Protected Vs Private

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Protected access modifier is similar to that of private access modifiers, the difference is that the class member declared as Protected are inaccessible outside the class but they can be accessed by any subclass(derived class) of that class.
        // Child class is able to access the inherited
        // protected data members of the base class
        cout << "id_protected is: "
             << id_protected << endl;
    // member function of the derived class can
    // access the protected data members of the base class
The class members declared as private can be accessed only by the functions inside the class. They are not allowed to be accessed directly by any object or function outside the class. Only the member functions or the friend functions are allowed to access the private data members of a class.
// C++ program to demonstrate private
        // member function can access private
        double area = 3.14 * radius * radius;
        cout << "Radius is: " << radius << endl;
        cout << "Area is: " << area;
    // creating object of the class
    // trying to access private data member
Output:
Radius is: 1.5
Area is: 7.065

Difference between Private and Protected
The class members declared as private can be accessed only by the functions inside the class.
Protected access modifier is similar to that of private access modifiers.
Only the member functions or the friend functions are allowed to access the private data members of a class.
The class member declared as Protected are inaccessible outside the class but they can be accessed by any subclass(derived class) of that class.
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I've been searching and I know the theoretic difference.
That's all fine and well, the question is, what's the practical difference between them? When would you use private and when would you use protected? Is there a standard or acceptable good practice over this one?
Up until now, to retain the concept of inheritance and polymorphism, I use public for anything that should be accessed from the outside (like constructors and main class functionality), and protected for internal methods (logic, helper methods etc). Am I on the right track?
(Note that this question is for me, but also for future reference as I haven't seen a question like this one SO).
Madara's Ghost
Madara's Ghost 161k●4949 gold badges●246246 silver badges●296296 bronze badges
Does it matter? Any language with OOP support has this concern. I happen to program in PHP, but I think the question applies for any OOP supporting language. – Madara's Ghost Dec 2 '11 at 7:57
Okay fair enough, just wondering if you had forgotten to tag. Now I see the oop tag. – Paul Bellora Dec 2 '11 at 7:58
I have to set the visibility (private/public/protected) for each and every property of the class? Or only some of them need to have a visibility type? If yes, how to decide which property need to have a visibility set in top of class? – user4271704 Oct 2 '16 at 18:07
offhand, the difference between protected and private seems obvious. Use protected if subclasses will use the method/variable, otherwise use private. Specifically, if subclasses would have to re-define a very similar private variable in the parent, just make it protected. – iPherian Oct 23 '16 at 4:59
There is another access specifier internal in C# language which restricts the access level of a class or its members within a physical assembly. Though, I'm not sure about its support or something similar in other languages. – RBT Feb 3 '17 at 22:59
No, you're not on the right track. A good rule of thumb is: make everything as private as possible. This makes your class more encapsulated, and allows for changing the internals of the class without affecting the code using your class.
If you design your class to be inheritable, then carefully choose what may be overridden and accessible from subclasses, and make that protected (and final, talking of Java, if you want to make it accessible but not overridable). But be aware that, as soon as you accept to have subclasses of your class, and there is a protected field or method, this field or method is part of the public API of the class, and may not be changed later without breaking subclasses.
A class that is not intended to be inherited should be made final (in Java). You might relax some access rules (private to protected, final to non-final) for the sake of unit-testing, but then document it, and make it clear that although the method is protected, it's not supposed to be overridden.
JB Nizet
JB Nizet 640k●8282 gold badges●11341134 silver badges●11981198 bronze badges
The real question here, is about private vs protected When do I want a property to be inherited, and when don't I? I can't really tell if a user sometimes in the future wants to take my class and extend it... – Madara's Ghost Dec 2 '11 at 8:04
Well, the question is not what the user wants to override, the question is what you want to allow to be overridden. Usually it helps to switch sides and try to think: If I used that class and created a subclass, what would I want to be able to override? Sometimes other users will still miss things... – Thorsten Dittmar Dec 2 '11 at 8:14
Protected fields are bad practice in inheritance!. Please beware of it. Here is why – RBT Feb 3 '17 at 23:04
Private fields are bad in practice in inheritance!. (over exaggerating) Please read my answer. – Larry Nov 4 '17 at 14:58
Let me preface this by saying I'm talking primarily about method access here, and to a slightly lesser extent, marking classes final, not member access.
