Cross Ref > OOP Details

General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

C++:   =0 in a virtual method AbstractMemberFunction is a pure virtual function makes this class Abstract class indicated by the "=0"and NonAbstractMemberFunction1 is a virtual function. Syntax Example: class AbstractClass { public: virtual void AbstractMemberFunction() = 0; virtual void NonAbstractMemberFunction1(); };  C++ Abstraction (=0 in a virtual method)

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A. In Dephi, class helpers allow you to extend a class without using inheritance. With a class helper, you do not have to create and use a new class descending from a class but instead you enhance the class directly and continue using it as you always have (even just with the DCU).

B. In general terms, developers sometimes use the term to refer to any class that helps out another class.

C++:  "Class Helpers" Not Supported However, developers sometimes use the term "class helper" to refer to code that helps out a class. Not truly the meaning we are using here, but you should be aware of the term's general usage.

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General Info: Class Constructor

Constructors are called when you instantiate an object from a class. This is where you can initialize variables and put code you wish executed each time the class is created. When you initially set the member fields and properties of an object, you are initializing the state of the object. The state of an object is the values of all it's member fields and properties at a given time.

Languages Focus

What is the syntax? Can you overload constructors? Is a special method name reserved for constructors?

C++:  "Constructors" Use Class name A member function with the same name as the class. Syntax Example:   class X { public: X(); // constructor for class X };

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General Info: Class Destructor

A special class method called when an object instance of a class is destroyed. With some languages they are called when the object instance goes out of scope, with some languages you specifically have to call the destructor in code to destroy the object, and others use a garbage collector to dispose of object instances at specific times.

Desctructors are commonly used to free the object instance but with languages that have a garbage collector object instances are disposed of when appropriate. Either way, destructors or their equivalent are commonly used to free up resources allocated in the class constructor.

Languages Focus

Are object instances freed with a garbage collector? Or, do you have to destroy object instances.

C++:   ~ClassName A member function with the same name as the class prefixed with a ~ (tilde). C++ destructors are automatically called when an object goes out of scope, or when you delete a dynamically allocated object. Every class can have only one destructor. Syntax Example: class Cyborg { public: // Constructor for class Cyborg Cyborg(); // Destructor for class Cyborg ~Cyborg(); };

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An element of coding where you define a common set of properties and methods for use with the design of two or more classes.

Both interfaces and abstract classes are types of abstraction. With interfaces, like abstract classes, you cannot provide any implementation. However, unlike abstract classes, interfaces are not based on inheritance. You can apply an Interface to any class in your class tree. In a real sense, interfaces are a technique for designing horizontally in a class hierarchy (as opposed to inheritance where you design vertically). Using interfaces in your class design allows your system to evolve without breaking existing code.

C++:  "Interfaces" No, but mimic it. You can mimic an interface by using a class that has only pure-virtual functions and no member variables.  C++ Interfaces (No, but mimic it.)

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A partial class, or partial type, is a class that can be split into two or more source code files and/or two or more locations within the same source file. Each partial class is known as a class part or just a part. Logically, partial classes do not make any difference to the compiler. The compiler puts the class together at compile time and treats the final class or type as a single entity exactly the same as if all the source code was in a single location.

Languages Focus

For languages that have implemented partial classes, you need to know usage details and restrictions. Can you split a class into two or more files? Can you split a class within a source code file into two or more locations? What are the details of inheritance? Does it apply to interfaces as well?

C++:  "Partial Classes" Not Supported In C++, you can split the implementation of a class among two or more source files. However, you must declare the member in the class declaration.

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Languages Focus

How do you prevent another class from inheriting and/or prevent a class from overriding a member.

C++:  Make the constructor a private member of the class. More Info / Comment

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General Info: Static Class / Static Member

A static member is a member you can have access to without instantiating the class into an object. For example, you can read and write static properties and call static methods without ever creating the class. Static members are also called class members (class methods, class properties, etc.) since they belong to the class and not to a specific object. A static class is a class that contains only static members. In the UML, these classes are described as utility classes.

Languages Focus

Languages that support static members usually at least support static member fields (the data). Some languages also support static methods, properties, etc. in which case the class member is held in memory at one location and shared with all objects. Finally, some languages support static classes which usually means the compiler will make sure a static class contains only static members.

C++:  "Static Members" static C++ supports static methods and static member fields. Syntax Example: class MyUtils { public: static void MyStaticMethod(); };  C++ Static Members (static)

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