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In C++, a coder doesn't know whether other coders will inherit his class. Should he make every function in that class virtual? Are there any drawbacks? Or is it just not acceptable at all?
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If you don't use virtual functions, you don't understand OOP yet. Because the virtual function is intimately bound with the concept of type, and type is at the core of object-oriented programming, there is no analog to the virtual function in a traditional procedural language.
Base classes can't inherit what the child has (such as a new function or variable). Virtual functions are simply functions that can be overridden by the child class if the that child class changes the implementation of the virtual function so that the base virtual function isn't called. A is the base class for B,C,D.
It is not mandatory for the derived class to override (or re-define the virtual function), in that case, the base class version of the function is used.
A virtual function allows derived classes to replace the implementation provided by the base class. The compiler makes sure the replacement is always called whenever the object in question is actually of the derived class, even if the object is accessed by a base pointer rather than a derived pointer.
In C++, you should only make a class inheritable from if you intend for it to be used polymorphically. The way that you treat polymorphic objects in C++ is very different from how you treat other objects. You don't tend to put polymorphic classes on the stack, or pass them by or return them from functions by value, since this can lead to slicing. Polymorphic objects tend to be heap-allocated, be passed around and returns by pointer or by reference, etc.
If you design a class to not be inherited from and then inherit from it, you cause all sorts of problems. If the destructor isn't marked virtual, you can't delete the object through a base class pointer without causing undefined behavior. Without the member functions marked virtual, they can't be overridden in a derived class.
As a general rule in C++, when you design the class, determine whether you want it be inherited from. If you do, mark the appropriate functions virtual and give it a virtual destructor. You might also disable the copy assignment operator to avoid slicing. Similarly, if you want the class not to be inheritable, don't give it any of these functions. In most cases it's a logic error to inherit from a class that wasn't designed to be inherited from, and most of the times you'd want to do this you can often use composition instead of inheritance to achieve this effect.
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