Instantiate subclass depending on string value

[grossespinne@gmail.com]

Hi everybody!
I have been thinking over the following problem:
there are three classes: PageBase, which is the base class, PageA and
PageB which are the subclasses of PageBase.
In the index.php file I have a variable which I like to hold either an
instance of PageA or PageB depending on the query string that is
passed to index.php (e.g: if I get the query index.php?conte nt=a, I
need an instance of PageA, but if I get the query: index.php?
content=b, an instance of PageB is needed)

Until now I found two solutions, but I am not satisfied with either of
them:
- use a switch structure
- or use eval:

$pageType = isset($_GET["page"]) ? $_GET["page"] : "PageBase";
$thePage = eval("return new $pageType();");

My question is whether a more object-oriented way exists to solve the
problem.

TIA

[=?ISO-8859-15?Q?Iv=E1n_S=E1nchez_Ortega?=]

Re: Instantiate subclass depending on string value

grossespinne@gm ail.com wrote:

My question is whether a more object-oriented way exists to solve the
problem.

You are looking for a Factory design pattern.

I suggest you to do a bit of research about that, and about design patterns
in general.

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[Malcolm Dew-Jones]

Re: Instantiate subclass depending on string value

grossespinne@gm ail.com wrote:
: Hi everybody!
: I have been thinking over the following problem:
: there are three classes: PageBase, which is the base class, PageA and
: PageB which are the subclasses of PageBase.
: In the index.php file I have a variable which I like to hold either an
: instance of PageA or PageB depending on the query string that is
: passed to index.php (e.g: if I get the query index.php?conte nt=a, I
: need an instance of PageA, but if I get the query: index.php?
: content=b, an instance of PageB is needed)

: Until now I found two solutions, but I am not satisfied with either of
: them:
: - use a switch structure
: - or use eval:

: $pageType = isset($_GET["page"]) ? $_GET["page"] : "PageBase";
: $thePage = eval("return new $pageType();");

: My question is whether a more object-oriented way exists to solve the
: problem.

No, that's one of the dirty little secrets of OOPs programming.
Somewhere you have to do what you do above, either something like a
switch, if/then/else, or as in your case something like an eval.

(BTW: I would recommend a switch (or if/then/else) over eval because that
prevents any chance of code injection. What if "page" is set to a string
like "system('rm /*');" which probably wouldn't run in your code, but
shows what I am getting at. You don't want to put that in your eval().)


However you normally hide the dirty details in a "factory" class that
returns an instance of the _base_ class. (In php that won't make much
difference, as long as each class implements the same methods then all is
ok, in a language like Java it must explicitly be the base class.)

(my syntax correct! I haven't used php for a little while).

$thePage = PageFactory->createPage( $pageType );

later, when you say

$thePage->someMethod($va r1,$vars) ;

then either
PageA->someMethod
runs or
PageB->someMethod
runs.


PageFactory is a class that you create. In this case it has one static
method that returns PageBase. The code is something like (my syntax is
not valid php today)

function createPage ( $type ) returns PageBase

if ($type = 'A') return (PageBase) new PageA();

elsif ($type = 'B') return (PageBase) new PageB();

else die "$type is not a known type of page";

end function

[Rik]

Re: Instantiate subclass depending on string value

On Sat, 23 Jun 2007 00:18:56 +0200, <grossespinne@g mail.comwrote:

Hi everybody!
I have been thinking over the following problem:
there are three classes: PageBase, which is the base class, PageA and
PageB which are the subclasses of PageBase.
In the index.php file I have a variable which I like to hold either an
instance of PageA or PageB depending on the query string that is
passed to index.php (e.g: if I get the query index.php?conte nt=a, I
need an instance of PageA, but if I get the query: index.php?
content=b, an instance of PageB is needed)
>
Until now I found two solutions, but I am not satisfied with either of
them:
- use a switch structure
- or use eval:
>
$pageType = isset($_GET["page"]) ? $_GET["page"] : "PageBase";
$thePage = eval("return new $pageType();");

First of all, what is wrong with dropping the eval, and just creating
objects on the fly? So:

$pageType = (isset($_GET['page'] &&
class_exists('P age'.strtoupper ($_GET['page'][1]) ? 'Page'.$_GET['page'][1]
: 'PageBase';
$thePage = new $pageType();

My question is whether a more object-oriented way exists to solve the
problem.

Indeed, like others said, check Factory patterns.
--
Rik Wasmus

[Jerry Stuckle]

Re: Instantiate subclass depending on string value

grossespinne@gm ail.com wrote:

Hi everybody!
I have been thinking over the following problem:
there are three classes: PageBase, which is the base class, PageA and
PageB which are the subclasses of PageBase.
In the index.php file I have a variable which I like to hold either an
instance of PageA or PageB depending on the query string that is
passed to index.php (e.g: if I get the query index.php?conte nt=a, I
need an instance of PageA, but if I get the query: index.php?
content=b, an instance of PageB is needed)
>
Until now I found two solutions, but I am not satisfied with either of
them:
- use a switch structure
- or use eval:
>
$pageType = isset($_GET["page"]) ? $_GET["page"] : "PageBase";
$thePage = eval("return new $pageType();");
>
My question is whether a more object-oriented way exists to solve the
problem.
>
TIA
>

Not every statement has to be object oriented. That's why PHP still has
switch statements. It's what I would do.

There are other ways to do it, i.e. factories. But I also believe in KISS.

--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

[grossespinne@gmail.com]

Re: Instantiate subclass depending on string value

On Jun 23, 4:14 am, Jerry Stuckle <jstuck...@attg lobal.netwrote:

grossespi...@gm ail.com wrote:

Hi everybody!
I have been thinking over the following problem:
there are three classes: PageBase, which is the base class, PageA and
PageB which are the subclasses of PageBase.
In the index.php file I have a variable which I like to hold either an
instance of PageA or PageB depending on the query string that is
passed to index.php (e.g: if I get the query index.php?conte nt=a, I
need an instance of PageA, but if I get the query: index.php?
content=b, an instance of PageB is needed)

>

Until now I found two solutions, but I am not satisfied with either of
them:
- use a switch structure
- or use eval:

>

$pageType = isset($_GET["page"]) ? $_GET["page"] : "PageBase";
$thePage = eval("return new $pageType();");

>

My question is whether a more object-oriented way exists to solve the
problem.

>

TIA

>
Not every statement has to be object oriented. That's why PHP still has
switch statements. It's what I would do.
>
There are other ways to do it, i.e. factories. But I also believe in KISS.
>
--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstuck...@attgl obal.net
=============== ===

Thanks to everybody for your help. I think I am going to use the
Factory method and implement it with the solution suggested by Rik.