Test for the existence of a null property

[wildernesscat]

Hello there,

I'm looking for a method to test, whether an object has a certain
property.
Consider the following snippet:

class A { var $aaa; }
$var = new A;

(Assuming that the structure of class A is unknown) I need a way to
check whether $var->aaa exists (test positive), and whether $var->xxx
exists (test negative). I tried boolean tests, isset(), is_null(), but
they can't tell the difference.
I guess I could convert the object to an array and test its indices,
but that wouldn't be practical for large object with many fields.

Any other ideas?

Thanks in advance,
Danny

[Jerry Stuckle]

wi***********@g mail.com wrote:

Hello there,

I'm looking for a method to test, whether an object has a certain
property.
Consider the following snippet:

class A { var $aaa; }
$var = new A;

(Assuming that the structure of class A is unknown) I need a way to
check whether $var->aaa exists (test positive), and whether $var->xxx
exists (test negative). I tried boolean tests, isset(), is_null(), but
they can't tell the difference.
I guess I could convert the object to an array and test its indices,
but that wouldn't be practical for large object with many fields.

Any other ideas?

Thanks in advance,
Danny

Danny,

Once of the concepts of OO programming is encapsulation (not fully
implemented in PHP), in which can't access $aaa. Anything outside of
the class should not have access to the internal variables of the class;
nor should they concern themselves with the internals of the class.
This limits the effects of changes to the class.

Exactly what problem are you trying to resolve?
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js*******@attgl obal.net
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[wildernesscat]

Hi Jerry,

I'm trying to write a function that checks whether a given object is of
a known class (named 'A' in this example). I want it to work even if
the object has been constructed manually, starting from 'new
StdClass()'.

Danny
Jerry Stuckle wrote:

wi***********@g mail.com wrote:

Hello there,

I'm looking for a method to test, whether an object has a certain
property.
Consider the following snippet:

class A { var $aaa; }
$var = new A;

(Assuming that the structure of class A is unknown) I need a way to
check whether $var->aaa exists (test positive), and whether $var->xxx
exists (test negative). I tried boolean tests, isset(), is_null(), but
they can't tell the difference.
I guess I could convert the object to an array and test its indices,
but that wouldn't be practical for large object with many fields.

Any other ideas?

Thanks in advance,
Danny

Danny,

Once of the concepts of OO programming is encapsulation (not fully
implemented in PHP), in which can't access $aaa. Anything outside of
the class should not have access to the internal variables of the class;
nor should they concern themselves with the internals of the class.
This limits the effects of changes to the class.

Exactly what problem are you trying to resolve?
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=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
js*******@attgl obal.net
=============== ===

[Marcin Dobrucki]

wildernesscat wrote:

Hi Jerry,

I'm trying to write a function that checks whether a given object is of
a known class (named 'A' in this example). I want it to work even if
the object has been constructed manually, starting from 'new
StdClass()'.

I think this is VERY different from what you asked in the first mail.
But to do that, you want something like this:

class Foo (
function Foo() {}
}

$f = new Foo();
....
if (is_a($f, 'Foo')) {
echo "yes";
}
....
echo "\$f is of type: " . get_class($f);

To do what you asked before (and hence break some main concepts in
OO), you can:

$vars = get_class_vars( 'Foo');

/M

[Chung Leong]

wi***********@g mail.com wrote:

Hello there,

I'm looking for a method to test, whether an object has a certain
property.
Consider the following snippet:

class A { var $aaa; }
$var = new A;

(Assuming that the structure of class A is unknown) I need a way to
check whether $var->aaa exists (test positive), and whether $var->xxx
exists (test negative). I tried boolean tests, isset(), is_null(), but
they can't tell the difference.
I guess I could convert the object to an array and test its indices,
but that wouldn't be practical for large object with many fields.

Any other ideas?

Psst! You can use array_key_exist s('aaa', $var) to test for the
existence of a property. I didn't tell you that by the way.

