C++: Why does a struct\class need a virtual method in order to be polymorphic?

Question

Following this question, I\'m wondering why a struct\\class in C++ has to have a virtual method in order to be polymorphic.

Forcing a virtual destructor makes sense,

Answer 1

I'm wondering why does a struct\class in C++ has to have a virtual method in order to be polymorphic?

Because that is what polymorphic class means.

In C++, runtime polymorphism is achieved through virtual functions. A base class declares some virtual functions which the many derived classes implement, and the clients use pointers (or references) of static type of base class, and can make them point to objects of derived classes (often different derived classes), and then later on, call the implementation of derived classes through the base pointers. That is how runtime polymorphism is achieved. And since the central role is played by the functions being virtual, which enables runtime polymorphism, that is why classes having virtual functions is called polymorphic class.

Answer 2

Because it is defined as such in the standard.

From 10.3/1 [class.virtual]

Virtual functions support dynamic binding and object-oriented programming. A class that declares or inherits a virtual function is called a polymorphic class.

It makes sense that if you use inheritance, then you have at least one virtual method. If you don't have any virtual method, then you could use composition instead.

Answer 3

Classes only need virtual methods in order to be dynamically polymorphic - for the reasons described by others. You can still have static polymorphism through templates, though.

Answer 4

Forcing a virtual destructor makes sense

Exactly. To destruct a virtual class manually (via delete) through its base class you need a virtual destructor. (Now, as I’ve been reminded in the comments, this isn’t usually needed: rather than use manual memory management, one would rely on modern smart pointers which also work correctly with non-virtual destructors.)

So any class which acts as a polymorphic base class usually needs either a virtual destructor or virtual functions anyway.

And since having runtime polymorphism adds an overhead (the class needs to store an additional pointer to its virtual method table), the default is not to add it, unless necessary anyway: C++’ design philosophy is “you only pay for what you need”. Making every class have a virtual method table would run afoul of this principle.

Answer 5

polymorphism is to allow your subclasses to override the default behaviour of base class functions, so unless you have virtual methods in your base class, you can't override methods in base.

Answer 6

Because the type of a polymorphic object in C++ is, basically, determined from the pointer to its vtable, which is the table of virtual functions. The vtable is, however, only created if there's at least one virtual method. Why? Because in C++, you never get what you didn't explicitly ask for. They call it "you don't have to pay for something you don't need". Don't need polymorphism? You just saved a vtable.