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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.
For example, the following C++ code establishes an explicit inheritance relationship between classes B and A, where B is both a subclass and a subtype of A and can be used as an A wherever a B is specified (via a reference, a pointer or the object itself).
Virtual inheritance is a C++ technique that ensures only one copy of a base class ' s member variables are inherited by grandchild derived classes. Without virtual inheritance, if two classes B and C inherit from a class A , and a class D inherits from both B and C , then D will contain two copies of A ' s member variables: one via B , and one ...
The following C++ code establishes an explicit inheritance relationship between classes B and A, where B is both a subclass and a subtype of A, and can be used as an A wherever a B is specified (via a reference, a pointer or the object itself).
If virtual inheritance and nonvirtual inheritance are mixed, there is a single virtual A, and a nonvirtual A for each nonvirtual inheritance path to A. C++ requires stating explicitly which parent class the feature to be used is invoked from i.e. Worker::Human.Age. C++ does not support explicit repeated inheritance since there would be no way ...
The g++ compiler implements the multiple inheritance of the classes B1 and B2 in class D using two virtual method tables, one for each base class. (There are other ways to implement multiple inheritance, but this is the most common.) This leads to the necessity for "pointer fixups", also called thunks, when casting. Consider the following C++ code:
The doctrine of composition over inheritance advocates implementing has-a relationships using composition instead of inheritance. For example, instead of inheriting from class Person, class Employee could give each Employee object an internal Person object, which it then has the opportunity to hide from external code even if class Person has ...
In the above example, the function Base<Derived>::interface(), though declared before the existence of the struct Derived is known by the compiler (i.e., before Derived is declared), is not actually instantiated by the compiler until it is actually called by some later code which occurs after the declaration of Derived (not shown in the above ...