On Visibility in OOP

I'm a big proponent of object oriented programming. OOP done right helps ease code maintenance and enables code re-use.

Starting in PHP, OOP enthusiasts got a whole bunch of new tools, and new tools keep coming into the language for us with each minor release. One feature that has had a huge impact on frameworks and libraries has been available since the earliest PHP 5 versions: visibility.

Theory

The visibility keywords include private, protected, and public, often referred to as PPP. There's an additional keyword I often lump in with them, final.

Public visibility is the default, and equivalent to the only visibility available to PHP prior to version 5: any member declared public is accessible from any scope. This means the following:


class Foo
{
    public $bar = 'bar';

    public function baz() 
    {
        // I can access within my own scope
        return $this->bar;
    }
}

class FooBar extends Foo
{
    public function doThat()
    {
        // I have access to members in my parent
        return $this->bar . $this->baz();
    }
}

$foo = new Foo();

// I can access public members from an instance
echo $foo->bar . $foo->baz();

Basically, public visibility means that I can access the member from within the object, within an extending class, or from simply an instance.

Protected visibility starts to tighten things down a little. With protected visibility, only the class itself, or an extending class, can access the member:


class Foo
{
    protected $bar = 'bar';

    protected function baz() 
    {
        // I can access within my own scope
        return $this->bar;
    }
}

class FooBar extends Foo
{
    public function doThat()
    {
        // I can access protected members in my parent
        return $this->bar . $this->baz();
    }
}

$foo = new FooBar();

// This works, as I'm calling a public member of an extending class:
$foo->doThat();

// But these are both illegal:
echo $foo->bar . $foo->baz();

Protected visibility is nice for hiding things from those consuming your class. It can be used to hide implementation details, and to prevent direct modification of public properties -- something important to consider, if a property may be the product of calculation, or if a particular type is required.

Private visibility locks things down further. With private visibility, the object member is only directly modifiable or callable within the declaring class.


class Foo
{
    private $bar = 'bar';

    private function baz() 
    {
        // I can access within my own scope
        return $this->bar;
    }
}

class FooBar extends Foo
{
    public function doThat()
    {
        // These are both illegal
        return $this->bar . $this->baz();
    }
}

$foo = new FooBar();

// These are also both illegal:
echo $foo->bar . $foo->baz();

Private visibility is generally of interest for locking down algorithms. For instance, if you know that a particular value or operation must not change, even in extending classes, declaring the member private ensures that extending classes cannot directly call it.

At any point, you can redeclare a property in an extending class using equal or more public visibility. The effect of doing so depends on what the visibility of the member was in the parent class.

  • In the case of a public property, if an extending class re-declares with public visibility, any access to the member within the extending class or an instance of the extending class will see only the new declaration.

    
    class Foo
    {
        public $bar = 'bar';
    
        public function baz() 
        {
            return $this->bar;
        }
    }
    
    class FooBar extends Foo
    {
        public $bar = 'foobar';
    }
    
    $foo = new FooBar();
    echo $foo->bar;   // "foobar"
    echo $foo->baz(); // "foobar"
            
  • In the instance of a protected property, if the extending class re-declares with either public or protected visibility, you get the same behavior as public -> public.

    
    class Foo
    {
        protected $bar = 'bar';
    
        public function baz() 
        {
            return $this->bar;
        }
    }
    
    class FooBar extends Foo
    {
        public $bar = 'foobar';
    }
    
    $foo = new FooBar();
    echo $foo->bar;   // "foobar"
    echo $foo->baz(); // "foobar"
            
  • In the instance of a private property, things get interesting. The private value or method will be used for any access made within code declared in the parent class, but not overridden in the child. However, if the child class overrides any code, the value of the re-declared instance will be used. This is far easier to understand via an example.

    
    class Foo
    {
        private $bar = 'bar';
        private $baz = 'baz';
    
        public function baz() 
        {
            return $this->bar;
        }
    }
    
    class FooBar extends Foo
    {
        protected $bar = 'foobar';
        private $baz = 'foobaz';
    
        public function myBaz() 
        {
            return $this->bar;
        }
    
        public function myBaz2()
        {
            return $this->baz;
        }
    }
    
    $foo = new FooBar();
    echo $foo->baz();    // "bar"
    echo $foo->myBaz();  // "foobar"
    echo $foo->myBaz2(); // "foobaz"
            

My personal takeaway from this is:

  • Use public for members that are safe for anything to call.
  • Use protected for anything you don't want called from instance methods, not important to the public API (implementation details), and anything you feel is safe for extending classes to muck about with.
  • Use private for any important implementation details that could adversely affect execution if overridden by an extending class.

