8/13/2023 0 Comments Java define virtual method![]() Now let’s come to the interesting part: at runtime, how it determines which method to call using vtable? We will consider 6 cases of declaring and instantiating instances of A, B and C. This new entry C:GetName() points to its own implementation of GetName() while original entry A:GetName() points to the implementation that it inherits (from class B) The effect of that is in vtable of class C, beside the inherted A:GetName() from class B, it has its own entry for C:GetName(). The property of redefining a member function declared. But in addition, class C says it wants to have a new entry for GetName() by specifying modifier new in GetName() declaration. A virtual function is nothing but a member function of a base class that you redefine in a derived class. The modifier override in class B GetName() declaration will overwrite entry A:GetName() that it inherits to point to its own implementation.Ĭlass C inherits from class B, so it also inherts the entries from class B’s vtable, which is A:GetName(). We identify that by A:GetName().Ĭlass B inherits from class A, so it also inherits the entries in class A’s vtable, which is A:GetName(). In Java, Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes. What happened at compile time and runtime?Īt compile time, the structure of classes and their vtable might look like this:Ĭlass A has one virtual method, so it has one vtable that contains an entry for that method. B overrides method GetName() from A, C hides method GetName() from B.Let’s say we have 3 classes: A, B and C.To understand behaviour of new and override modifiers with vtable, it’s easier to use an example: vtable only contains entries for virtual methods declared in the class. In the diagram, a1 and a2 are 2 object instances of class A, both of them have a virtual method table pointer ( vptr) to point to the same vtable of class A. That means all object instances of the same class will share the same vtable. When created, every object instance of that class will have a pointer to vtable of the class (the pointer will always be at a fixed address from beginning of object address, so the runtime always knows where to find the vtable). ![]() Each class that has at least one virtual method will have its own vtable. ![]() Under the hood, C# uses virtual method table ( vtable, dispatch table) to determine at run time which method from which class in the hierarchy to call. With virtual modifier in base class, child class can use either new modifier or override modifier to hide or override the inherited method from base class. Different from Java, C# doesn’t enable method overriding by default, so the base class must use this modifier to allow base class method to be overridden. In C#, base class can use virtual modifier for a method to indicate that that method can be overridden by sub class. This is a fundamental concept in C# but I find it difficult to find explanation that really goes into details of how it’s implemented in C#, so I decided to write a post to document what I understand about this. ![]() What happened at compile time and runtime?. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |