Functions like `get_Fuel_Type()` can be added to the virtual class `Parts` by the derived class `Car`.
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Since initialization order corresponds to textual order of variable definitions, such a variable would not be initialized to the value prescribed by its initializer and would contain the default value. Another potential trap is when a method that is overridden in the derived class is called in the base class constructor, which can lead to behavior the programmer would not expect when an object of the derived class is created. According to the initialization order, the body of the base class constructor is executed before variable initializers are evaluated and before the body of the derived class constructor is executed. The overridden method called from the base class constructor can, however, reference variables defined in the derived class, but these are not yet initialized to the values specified by their initializers or set in the derived class constructor.
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The concept of the virtual function solves the following problem: In object- oriented programming, when a derived class inherits from a base class, an object of the derived class may be referred to via a pointer or reference of the base class type instead of the derived class type. If there are base class methods overridden by the derived class, the method actually called by such a reference or pointer can be bound either 'early' (by the compiler), according to the declared type of the pointer or reference, or 'late' (i.e., by the runtime system of the language), according to the actual type of the object referred to. Virtual functions are resolved 'late'.
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Classes can be derived from one or more existing classes, thereby establishing a hierarchical relationship between the derived-from classes (base classes, parent classes or ') and the derived class (child class or subclass) . The relationship of the derived class to the derived-from classes is commonly known as an is-a relationship. For example, a class 'Button' could be derived from a class 'Control'. Therefore, a Button is a Control.
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Implicit type conversion complicates function overloading because if the types of parameters do not exactly match the signature of one of the overloaded functions, but can match after type conversion, resolution depends on which type conversion is chosen. These can combine in confusing ways: An inexact match declared in an inner scope can mask an exact match declared in an outer scope, for instance. For example, to have a derived class with an overloaded function taking a or an , using the function taking an from the base class, in C++, one would write: class B { public: void F(int i); }; class D : public B { public: using B::F; void F(double d); }; Failing to include the results in an parameter passed to in the derived class being converted to a double and matching the function in the derived class, rather than in the base class; Including results in an overload in the derived class and thus matching the function in the base class.
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The member functions of such an abstract base class are normally explicitly defined in the derived class, not inherited implicitly. C++ virtual inheritance exhibits an ambiguity resolution feature called dominance.
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When we derive a class from this base class, we inherit all the member variables and member functions that weren't overridden (no constructors or destructors). If the derived class calls an inherited function which then calls another member function, that function will never call any derived or overridden member functions in the derived class. 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 VTBL machinery) at the disadvantage of not being able to make this choice at runtime.
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A member function can also be made "pure virtual" by appending it with = 0 after the closing parenthesis and before the semicolon. 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.
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A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class if the derived class is not abstract. Classes containing pure virtual methods are termed "abstract" and they cannot be instantiated directly. A subclass of an abstract class can only be instantiated directly if all inherited pure virtual methods have been implemented by that class or a parent class. Pure virtual methods typically have a declaration (signature) and no definition (implementation).
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Since multiple constructors will be allowed to execute, this will mean that each delegating constructor will be executing on a fully constructed object of its own type. Derived class constructors will execute after all delegation in their base classes is complete. For base-class constructors, C++11 allows a class to specify that base class constructors will be inherited. Thus, the C++11 compiler will generate code to perform the inheritance and the forwarding of the derived class to the base class.
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Disposal in the presence of inheritance and composition of objects that hold resources have analogous problems to destruction/finalization (via destructors or finalizers). Further, since the dispose pattern usually does not have language support for this, boilerplate code is necessary. Firstly, if a derived class overrides a `dispose` method in the base class, the overriding method in the derived class generally needs to call the `dispose` method in the base class, in order to properly release resources held in the base. Secondly, if an object has a "has a" relationship with another object that holds a resource (i.e.
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The enclosing class can be instantiated, rather a new derived class, which provides the definition of the method, would need to be created in order to create an instance of the class. Starting with Java 8, the lambda expression was included in the language, which could be viewed as a function declaration.
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There is no need (and no way) for the derived class to explicitly call the base destructor. The C# compiler compiles destructors to the appropriate CLR representation. For this version that probably means an instance finalizer that is distinguished in metadata. CLR may provide static finalizers in the future; we do not see any barrier to C# using static finalizers.
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SystemVerilog's polymorphism features are similar to those of C++: the programmer may specifically write a `virtual` function to have a derived class gain control of the function. See virtual function for further info. Encapsulation and data hiding is accomplished using the `local` and `protected` keywords, which must be applied to any item that is to be hidden. By default, all class properties are public.
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When the reference or pointer invoke the virtual inner classes, the derived class's implementation will be called if the object is of the derived class type. The type of the outer class determines the run time of the inner virtual class. A method with an object argument has access to the object's virtual classes. The method can use the virtual classes of its arguments to create instances and declare variables.
