. age
>>>> >>> = 0). Later this man goes to sleep (serialize); after he wakes up
>>>> >>> (unserialize) the constructor is not called again, as this would
>>>> >>> reset
>>>> >>> the man to his initial properties, and surely after a good night of
>>>> >>> sleep one may feel younger, but the age has not been reset.
>>>> >>>
>>>> >>
>>>> >> It is creating a new object. Before unserialize(), the object does
>>>> >> not
>>>> >> exist - only a bunch of data. And that data is NOT an object! You
>>>> >> cannot call an object method on the data, for instance. It is no
>>>> >> different than if the data were stored in a file and loaded in Java or
>>>> >> C++, for instance. To create the object requires a call to a
>>>> >> constructor.
>>>> >
>>>> > If you really need to think this way, just take
>>>> > Serializable::serialize() resp. __sleep() as a destructor and
>>>> > Serializable::unserialize() resp. __wakeup() as a constructor.
>>>> > Actually, the documentation of the Serializable interface explaines it
>>>> > that way.
>>>> >
>>>>
>>>> Then it is wrong in another respect, because the destructor is called
>>>> after __sleep(), when the object goes out of scope.
>>>>
>>>> >>>> If you think otherwise, I suggest you learn more about
>>>> >>>> how OO
>>>> >>>> is supposed to work. PHP is not a good example.
>>>> >>>
>>>> >>> I refrain from commenting this statement.
>>>> >>>
>>>> >>
>>>> >> If you think otherwise, I suggest you learn more about how OO is
>>>> >> supposed to work. PHP is not a good example.
>>>> >
>>>> > No comment.
>>>> >
>>>>
>>>> No matter how you look at it, PHP's implementation violates OO
>>>> principles and works differently than any other OO language.
>>>
>>> You have claimed that often enough, but yet you have not brought any
>>> proof.
>>>
>>> E.g. what happens in Java if an object that implements the
>>> java.io.Serializable interface is given as argument to
>>> java.io.OutputStream.writeObject()? Is the destructor called?
>>>
>>
>> Yes, it is.
>
> I strongly doubt that. On one hand this would mean you can't use the
> object as soon as it has been written to the stream, and on the other
> hand there are no *destructors* in Java (finalize() is similar to a
> destructor, but it is not guaranteed that it will be called).
> Furthermore it depends on the garbage collector, when an object will be
> *destroyed*.
>
>> But when the new object is created, a constructor is
>> called. You cannot unserialize an object without a constructor being
>> called first.
>
> I doubt that, too. After all, java.io.ObjectInputStream.readObject()
> returns an object, which of course has to be *created* -- but there's no
> need to call its *constructor*.
>

No, because the constructor is called automatically.

> However, I believe our dissent mainly stems from different use of terms.
> I am accustomed to understand constructor/destructor as user defined
> functions (which can be defined for custom initialization/finalization
> tasks), while this may not be the common definition.
>

They may be user defined, or they may be defined by the system (as in
the absence of a user-defined constructor).

> When you're talking about OO principles wrt. to
> construction/destruction, you're most likely referring to RAII, which is
> not necessarily a general OO principle, and might not be possible to be
> cleanly implemented in garbage collected languages generally (consider
> cyclic references).
>

No, I am not referring to RAII. I am referring to basic OO principles.

> Anyway, I somewhat consider this discussion as harping on about
> principles, and I still don't see that PHP lacks the really important
> concepts regarding object (de)serialization and cloning.
>

Just that it doesn't follow OO principles - which has caused me problems
in the past.


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==================
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Jerry Stuckle
jstucklex(at)attglobal(dot)net
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