Question

So lets say I have a class that is composed of other classes.

class HttpRequest
{
public $session = new Session();
// .. the rest of the HttpRequest code
}

Now, I want to have access to Session class through HttpRequest class so Im using composition.
But does this breaks laws of OOP Encapsulation or Data hidding that states that all properties should be protected, and accessed through setter and getter methods?

Is this wrong:

$request = new HttpRequest();
$request->session->set('id', 5);

or should I use this:

$request = new HttpRequest();
$session = $request->getSession();
$session->set('id', 5);

Encapsulation states that properties shoud be protected.
How to provide access to inner classes then? Is the first example wrong as far as proper OOP goes?

Answer 1

There are valid reasons to not allow direct access to the object:

• Allows for manipulation of the object outside of the object itself. If you make the property public, any part of your code could overwrite $session on the HttpRequest class, and you’d have a tough time tracking it down. Encapsulation from a data protection standpoint is there to ensure that only the object’s methods can directly alter the object.
• Allows you to gracefully handle the case in which that variable is not set. If, for some reason, $session does not get set on your class – you’ll immediately have a fatal when you try to call a method on it. If you wrap it in a getter, you could check for that condition and create a new instance of the class on the fly.
• Follows true “OO” paradigms

However, in some cases I would say it is okay to do this. Particularly if you know that the property will always be set (and the only way in which it would not be set is not a supported way to use the object).

It also makes sense depending on how the property is going to be accessed. Symfony2 uses this in their Request classes. It feels natural in that case, as the “query” “post” and “request” vars are all “ParameterBag”s (glorified arrays). However, they do expose a getter for the Session object – likely because of it’s use case.

In short: it really depends on how you’ll be using the variable. In this particular case, I’d say it doesn’t much matter.