"mark it private unless you have a good reason not to"
made sense in days when it was written, before open source dominated the developer library space and VCS/dependency mgmt. became hyper collaborative thanks to Github, Maven, etc. Back then there was also money to be made by constraining the way(s) in which a library could be utilized. I spent probably the first 8 or 9 years of my career strictly adhering to this "best practice".
Today, I believe it to be bad advice. Sometimes there's a reasonable argument to mark a method private, or a class final but it's exceedingly rare, and even then it's probably not improving anything.
These are the three biggest rationalizations I've heard for marking methods private by default:
I can't count the number of times I've been wrong about whether or not there will ever be a need to override a specific method I've written. Having worked on several popular open source libs, I learned the hard way the true cost of marking things private. It often eliminates the only practical solution to unforseen problems or use cases. Conversely, I've never in 16+ years of professional development regretted marking a method protected instead of private for reasons related to API safety. When a developer chooses to extend a class and override a method, they are consciously saying "I know what I'm doing." and for the sake of productivity that should be enough. period. If it's dangerous, note it in the class/method Javadocs, don't just blindly slam the door shut.
Marking methods protected by default is a mitigation for one of the major issues in modern SW development: failure of imagination.
This one is more reasonable, and depending on the target audience it might even be the right thing to do, but it's worth considering what the cost of keeping the API "clean" actually is: extensibility. For the reasons mentioned above, it probably makes more sense to mark things protected by default just in case.
This is reasonable too, but as a consumer I'd go with the less restrictive competitor (assuming no significant quality differences exist) every time.
I'm not saying never mark methods private. I'm saying the better rule of thumb is to "make methods protected unless there's a good reason not to".
This advice is best suited for those working on libraries or larger scale projects that have been broken into modules. For smaller or more monolithic projects it doesn't tend to matter as much since you control all the code anyway and it's easy to change the access level of your code if/when you need it. Even then though, I'd still give the same advice :-)
Nick
Nick 7,895●33 gold badges●3636 silver badges●6262 bronze badges
W.r.t. your Rationalization # 1 - When I mark something as protected I choose to do that explicitly as I feel that this behavior isn't final and might be a case where my child classes would want to override it. If I'm not so sure then I would want to make it private by default. Specially in a language like C# which keeps a method non-virtual by default, adding just protected access specifier makes no sense.You have to add both virtual and protected keywords to make your intent very clear. Also, people prefer association over inheritance so protected as default is difficult to perceive – RBT Feb 4 '17 at 0:08
The combination of keywords necessary to make a method overridable is a language detail IMHO. The counter argument I present to rationalization #1 is squarely aimed at the case you mention "If I'm not so sure...". Practically speaking, if you can't think of a reason why it would be dangerous then theres more to be gained by opting for extensibility. When you say 'association' I'm taking that to be a synonym for composition, in which case I don't see it mattering either way. – Nick Feb 26 '17 at 22:54
I don't remember how many times I had to "clone" the underlying classes just because I wanted to override 1 or 2 or the methods. And like what you said, with the social trend that we're sharing more code than ever before, it makes much more sense to go with protected than private by default. i.e. be more open to your own logics. – shinkou Jan 8 '18 at 17:49
Agree. I just wanted to tweak Java's BufferedReader to handle custom line delimiters. Thanks to private fields I have to clone the entire class rather than simply extending it and overriding readLine(). – Ron Dunn May 26 '18 at 23:32
The comments here seem to be overwhelmingly supportive towards using private fields. Well, then I have something different to say.
