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Re: Object constructors/destructors [message #185092 is a reply to message #185085] Tue, 25 February 2014 23:18 Go to previous messageGo to previous message
Christoph Michael Bec is currently offline  Christoph Michael Bec
Messages: 207
Registered: June 2013
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Senior Member
Jerry Stuckle wrote:

> On 2/25/2014 4:55 PM, Christoph Michael Becker wrote:
>> Jerry Stuckle wrote:
>>
>>> On 2/25/2014 4:19 PM, Christoph Michael Becker wrote:
>>>> Jerry Stuckle wrote:
>>>>
>>>> > On 2/25/2014 2:19 PM, Christoph Michael Becker wrote:
>>>> >> Jerry Stuckle wrote:
>>>> >>
>>>> >>> I have my own problems with PHP - for instance, it is entirely
>>>> >>> possible
>>>> >>> to create an object without calling a constructor - a violation
>>>> >>> of OO
>>>> >>> principles. But I work around it.
>>>> >>
>>>> >> How is it possible to create an object without its constructor (if
>>>> >> defined) being called?
>>>> >>
>>>> >
>>>> > Load the object from the $_SESSION. The constructor will not be
>>>> > called.
>>>> >
>>>> > However, the constructor is called when the original object was
>>>> > created,
>>>> > and the destructor called when the original object goes out of scope.
>>>> > The destructor is also called when the object loaded from the
>>>> > $_SESSION
>>>> > goes out of scope.
>>>> >
>>>> > The result is one constructor call and two destructor calls. A
>>>> > violation of OO principles.
>>>>
>>>> Um, I'm not sure whether this is a violation of OOP principles.
>>>> Storing
>>>> an object in the session requires serialization of the object, what is
>>>> somewhat comparable to cloning an object. So actually, you're not
>>>> working with the *same* object.
>>>>
>>>> Anyway, you can work around these issues by using the magic methods
>>>> __sleep() and __wakeup(), as you probably know. :)
>>>>
>>>
>>> Yes, it is a violation. When the script starts, the object is
>>> serialized in the $_SESSION. But it does not exist in the script.
>>> Creating it requires a constructor call.
>>
>> It requires the object to be created in memory somehow, but the defined
>> constructor function is not called in this case.
>>
>
> Which is a violation of OO principles.
>
>>> OO principles require exactly
>>> one constructor call and one destructor call for every object created.
>>> Note these may be explicit or implicit, and the functions may be noops,
>>> but the calls must still be made.
>>
>> When you're cloning an object, its constructor will not be called either.
>>
>
> It is in every other language. For instance, in Java, clone() acts like
> a copy constructor. C++ doesn't have a clone() method, but it does have
> a copy constructor.

Isn't the copy constructor called on other occassions as the normal
constructor?

If so, then PHP doesn't handle that too different. While
__constructor() would be the counterpart of a normal constructor,
__clone() would be the counterpart of a copy constructor.

>>> That is not the case here; you get
>>> one constructor call and two destructor calls.
>>
>> 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.

Therefore there are __sleep() and wakeup(). In __sleep() you can close
the log file (if it's still open), and in __wakeup() you can open it again.

Anyway, consider the following snippet:

<?php

class Foo {
function __construct() {
$this->bar = 'foo';
}
function setBar($bar) {
$this->bar = $bar;
}
}

$foo = new Foo();
$foo->setBar('bar');
$bar = unserialize(serialize($foo));
echo $bar->bar;

What output would you expect?

> PHP's method is only a shallow copy. Copy constructors and the like
> were created when a deep copy is required (quite often in other languages).

You can force a deep copy yourself by respectively defining __clone().

>>> I know of no other OO language which would allow this.
>>
>> Others may.
>>
>
> None that I know of. Please name one.

Obviously, I was too terse here. I meant: "I can't name one, but others
may be able to name one."

--
Christoph M. Becker

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