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Virtual classes solve the extensibility problem of extending data abstraction with new functions and representations. Like virtual functions, virtual classes follow the same rules of definition, overriding, and reference. When a derived class inherits from a base class, it must define or override the virtual inner classes it inherited from the base class. An object of the child class may be referred to by a reference or pointer of the parent class type or the child class type.
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More generally, the problem illustrates the difficulties which can occur when a base class contains methods which mutate an object in a manner which may invalidate a (stronger) invariant found in a derived class, causing the Liskov substitution principle to be violated. The existence of the circle–ellipse problem is sometimes used to criticize object-oriented programming. It may also imply that hierarchical taxonomies are difficult to make universal, implying that situational classification systems may be more practical.
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In a language which allows covariant return types, a derived class can override the method to return a more specific type: alt=UML diagram class CatShelter extends AnimalShelter { Cat getAnimalForAdoption() { return new Cat(); } } Among mainstream OO languages, Java and C++ support covariant return types, while C# does not. Adding the covariant return type was one of the first modifications of the C++ language approved by the standards committee in 1998. Scala and D also support covariant return types.
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Class members are public by default, and classes themselves are final by default, meaning that creating a derived class is disabled unless the base class is declared with the keyword. In addition to the classes and member functions (equivalent to methods) of object-oriented programming, Kotlin also supports procedural programming with the use of functions. Kotlin functions (and constructors) support default arguments, variable-length argument lists, named arguments and overloading by unique signature. Class member functions are virtual, i.e.
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The UML graphical representation of a Generalization is a hollow triangle shape on the superclass end of the line (or tree of lines) that connects it to one or more subtypes. The generalization relationship is also known as the inheritance or "is a" relationship. The superclass (base class) in the generalization relationship is also known as the "parent", superclass, base class, or base type. The subtype in the specialization relationship is also known as the "child", subclass, derived class, derived type, inheriting class, or inheriting type.
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Ordinarily, when a function in a derived class overrides a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference in the number or type of parameters between two or more definitions of that function. Hence, at compile time, it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is called dynamic dispatch.
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In some cases, when a subclass introduces a method with the same name and signature as a method already present in the base class, problems can occur. In Java, this will mean that the method in the derived class will implicitly override the method in the base class, even though that may not be the intent of the designers of either class. To mitigate this, C# requires that if a method is intended to override an inherited method, the `override` keyword must be specified. Otherwise, the method will "hide" the inherited method.
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If the keyword is absent, compiler warning to this effect is issued, which can be silenced by specifying the `new` keyword. This avoids the problem that can arise from a base class being extended with a non-private method (i.e. an inherited part of the namespace) whose signature is already in use by a derived class. Java has a similar compiler check in the form of the `@Override` method annotation, but it is not compulsory, and in its absence, most compilers will not provide comment (but the method will be overridden).
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In class-based programming, downcasting or type refinement is the act of casting a reference of a base class to one of its derived classes. In many programming languages, it is possible to check through type introspection to determine whether the type of the referenced object is indeed the one being cast to or a derived type of it, and thus issue an error if it is not the case. In other words, when a variable of the base class (parent class) has a value of the derived class (child class), downcasting is possible.
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The appearance of this anti-pattern in programs is usually because few programming languages provide a feature to contractually ensure that a super method is called from a derived class. One language that does have this feature, in a quite radical fashion, is BETA. The feature is found in a limited way in for instance Java and C++, where a child class constructor always calls the parent class constructor. Languages that support before and after methods, such as Common Lisp (specifically the Common Lisp Object System), provide a different way to avoid this anti-pattern.
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As a rule of thumb, if any function in the class is virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extension copy constructors, cannot be virtual. Nonetheless a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case, a common solution is to create a clone() (or similar) virtual function that creates and returns a copy of the derived class when called.
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The technique was formalized in 1989 as "F-bounded quantification." The name "CRTP" was independently coined by Jim Coplien in 1995, who had observed it in some of the earliest C++ template code as well as in code examples that Timothy Budd created in his multiparadigm language Leda. It is sometimes called "Upside- Down Inheritance" due to the way it allows class hierarchies to be extended by substituting different base classes. The Microsoft Implementation of CRTP in Active Template Library (ATL) was independently discovered, also in 1995, by Jan Falkin, who accidentally derived a base class from a derived class.
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Java methods are virtual by default (although they can be sealed by using the `final` modifier to disallow overriding). There is no way to let derived classes define a new, unrelated method with the same name. This means that by default in Java, and only when explicitly enabled in C#, new methods may be defined in a derived class with the same name and signature as those in its base class. When the method is called on a superclass reference of such an object, the "deepest" overridden implementation of the base class' method will be called according to the specific subclass of the object being referenced.
