Returning of new $this with abstract classes

Question

I found some trouble with my code and do not understand why it's doing as it is. Can anyone explain me?

Let we have:

abstract class AbstractThing
{
    public function search(...)
    {
        $ret = false;

        $data = $database->query(...);
        foreach($data as $values)
        {
            $item  = new $this;
            $item->fill_with_values($values);

            $ret []= $item;
        }

        return $ret;
    }
}

It's works as intended and return object instances at success searches:

class Thing extends AbstractThing
{
    // ...
}

$thing = new Thing;
$things = $thing->search(...); // Thing[] on success, false on failure

But if I wish to shorten code very slightly, it breaks:

abstract class AbstractThing
{
    public function search(...)
    {
        $ret = false;

        $data = $database->query(...);
        foreach($data as $values) {
            $ret []= (new $this)->fill_with_values($values);
        }

        return $ret;
    }
}

This return boolean true. Why? It works well on the classes that are not inherited from abstract class.

score 1 · Answer 1 · answered Sep 29 '17 at 02:21

The code does 2 different things:

This adds $item to your "$ret" array:

        $item  = new $this;
        $item->fill_with_values($values);

        $ret []= $item;

This adds the returned value of "fill_with_values" to your array:

$ret []= (new $this)->fill_with_values($values);

The equivalent of the above code would be:

        $item  = new $this;
        $return = $item->fill_with_values($values);
        $ret []= $return;

If I knew what was going on in your "fill_with_values" method I could tell you why it is a boolean, but the code does not do the same thing. Hope that makes sense.

Oh, sorry, forgot to said: fill_with_values() returns $this – Sep 29 '17 at 03:39 — , Sep 29 '17 at 03:39

Cy Rossignol · Accepted Answer · 2017-09-29T03:03:58.127

1

When we assign:

$ret []= (new $this)->fill_with_values($values);

...we're not setting $ret[] = (new $this). Instead, this statement pushes the return value of fill_with_values() into the array because it executes last.

It looks like you're trying to implement something similar to the factory method pattern. Consider this:

abstract class AbstractThing
{ 
    ...
    public static function fill($values) 
    { 
        $instance = new static; 
        $instance->fill_with_values($values);

        return $instance; 
    }
}

Then we can actually do what you're trying to accomplish in your question like this:

$ret[] = static::fill($values);

This works because the return value of fill() is the instance of the class, not the return value of fill_with_values(). The static keyword in this context uses late static binding to resolve the type of the class that executes the code (Thing in this case) instead of the class that declares it, so it works through inheritance. See this question for more information.

edited Sep 29 '17 at 03:03

answered Sep 29 '17 at 02:22

Cy Rossignol

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1

Thanks a lot, I understand (and used before) mechanics with the static methods. Btw, `fill_with_values()` returns `$this`, it can't return `true`. At both calls data is equal. I just curious about this. – Sep 29 '17 at 03:48
Hmm... something else must be happening inside `fill_with_values()` or `search()`. I just tested your second example (minus the database query) using a `fill_with_values()` that returns `$this` and I see the expected array of `Thing`s. – Cy Rossignol Sep 29 '17 at 04:11
Narrowed the "trouble" a little: `$ret = new $this; $ret->_fill($data);` works, `$ret = (new $this)->_fill($data)` raises `Call to a member function _fill() on boolean` error. So, in some way (new $this) returns `true`. – Sep 29 '17 at 04:25
Perplexing...which version of of PHP does this occur in? It sounds like a bug in the language. Here are a couple of debugging suggestions, but based on the info on this question it seems like the problem may be elsewhere: try `var_dump(new $this)`...is it a boolean? Check `AbstractThing` subclasses for a method that overrides these methods. – Cy Rossignol Sep 29 '17 at 05:29
Funny that I can't reproduce it from scratch with the tiny classes. Using `PHP 7.0.15-0ubuntu0.16.04.4 (cli) ( NTS )` on Docker. Thanks a lot for your replies! – Sep 29 '17 at 06:43

score 1 · Answer 3 · answered Sep 29 '17 at 16:37

1

Ok, finally it was my own error. There was really possibility to return TRUE from fill_with_values() function at some point. Sorry all for bad questions and thanks for answers!

answered Sep 29 '17 at 16:37

Returning of new $this with abstract classes

3 Answers3