[Jerry Stuckle]

wildernesscat wrote:

Hi Jerry,

I'm trying to write a function that checks whether a given object is of
a known class (named 'A' in this example). I want it to work even if
the object has been constructed manually, starting from 'new
StdClass()'.

Danny

Yes, that's a lot different than your original question.

You can use is_a() as Marcin suggested. But what's to stop me from
creating my own class 'A'?

The real question is - why do you need this "feature"? It violates
several principles of OO.

OO principles include that you don't care WHAT the object is - all you
care about is its interface (functions). If you need to depend on a
specific class, you need to rethink your design.

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=============== ===
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Jerry Stuckle
JDS Computer Training Corp.
js*******@attgl obal.net
=============== ===

[wildernesscat]

Hi Marcin,

>From my experience, the is_a() function does not work unless the object

has been created explicitly using its class name (Foo in your example).
If the object has been created via StdClass, is_a() doesn't recognize
it.

As for using get_class_vars( ), I'm getting into O(n2) here. You
suggested that I scan the properties of one object and compare them to
the other one, right? I was looking for a O(n) solution.

Regards,
Danny

I think this is VERY different from what you asked in the first mail.
But to do that, you want something like this:

class Foo (
function Foo() {}
}

$f = new Foo();
...
if (is_a($f, 'Foo')) {
echo "yes";
}
...
echo "\$f is of type: " . get_class($f);

To do what you asked before (and hence break some main concepts in
OO), you can:

$vars = get_class_vars( 'Foo');

[wildernesscat]

That's interesting. I didn't know that array_key_exist s() works on
objects, but someone suggested I used property_exists (), and that seems
to solve the riddle.

Psst! You can use array_key_exist s('aaa', $var) to test for the
existence of a property. I didn't tell you that by the way.

[wildernesscat]

Hi Jerry,

You can use is_a() as Marcin suggested. But what's to stop me from
creating my own class 'A'?

As I told Marcin, is_a() will not be of much use here. I need an
in-depth comparison, and not just resemblance in names.

The real question is - why do you need this "feature"? It violates
several principles of OO.

Actually, I don't think that I violate any major OO principles. This is
almost like asking an object whether it supports certain interfaces.
I'll pinpoint my question. I want to know whether a certain object has
the _public_ properties of a given class. Does that make sense? Suppose
I have an object that has public properties $aaa, $bbb, and $ccc - I'd
like to know whether class A also has them.

OO principles include that you don't care WHAT the object is - all you
care about is its interface (functions). If you need to depend on a
specific class, you need to rethink your design.

Regards,
Danny

[Jerry Stuckle]

wildernesscat wrote:

Hi Jerry,

>>You can use is_a() as Marcin suggested. But what's to stop me from
creating my own class 'A'?

As I told Marcin, is_a() will not be of much use here. I need an
in-depth comparison, and not just resemblance in names.

>>The real question is - why do you need this "feature"? It violates
several principles of OO.

Actually, I don't think that I violate any major OO principles. This is
almost like asking an object whether it supports certain interfaces.
I'll pinpoint my question. I want to know whether a certain object has
the _public_ properties of a given class. Does that make sense? Suppose
I have an object that has public properties $aaa, $bbb, and $ccc - I'd
like to know whether class A also has them.

>>OO principles include that you don't care WHAT the object is - all you
care about is its interface (functions). If you need to depend on a
specific class, you need to rethink your design.

Regards,
Danny

Danny,

Again, that's a completely different question.

No, there's nothing wrong to determine of a class supports specific
properties. But that's not the same as expecting a specific class.

And in many cases classes shouldn't have public properties - they should
have private properties and public access methods. It isolates the
interface from the implementation.

I see a lot of public properties in PHP code - mainly because before
PHP5 you couldn't define private properties. However, if you look at
C++, Java and Smalltalk, you'll find almost no public properties - only
methods.

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Jerry Stuckle
JDS Computer Training Corp.
js*******@attgl obal.net
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