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[7]: §15.7.3 [8] [9] If that function is a pure virtual function, then undefined behavior occurs. [7]: §13.4.6 [8] This is true even if the class contains an implementation for that pure virtual function, since a call to a pure virtual function must be explicitly qualified. [10]
Therefore, calling virtual functions is inherently slower than calling non-virtual functions. An experiment done in 1996 indicates that approximately 6–13% of execution time is spent simply dispatching to the correct function, though the overhead can be as high as 50%. [ 5 ]
For example, in C++ a method is known as a member function. C++ also has the concept of virtual functions which are member functions that can be overridden in derived classes and allow for dynamic dispatch.
In inheritance hierarchies, the declaration of a virtual destructor in the base class ensures that the destructors of derived classes are invoked properly when an object is deleted through a pointer-to-base-class. Objects that may be deleted in this way need to inherit a virtual destructor. A destructor should never throw an exception. [7]
The C++ examples in this section demonstrate the principle of using composition and interfaces to achieve code reuse and polymorphism. Due to the C++ language not having a dedicated keyword to declare interfaces, the following C++ example uses inheritance from a pure abstract base class .
C++ destructors for local ... mimics the syntax for overriding virtual functions. Because C++ templates ... C++ code, any function declaration or ...
C++ does not have the keyword super that a subclass can use in Java to invoke the superclass version of a method that it wants to override. Instead, the name of the parent or base class is used followed by the scope resolution operator. For example, the following code presents two classes, the base class Rectangle, and the derived class Box.
To elaborate on the above example, consider a base class with no virtual functions. Whenever the base class calls another member function, it will always call its own base class functions. When we derive a class from this base class, we inherit all the member variables and member functions that were not overridden (no constructors or destructors).