Object-oriented – In OOP, isn’t the ‘protected’ keyword required

All answers so far have focused on the topic of your question as stated, which is "what is the difference between c and c++". In reality, it sounds like you know what difference is, you just don't understand why you would need that difference. So then, other answers attempted to explain OO and encapsulation.

I wanted to chime in with yet another answer, because based on the details of your question, I believe you need to take several steps back.

You don't understand the purpose of C++ or OO, because to you, it seems that your application simply needs to store data. This data is stored in variables. "Why would I want to make a variable inaccessible? Now I can't access it anymore! By making everything public, or better yet global, I can read data from anywhere and there are no problems." - And you are right, based on the scale of the projects you are currently writing, there are probably not that many problems (or there are, but you just haven't become aware of them yet).

I think the fundamental question you really need to have answered is: "Why would I ever want to hide data? If I do that, I can't work with it!" And this is why:

Let's say you start a new project, you open your text editor and you start writing functions. Every time you need to store something (to remember it for later), you create a variable. To make things simpler, you make your variables global. Your first version of your app runs great. Now you start adding more features. You have more functions, certain data you stored from before needs to be read from your new code. Other variables need to be modified. You keep writing more functions. What you may have noticed (or, if not, you absolutely will notice in the future) is, as your code gets bigger, it takes you longer and longer to add the next feature. And as your code gets bigger, it becomes harder and harder to add features without breaking something that used to work. Why? Because you need to remember what all your global variables are storing and you need to remember where all of them are being modified. And you need to remember which function is okay to call in what exact order and if you call them in a different order, you might get errors because your global variables aren't quite valid yet. Have you ever run into this?

How big are your typical projects (lines of code)? Now imaging a project 5000 to 50000 times as big as yours. Also, there are multiple people working in it. How can everyone on the team remember (or even be aware of) what all those variables are doing?

What I described above is an example of perfectly coupled code. And since the dawn of time (assuming time started Jan 1, 1970), human kind has been looking for ways to avoid these problems. The way you avoid them is by splitting up your code into systems, subsystems and components and limiting how many functions have access to any piece of data. If I have 5 integers and a string that represent some kind of state, would it be easier for me to work with this state if only 5 functions set/get the values? or if 100 functions set/get these same values? Even without OO languages (i.e. C), people have been working hard on isolating data from other data and creating clean separation boundaries between different parts of the code. When the project gets to a certain size, ease of programming becomes not, "can I access variable X from function Y", but "how do I make sure ONLY functions A, B, C and no one else is touching variable X".

This is why OO concepts have been introduced and this is why they are so powerful. They allow you to hide your data from yourself and you want to do it on purpose, because the less code that sees that data, the less chance there is, that when you add the next feature, you will break something. This is the main purpose for the concepts of encapsulation and OO programming. They allow you to break our systems/subsystems down into even more granular boxes, to a point where, no matter how big the overall project is, a given set of variables may only be accessed by 50-200 lines of code and that's it! There's obviously much more to OO programming, but, in essence, this is why C++ gives you options of declaring data/functions as private, protected or public.

The second greatest idea in OO is the concept of abstraction layers. Although procedural languages can also have abstractions, in C, a programmer must make a conscious effort to create such layers, but in C++, when you declare a class, you automatically create an abstraction layer (it's still up to you whether or not this abstraction will add or remove value). You should read/research more about abstraction layers and if you have more questions, I'm sure this forum will be more than happy to answer those as well.

Yes.

A situation where a non-virtual protected method makes sense might be something like bool IsInitialized(). The class is designed to be sub-classed. The sub-class might provide new methods, or overriding methods. In either case, before these methods can be run, it is important to check that the class is initialized. So the IsInitialized() method is provided for that purpose. It is intended to be used by the new or overriding methods in the sub-class only, and not the calling code. It is also intended that IsInitialized() not be overridden. The way to provide this is by declaring IsInitialized() to be protected and non-virtual.