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Subject: Re: [PHP-DEV] Class Properties in Interfaces?
From: jeff@procata.com (Jeff Moore)


On May 6, 2008, at 12:21 PM, Lars Strojny wrote:

> I think this is too unspecific. At least the visibility, setter and/or
> getter and type-hint (assuming we will have type hints) should be
> defined. Otherwise defining properties in interfaces become useless as
> it does not tell somebody more except "there is a property".
>
> interface Something
> {
>     public $property {
>         string public function get();
>         string protected function set(string $value);
>     }
> }


Hi Lars,

It isn't necessary to specify the visibility in an interface because  
all interface members are public.

Specifying the accessor methods in an interface reveals too much of  
the implementation.  Properties (the kind with accessor methods) are  
themselves an abstract concept.  You want to be able to take an  
interface definition such as:

interface foo {
	public $bar;  // if you prefer this to abstract $foo;
}

and implement it like this:

class DoIt implements foo {
	public $bar;
}

or to implement it using setters, using your notation:

class DoIt2 implements foo {
	public $foo {
		public function get();
		protected function set($value);	
	}
}

The whole point of this kind of property is that the caller is never  
supposed to know if they are just accessing a object instance variable  
or an accessor method.  This is called the uniform access principle.
http://en.wikipedia.org/wiki/Uniform_access_principle

Here is how C# handles properties in an interface:
http://msdn.microsoft.com/en-us/library/64syzecx.aspx

"The accessor of an interface property does not have a body. Thus, the  
purpose of the accessors is to indicate whether the property is read- 
write, read-only, or write-only."

So, yeah, "there is a property named foo" is about all an interface  
should say.

Best Regards,

Jeff