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In C++, the "base" function is called. Specifically, the most derived function that is not more derived than the current constructor or destructor's class is called. [7]: §15.7.3 [8] [9] If that function is a pure virtual function, then undefined behavior occurs.
However, if base class member functions use CRTP for all member function calls, the overridden functions in the derived class will be selected at compile time. This effectively emulates the virtual function call system at compile time without the costs in size or function call overhead (VTBL structures, and method lookups, multiple-inheritance ...
Suppose a pure virtual method is defined in the base class. If a deriving class inherits the base class virtually, then the pure virtual method does not need to be defined in that deriving class. However, if the deriving class does not inherit the base class virtually, then all virtual methods must be defined.
By including, in the class definition, one or more abstract methods (called pure virtual functions in C++), which the class is declared to accept as part of its protocol, but for which no implementation is provided. By inheriting from an abstract type, and not overriding all missing features necessary to complete the class definition. In other ...
A class containing a pure virtual function is called an abstract class. Objects cannot be created from an abstract class; they can only be derived from. Any derived class inherits the virtual function as pure and must provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created.
[26] [27] In C++, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). [25] A class consisting of only pure virtual methods is called a pure abstract base class (or pure ABC) in C++ and is also known as an interface by users of the language. [13]
In object-oriented programming, a virtual base class is a nested inner class whose functions and member variables can be overridden and redefined by subclasses of an outer class. [1] Virtual classes are analogous to virtual functions. The run time type of a virtual class depends on the run time type of an object of the outer class. (Just like ...
When an object is created, a pointer to this table, called the virtual table pointer, vpointer or VPTR, is added as a hidden member of this object. As such, the compiler must also generate "hidden" code in the constructors of each class to initialize a new object's virtual table pointer to the address of its class's virtual method table.