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Thu, 09 Sep 2010

Protected Attributes Make No Sense


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In C++, you can declare an attribute or a method as "protected". Which means that it is private, but subclasses can still access them.

This makes just as much sense as saying you only get access to these attributes if you wear a funny yellow hat.

I'll try to explain why.

Encapsulation has the advantage that you can change the implementation of a class without changing the API. If you write a library, that means you don't break your user's code if you change some internals.

However your users can not only instantiate your classes, but they can also inherit from them. Which means they can put a funny yellow hat on. There is no restriction as to who can inherit from your classes. Which makes protected attributes part of the public interface.

So, if you want to change the implementation of a class, you can't change how a protected attribute or method works, and what data it stores. If you do, you break all subclasses. Which not only exist in your own source tree, but in your user's source tree too. If you think that changing the implementation will also change protected attributes or methods, make them private.

So if protected methods are essentially part of the public API anyway, why not declare them as public? Why require your user to wear a funny yellow hat (aka declare an inheritance relation) while using your class? There is no good reason.

When seasoned C++ programmers hear that, they often respond with "but if I declare my private data as private a subclass doesn't have access to it! How can it work properly?". The answer is simple: You must provide a sufficiently powerful public API. Your users will be grateful.

One more thing: people often come up with an analogy to the real world, and claim that your kids also have access to your house or flat. This analogy is broken, because in the real world the parents have to take actions to get children (or give somebody the same status as your children). In the programming world, every outsider can write a child class of your classes.

(The reason I posted this in the Perl 6 section of this blog is that Perl 6 has no protected attributes. Which has spawned many interesting discussions, which were the precursor to this blog post.)

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