How to cast custom post type into separate class instead of WP_Post

Question

I'd like to create a subclass of WP_Post and add some "model functionality" to it. How can i force WP to create objects of that child class instead of WP_Post itself, when i query for my custom post type?

Example:

Let's assume i have two custom post types: Book and Review. Each Book can have many Reviews. On my Book, I want a method to sum up all its reviews. I'd define the following class:

class Book extends WP_Post
{
    public function reviewsSummary()
    {
        // Retrieve all reviews for $this book
        // Sum up their ratings
        // Return that sum
    }
}

Is there a way, for example when calling register_post_type(), to force WordPress into casting all posts of type "book" into my Book class instead of WP_Post?

Could look something like this:

register_post_type('book', [
    …,
    'class' => Acme\Models\Book::class
]);

Answer 1

WP_Post class in WP is final, so it explicitly forbids subclassing. It is also quite vague in practice, a lot of code around will happily take/produce post–like objects as long as they have established data structure.

It is challenging to recommend an alternative without knowing more about your specific needs. In a general and assuming use in templates the typical would be to create a template tag function that will calculate score for review post, provided to it as argument and/or current in the Loop.

Answer 2

How to "subclass" WP_Post

Thank you, @Rarst for pointing out the issue that you can't subclass WP_Post, which is a major problem in this case.

Workaround

There is, however, a way of working around it using a wrapper class that has a real WP_Post object stored as an attribute and exposes it via magic methods in order to behave like one.

Two examples of that approach can be found here and here.

Warning: In many cases, this approach might not work for different reasons, one being that these objects won't pass an instanceof WP_Post check.

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Where to hook in

So let's assume we could replace WP_Post somehow. The thing i was actually looking for was where to hook in in order to replace WP_Post. The answer to this question is the posts_results filter and was given here: https://stackoverflow.com/a/40631962/3919281