object default value in constructor

[Janez Cerar]

I'm trying to write a class with a constructor that takes a single object
parameter. So far so good, but I can't for the life of me make the parameter
optional. I understand how to do it with a numerical or textual parameter,
but not with an object. I tried null, but it doesn't work.

class A
{
}

class B
{
function B($a = ???)
{
if (!isset($a)) $a = new A;
}
}

Any ideas?

Jaka

[Nikolai Chuvakhin]

Re: object default value in constructor

"Janez Cerar" <janez.cerar@em ail.removethis. si> wrote
in message news:<9KTlb.485 2$2B6.896815@ne ws.siol.net>...[color=blue]
>
> I'm trying to write a class with a constructor that takes a single object
> parameter.[/color]

Why? Isn't it easier to just extend the class?
[color=blue]
> So far so good, but I can't for the life of me make the parameter
> optional.
>
> class A
> {
> }
>
> class B
> {
> function B($a = ???)
> {
> if (!isset($a)) $a = new A;
> }
> }[/color]

Try this instead:

class A {
function A () {}
}
class B extends A {}

Since you defined no constructor for B, new B will simply call the
constructor for A (in PHP 4, at least).

Cheers,
NC

[Phil Roberts]

Re: object default value in constructor

With total disregard for any kind of safety measures nc@iname.com
(Nikolai Chuvakhin) leapt forth and uttered:
[color=blue]
> Since you defined no constructor for B, new B will simply call
> the constructor for A (in PHP 4, at least).[/color]

Not strictly true. The parent constructor is not called by default,
it has be called explicitly.

class ClassOne {
var $number;
function ClassOne() {
$this->number = 2;
}
}

class ClassTwo extends ClassOne {
function ClassTwo() {
# Call the parent constructor
parent::ClassOn e();
}

function classMethod() {
return $this->number;
}
}

$obj =& new ClassTwo;

# Will echo '2';
echo $obj->classMethod( );

--
There is no signature.....

[André Næss]

Re: object default value in constructor

Janez Cerar:
[color=blue]
> I'm trying to write a class with a constructor that takes a single object
> parameter. So far so good, but I can't for the life of me make the
> parameter optional. I understand how to do it with a numerical or textual
> parameter, but not with an object. I tried null, but it doesn't work.
>
> class A
> {
> }
>
> class B
> {[/color]
function B($a = null)[color=blue]
> {[/color]
if(!$a) { $a = new A(); }[color=blue]
> }
> }[/color]

isset() checks if a variable is set or not. A variable which is set to null
is of course set. null is equivalent to false.

André Næss

[Nikolai Chuvakhin]

Re: object default value in constructor

Phil Roberts <philrob@HOLYfl atnetSHIT.net> wrote
in message news:<Xns941ECB 5CAAA26philrobe rts@206.127.4.2 2>...[color=blue]
>
> With total disregard for any kind of safety measures nc@iname.com
> (Nikolai Chuvakhin) leapt forth and uttered:
>[color=green]
> > Since you defined no constructor for B, new B will simply call
> > the constructor for A (in PHP 4, at least).[/color]
>
> Not strictly true. The parent constructor is not called by default,
> it has be called explicitly.[/color]

The parent constructor IS called by default, if the child class
has no constructor defined:

...in PHP 4... If a class has no constructor, the constructor
of the base class is being called, if it exists.

http://php.net/manual/en/language.oop.constructor.php

This is really easy to test:

class A {
var $one;
var $two;
function A () {
$this->one = 'String One';
$this->two = 'Sting Two';
}
}

class B extends A {
function show () {
echo 'Field one = ', $this->one, "\r\n";
echo 'Field two = ', $this->two, "\r\n";
}
}

$b = new B();
$b->show();

The above code snippet prints:

Field one = String One
Field two = Sting Two

Cheers,
NC