Are private fields good in principle? Yes. But saying that a golden rule is make everything private when you're not sure is definitely wrong! You won't see the problem until you run into one. In my opinion, you should mark fields as protected if you're not sure.
There are two cases you want to extend a class:
There's nothing wrong with private fields in the first case. The fact that people are abusing private fields makes it so frustrating when you find out you can't modify shit.
Consider a simple library that models cars:
The library author thought: there's no reason the users of my library need to access the implementation detail of assembleCar() right? Let's mark screw as private.
Well, the author is wrong. If you want to modify only the assembleCar() method without copying the whole class into your package, you're out of luck. You have to rewrite your own screw field. Let's say this car uses a dozen of screws, and each of them involves some untrivial initialization code in different private methods, and these screws are all marked private. At this point, it starts to suck.
Yes, you can argue with me that well the library author could have written better code so there's nothing wrong with private fields. I'm not arguing that private field is a problem with OOP. It is a problem when people are using them.
The moral of the story is, if you're writing a library, you never know if your users want to access a particular field. If you're unsure, mark it protected so everyone would be happier later. At least don't abuse private field.
Larry
Larry 797●88 silver badges●1515 bronze badges
Using private fields is mostly saying that inheritance is the wrong abstraction to alter a certain behavior of a class/object (if used consciously). Altering will usually silently violate LSP as behavior is changed without the API changing. Great answer, +1. – Madara's Ghost Nov 4 '17 at 18:51
Preach! Private is the bane of my existence when trying to write Minecraft mods. Nothing more annoying than having to use Reflection or ASM to fix that. – RecursiveExceptionException Aug 20 '18 at 21:25
As I understood Nick's answer he was referring mostly to methods not properties. I totally agree we shouldn't declare methods private every time and usage of those needs to be thoughtful, but cmon why would you advise to declare properties protected over private just because youd want to easier "rewrite" the class. Totally share your frustration as I'm working with 3rd p's daily, but let's encourage people to create solid architecture and not just saying "the code would end up being crap so let's have protected from the beginning so we can easily rewrite it". Although I share your frustration. – sean662 Apr 22 '20 at 18:45
I read an article a while ago that talked about locking down every class as much as possible. Make everything final and private unless you have an immediate need to expose some data or functionality to the outside world. It's always easy to expand the scope to be more permissible later on, but not the other way around. First consider making as many things as possible final which will make choosing between private and protected much easier.
Now if you're left with a final class, then make everything private unless something is absolutely needed by the world - make that public.
If you're left with a class that does have subclass(es), then carefully examine every property and method. First consider if you even want to expose that property/method to subclasses. If you do, then consider whether a subclass can wreak havoc on your object if it messed up the property value or method implementation in the process of overriding. If it's possible, and you want to protect your class' property/method even from subclasses (sounds ironic, I know), then make it private. Otherwise make it protected.
Disclaimer: I don't program much in Java :)
Anurag
Anurag 134k●3535 gold badges●214214 silver badges●257257 bronze badges
To clarify: A final class in Java is a class which cannot be inherited from. A final method is non-virtual, i.e. not overridable by a subclass (Java methods are virtual by default). A final field/parameter/variable is an immutable variable reference (in the sense that it cannot be modified after it was initialized). – M.Stramm Dec 3 '15 at 13:24
Funnily enough, I've seen the exact opposite advice, that nothing should be final unless it is certain that overriding the thing in question has the potential to break an invariant. That way anybody is free to extend your classes as needed. – GordonM Aug 8 '17 at 9:03
Can you name one case in your programming career where marking a method parameter "final" made a difference to your understanding? (other than if required by the compiler) – user949300 Aug 23 '17 at 19:50
When would you use private and when would you use protected?
Private Inheritance can be thought of Implemented in terms of relationship rather than a IS-A relationship. Simply put, the external interface of the inheriting class has no (visible) relationship to the inherited class, It uses the private inheritance only to implement a similar functionality which the Base class provides.
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