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In C++03, constructors of a class are not allowed to call other constructors in an initializer list of that class. Each constructor must construct all of its class members itself or call a common member function, as follows: class SomeType { public: SomeType(int new_number) { Construct(new_number); } SomeType() { Construct(42); } private: void Construct(int new_number) { number = new_number; } int number; }; Constructors for base classes cannot be directly exposed to derived classes; each derived class must implement constructors even if a base class constructor would be appropriate. Non-constant data members of classes cannot be initialized at the site of the declaration of those members. They can be initialized only in a constructor.
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Similar to C#, Kotlin allows a user to add functions to any class without the formalities of creating a derived class with new functions. Instead, Kotlin adds the concept of an extension function which allows a function to be "glued" onto the public function list of any class without being formally placed inside of the class. In other words, an extension function is a helper function that has access to all the public interface of a class which it can use to create a new function interface to a target class and this function will appear exactly like a function of the class, appearing as part of code completion inspection of class functions. For example: package MyStringExtensions fun String.
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As an example, an abstract base class `MathSymbol` may provide a pure virtual function `doOperation()`, and derived classes `Plus` and `Minus` implement `doOperation()` to provide concrete implementations. Implementing `doOperation()` would not make sense in the `MathSymbol` class, as `MathSymbol` is an abstract concept whose behaviour is defined solely for each given kind (subclass) of `MathSymbol`. Similarly, a given subclass of `MathSymbol` would not be complete without an implementation of `doOperation()`. Although pure virtual methods typically have no implementation in the class that declares them, pure virtual methods in C++ are permitted to contain an implementation in their declaring class, providing fallback or default behaviour that a derived class can delegate to, if appropriate.
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In programming, situations arise where it is necessary to add functionality to an existing class—for instance by adding a new method. Normally the programmer would modify the existing class's source code, but this forces the programmer to recompile all binaries with these new changes and requires that the programmer be able to modify the class, which is not always possible, for example when using classes from a third-party assembly. This is typically worked around in one of three ways, all of which are somewhat limited and unintuitive : # Inherit the class and then implement the functionality in an instance method in the derived class. # Implement the functionality in a static method added to a helper class.
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This is an all-or-nothing feature: either all of that base class's constructors are forwarded or none of them are. Also, an inherited constructor will be shadowed if it matches the signature of a constructor of the derived class, and restrictions exist for multiple inheritance: class constructors cannot be inherited from two classes that use constructors with the same signature. The syntax is as follows: class BaseClass { public: BaseClass(int value); }; class DerivedClass : public BaseClass { public: using BaseClass::BaseClass; }; For member initialization, C++11 allows this syntax: class SomeClass { public: SomeClass() {} explicit SomeClass(int new_value) : value(new_value) {} private: int value = 5; }; Any constructor of the class will initialize `value` with 5, if the constructor does not override the initialization with its own. So the above empty constructor will initialize `value` as the class definition states, but the constructor that takes an int will initialize it to the given parameter.
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Typically, the base class template will take advantage of the fact that member function bodies (definitions) are not instantiated until long after their declarations, and will use members of the derived class within its own member functions, via the use of a cast; e.g.: template struct Base { void interface() { // ... static_cast(this)->implementation(); // ... } static void static_func() { // ... T::static_sub_func(); // ... } }; struct Derived : Base { void implementation(); static void static_sub_func(); }; In the above example, note in particular that the function Base::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 example), so that at the time the function "implementation" is instantiated, the declaration of Derived::implementation() is known. This technique achieves a similar effect to the use of virtual functions, without the costs (and some flexibility) of dynamic polymorphism.
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We can re-declare our classes as follows: struct Animal { virtual ~Animal() = default; virtual void Eat() {} }; // Two classes virtually inheriting Animal: struct Mammal: virtual Animal { virtual void Breathe() {} }; struct WingedAnimal: virtual Animal { virtual void Flap() {} }; // A bat is still a winged mammal struct Bat: Mammal, WingedAnimal {}; The `Animal` portion of `Bat::WingedAnimal` is now the same `Animal` instance as the one used by `Bat::Mammal`, which is to say that a `Bat` has only one, shared, `Animal` instance in its representation and so a call to `Bat::Eat` is unambiguous. Additionally, a direct cast from `Bat` to `Animal` is also unambiguous, now that there exists only one `Animal` instance which `Bat` could be converted to. The ability to share a single instance of the `Animal` parent between `Mammal` and `WingedAnimal` is enabled by recording the memory offset between the `Mammal` or `WingedAnimal` members and those of the base `Animal` within the derived class. However this offset can in the general case only be known at runtime, thus `Bat` must become (`vpointer`, `Mammal`, `vpointer`, `WingedAnimal`, `Bat`, `Animal`).
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