Posted by Malcolm Dew-Jones on 06/23/07 00:02

grossespinne@gmail.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?content=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

$thePage->someMethod($var1,$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

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