Encyclopedia of Computer Science and Technology

88 classclassA class is a data type that combines both a data structureand methods for manipulating the data. For example, astring class might consist of an array to hold the charactersin the string and methods to compare strings, combinestrings, or extract portions of a string (see charactersand strings).As with other data types, once a class is declared,objects (sometimes called instances) of the class can becreated and used. This way of structuring programs iscalled object-oriented programming because the classobject is the basic building block (see object-orientedprogramming).Object-oriented programming and classes provide severaladvantages over traditional block-structured languages.In a traditional BASIC or even Pascal program, there isno particular connection between the data structure andthe procedures or functions that manipulate it. In a largeprogram one programmer might change the data structurewithout alerting other programmers whose code assumesthe original structure. On the other hand, someone mightwrite a procedure that directly manipulates the internaldata rather than using the methods already provided. Eithertransgression can lead to hard-to-find bugs.With a class, however, data and procedures are boundtogether, or encapsulated. This means that the data in aclass object can be manipulated only by using one of themethods provided by the class. If the person in chargeof maintaining the class decides to provide an improvedimplementation of the data structure, as long as the dataparameters expected by the class methods do not change,code that uses the class objects will continue to functionproperly.A class encapsulates (or hides) its internal information from therest of the program. When the program calls MyCircle.GetPosition,the GetPosition member function of the MyCircle Circle class objectretrieves the private Position data and sends it back to the callingstatement, where it is assigned to the variable P. Private data cannotbe directly accessed or changed by an outside caller.Most languages that use classes also allow for inheritance,or the ability to create a new class that derives dataand methods from a “parent” class and then modifies orextends them. For example, a class that provides supportfor 3D graphics could be derived from an existing class for2D graphics by adding data items such as a third (Z) coordinateand replacing a method such as “line” with a versionthat works with three coordinates instead of two.In designing classes, it is important to identify theessential features of the physical situation you are trying tomodel. The most general characteristics can be put in the“base class” and the more specialized characteristics wouldbe added in the inherited (derived) classes.Classes and C++Classes first appeared in the Simula 67 language, whichintroduced the terms class and object (see Simula). As thename suggests, the language was used mainly for simulationand modeling, but its object-oriented ideas wouldprove influential. The Smalltalk language developed atXerox PARC in the 1970s ran on the Alto computer, whichpioneered the graphic user interface that would becomepopular with the Macintosh in the 1980s. Smalltalk usedclasses to build a seamless and extensible operating systemand environment (see Smalltalk).However it was Bjarne Stroustrup’s C++ language thatbrought classes into the programming mainstream (seec++). C++ essentially builds its classes by extending theC struct so that it contains both methods (class functions)and data. An access mechanism allows class variables to bedesignated as completely accessible (public), which is rare,accessible only by derived classes (protected), or accessibleonly within the class itself (private). The creation of a newobject of the class is specified by a constructor function,which typically allocates memory for the object and setsinitial default values. The corresponding destructor functionfrees up the memory when the object no longer exists.C++ allows for multiple inheritance, meaning that a classcan be derived from more than one parent or base class.The language also provides two powerful mechanisms forextending functionality. The first, called virtual functions,allows a base class and its derived classes to have functionsbased on the same interface. For example, a base graphicsclass might have virtual line, circle, setcolor, and otherfunctions that would be implemented in derived classes for3D objects, 3D solid objects, and so on. When the programcalls a method in a virtual class, the compiler automaticallysearches the class’s “family tree” until it finds the class thatcorresponds to the actual data type of the object.A template specifies how to create a class definitionbased on the type of data to be used by the class. In otherwords, where a regular procedure takes and manipulatesdata parameters and returns data, a template takes dataparameters and returns a definition of a class for workingwith that data (see template).Other languages of the 1980s and later have embracedclasses. Examples include descendants of the Algol familyof languages (see Pascal, Ada, c++’s close cousin—Java),and Microsoft’s Visual Basic. (There is even a version ofCOBOL with classes.)