Those paying attention will note that I skipped final. Actually, I saved that for last. Marking a class or method final tells PHP that the class or method may not be extended or re-declared/overridden. At all. I lump this with visibility, because it's another way of locking down access to an API; marking something final is saying, "you cannot extend this", similar to using private, but without even the possibility of redeclaring.

Applied

What got me to thinking about all this was a turn of events with Zend Framework 2. We've had an annotation parser since last summer. Ralph Schindler developed it in order to facilitate automatic discovery of injection points for our Dependency Injection container. Classes could mark a method with the "Inject" annotation, and the various DI compilers would know that that method needed to be injected.


use Zend\Di\Definition\Annotation\Inject;

class Foo
{
    protected $bar;

    /**
     * @Inject()
     * @param  Bar $bar
     * @return void
     */
    public function setBar(Bar $bar)
    {
        $this->bar = $bar;
    }
}

class Bar {}

Recently, part of our Forms RFC included a feature to allow creating forms and their related input filters by using annotations. Basically, this allows developers to hint on their domain entities how specific properties should be filtered, validated, and potentially represented at the form level.


use Zend\Form\Annotation;

class Foo
{
    /**
     * @Annotation\Filter({"name":"StringTrim"})
     * @Annotation\Validator({"name":"Between","options":{"min":5,"max":20}})
     * @Annotation\Attributes({"type":"range"})
     */
    protected $bar;
}

One developer testing the support wanted to use a combination of Doctrine annotations and ZF2 form annotations -- that way his entities could also describe validation and representation.

I did some work to make this happen, and everybody was happy. Except then that same developer went to use that entity with Doctrine, and Doctrine's annotation parser started raising exceptions on all the ZF2 annotations.

After some debate, I realized: (a) we were basically just making up syntax for our annotations; it'd be better to use an established syntax; but (b) we should still retain the ability to use arbitrary syntax, as we can't really know what sorts of annotations developers may already be using.

So, we decided to make our annotation component depend on the annotations support in Doctrine\\Common, and to use the annotation syntax they utilize. ZF2 would provide some code to make it possible to plug in arbitrary parsers, and use the Doctrine\\Common annotation parser to parse annotations officially supported by ZF2.

However, when I went to start making this happen, I ran into immediate issues.

Remember how this post is about visibility? Well, the class I was directly interested in, Doctrine\Common\Annotations\DocParser, not only contains private members, but is marked final.

My immediate response was to start dissecting the class, cutting and pasting the bits interesting to my solution into a new class in ZF2. I went down this route for several hours, gradually pulling in more and more methods as I discovered how far down the rabbit hole I needed to go to accomplish my task.

But at the back of my head, I kept thinking this was a bad idea. If any patches ever came in for the original class, I'd need to port them into our ZF2 solution. And I couldn't help but think that I'd miss a crucial piece.

So I started playing with its public API, to see if there were any shortcuts I might be able to take. And there were.

The class has a public parse() method. Based on how Doctrine uses the code, I assumed I needed to pass a full PHP docblock in -- which ran counter to how I wanted to use the code. I wanted to pass in an annotation at a time. But when I looked closer, I realized that the parser didn't require a full docblock; any fragment would do.

To make a long story short: I was able to feed the parser a single annotation at a time from ZF2's AnnotationScanner. This allowed me to build a very simple class that allows registering a set of annotations it can handle, and feeding it a single annotation string at a time to decide (a) if it supports it, and (b) to parse it and return the associated annotation object.

In sum: because the class in question was marked final and had private members, I found myself forced to think critically about what I wanted to accomplish, and then thoroughly understand the public API to see how I might accomplish that task without the ability to extend.

Conclusions

Doctrine has a policy that encourages poka-yoke solutions: code should be executable in a specific way. The policy was developed to both aid users (having multiple ways of doing something is often confusing), as well as to ease maintenance (fewer extension points means less liklihood of developers doing hard-to-debug things in extending code and reporting it back to the project). These have led them to heavily use private and final visibility.

I've said it before, and I'll say it again: I feel that frameworks and libraries should use private and final sparingly. Over the years, I've seen code repurposed in simply wondrous ways -- largely due to keeping the code as open as possible to extension. I like to enable my users as much as possible.

That said, I can also see Doctrine's argument -- and can see where, while it can often be frustrating, it can also lead to potentially more sound and elegant solutions.

I'll probably continue shying away from private and final visibility, but I do plan to experiment with it more in the future. What about you?

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