Hello Thies,
Return NULL wouldn't work because a ctor always returns NULL by
default so we nee a trick. Would reallowing $this = NULL in ctors
only and direct return and disallowing any other value be an option
for (perhaps the executor can capture that event)? Would you like
that?
Aparat from that there were other possible solutions mentioned so
far:
The factory solution prevents inheritance on overloaded objects.
If it would work the derived class needs to somehow be able to
create a derived instance and still use the inherited factory from
its derived factory. That is it somehow needs to tell the inherited
factory which class entry to use.
Overriding new could be a solution for php 5.1:
class SQLiteDatabase {
// Suppose __new gets executed right after the object
// was created and contains the only reference to the
// newly created object in its first parameter. And
// let it return the instance or NULL. New obviously
// needs to be static.
// A small side-problem: __new must not be called from
// any other place than new.
static function __new($instance) {
if ($instance->ctorOk()) {
return $instance;
} else {
return NULL;
}
}
private function ctorOk() {
// just verify the object state once in __new
}
}
Another __new solution would be to let it execute the real
allocating "new" itself (more like c++ and others) how would
we differentiate those two news now? Anywhere i suggest the
code looking something like this:
static function new($file /* params from the new call*/) {
$instance = new SQLiteDatabase($file);
if ($instance->ctorOk()) {
return $instance;
} else {
return NULL;
}
}
Also the above second new solution has again the inheritance
problem: Every derived class would have to overwrite __new or
we need a dynamic allocating new and pass the requested class
to it:
static function new($class, $file) {
$instance = new $class($file);
if ($instance->ctorOk()) {
return $instance;
} else {
return NULL;
}
}
Any ideas from you Thies?
best regards
marcus
Thursday, April 15, 2004, 7:36:46 AM, you wrote:
> Am 14.04.2004 um 21:53 schrieb Marcus Boerger:
>>> Personally I'd much prefer a way of returning a value from a
>>> constructor, i.e. to be able to 'return null;' or a similar language
>>> construct so I could do 'if ($db = new SQLiteDatabase)'
>>> It would also mean that I would run into a 'calling method on a
>>> non-object' error if I don't check it (which is fine).
>>
>> In no language i know (c++, delphi, java as the popular ones) a ctor
>> can return a value especially not it's own value. The problem is that
>> when the ctor is called the object is already created. For most
>> languages
>> that means some memory has been allocated and for php overloaded
>> objects
>> like the SQLite class it also means that the memory has been
>> initialized.
>>
>> Now in PHP 4 there was the ugly trick of setting "$this=NULL".
>>
> i'm sure ZE2 can be tricked to support "return NULL" from a constructor
> so that:
> $a = new bla();
> if (! $a) ...
> if ctors are the only place that *cannot* life without exceptions it
> would be well worth *fixing* (yes, i said fix), as adding
> "understanding exceptions" to the list of pre-requisites for
> learning/using php would just be a poor decision looking at what made
> php as popular as it is today.
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