Should I follow this advice?

How I Learned to Stop Worrying and Make Everything virtual
There is one small twist to overriding methods in C++ and it has to do with the keyword virtual.
Only methods that are declared as virtual in the superclass can be overridden properly by
subclasses. The keyword goes at the beginning of a method declaration as shown in the modifi ed
version of Super that follows:
class Super
virtual void someMethod();
int mProtectedInt;
int mPrivateInt;
The virtual keyword has a few subtleties and is often cited as a poorly designed part of the
language. A good rule of thumb is to just make all of your methods virtual. That way, you won’t
have to worry about whether or not overriding the method will work. The only drawback is a very
tiny performance hit. The subtleties of the virtual keyword are covered toward the end of this
chapter, and performance is discussed further in Chapter 24.
Even though it is unlikely that the Sub class will be extended, it is a good idea to make its methods
virtual as well, just in case.
class Sub : public Super
virtual void someOtherMethod();

I didn't write this. It is from Professional C++ 2nd Edition.
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There are very many very bad books about C++. That sounds like one of them.
I don't think the book is bad. I love the way they teach you the syntax of the STL.

It just gives some advice that sounds a bit off sometimes. What do you think about this:

Even with the preceding measures and the best design principles, the C++ language is fundamentally
unfriendly to the principle of abstraction. The syntax requires you to combine your public interfaces and private (or protected) data members and methods together in one class definition,
thereby exposing some of the internal implementation details of the class to its clients. The
downside of this is that if you have to add new non-public methods or data members to your class,
all the clients of the class have to be recompiled. This can become a burden in bigger projects.
The good news is that you can make your interfaces a lot cleaner and hide all implementation details,
resulting in stable interfaces. The bad news is that it takes a bit of hacking. The basic principle is to
define two classes for every class you want to write: the interface class and the implementation class.
The implementation class is identical to the class you would have written if you were not taking
this approach. The interface class presents public methods identical to those of the implementation
class, but it only has one data member: a pointer to an implementation class object. The interface
class method implementations simply call the equivalent methods on the implementation class object.
The result of this is that no matter how the implementation changes, it has no impact on the public
interface class. This reduces the need for recompilation. None of the clients that use the interface
class need to be recompiled if the implementation (and only the implementation) changes.
To use this approach with the Spreadsheet class, simply rename the old Spreadsheet class
to SpreadsheetImpl. Here is the new SpreadsheetImpl class (which is identical to the old
Spreadsheet class, but with a different name):
I just found a review for the book:

This book has too many 5 star ratings, which is way too overrated, so I decided to write this review.

- Contains a lot of information on various aspects of C++ programming, which can be used by people of various levels.
- Provides a more practical approach to teaching and tries to show how real programmers use the language.
- Uses C+11 and provides modern approaches to programming in C++.

- Lacks focus. The first few chapters are for beginners, while later chapters are focused for more experienced programmers.
- Teaches some bad programming habits. For example, the virtual keyword should not be used before everything as the authors recommend.

This book is not bad. The title is misleading because it sounds like it is for more experienced programmers. However, the problem is that
this book is not for beginners either, although it tries to teach basic syntax in the first few chapters. Which puts it in a position where
you probably will learn from this book, but you will either not be able to grasp all the concepts or have to skip chapters because you already
understand the information. It would have been a much better book if it was more focused, rather than trying to accommodate for all levels.
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