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62-457: Simula 67 introduced objects , classes , inheritance and subclasses , virtual procedures , coroutines , and discrete event simulation , and featured garbage collection . Other forms of subtyping (besides inheriting subclasses) were introduced in Simula derivatives. Simula is considered the first object-oriented programming language . As its name suggests, the first Simula version by 1962

124-623: A UNIVAC 1107 in August 1963 at a considerable discount, on which Dahl implemented the SIMULA I under contract with UNIVAC. The implementation was based on the UNIVAC ALGOL 60 compiler. SIMULA I was fully operational on the UNIVAC 1107 by January 1965. In the following few years, Dahl and Nygaard spent a lot of time teaching Simula. Simula spread to several countries around the world and SIMULA I

186-409: A class defines the shared aspects of objects created from the class. The capabilities of a class differ between programming languages , but generally the shared aspects consist of state ( variables ) and behavior ( methods ) that are each either associated with a particular object or with all objects of that class. Object state can differ between each instance of the class whereas the class state

248-447: A destructor . An object expresses data type as an interface – the type of each member variable and the signature of each member function (method). A class defines an implementation of an interface, and instantiating the class results in an object that exposes the implementation via the interface. In the terms of type theory, a class is an implementation‍—‌a concrete data structure and collection of subroutines‍—‌while

310-457: A simulation package for doing discrete event simulations . This simulation package is based on Simula's object-oriented features and its coroutine concept. Sam, Sally, and Andy are shopping for clothes. They must share one fitting room. Each one of them is browsing the store for about 12 minutes and then uses the fitting room exclusively for about three minutes, each following a normal distribution. A simulation of their fitting room experience

372-603: A UITableView is a UIScrollView is a UIView is a UIResponder is an NSObject. In object-oriented analysis and in Unified Modelling Language (UML), an association between two classes represents a collaboration between the classes or their corresponding instances. Associations have direction; for example, a bi-directional association between two classes indicates that both of the classes are aware of their relationship. Associations may be labeled according to their name or purpose. An association role

434-445: A button, and all the buttons together compose the interface (other television sets that are the same model as yours would have the same interface). In its most common form, an interface is a specification of a group of related methods without any associated implementation of the methods. A television set also has a myriad of attributes , such as size and whether it supports color, which together comprise its structure. A class represents

496-405: A class from its interface: the internal structure is made private, while public accessor methods can be used to inspect or alter such private data. Access specifiers do not necessarily control visibility , in that even private members may be visible to client external code. In some languages, an inaccessible but visible member may be referred to at runtime (for example, by a pointer returned from

558-422: A collection of objects, such as instances of Body , Engine , Tires , etc. Object modeling languages such as UML include capabilities to model various aspects of "part of" and other kinds of relations – data such as the cardinality of the objects, constraints on input and output values, etc. This information can be utilized by developer tools to generate additional code besides the basic data definitions for

620-501: A concrete sub class. An abstract class is either labeled as such explicitly or it may simply specify abstract methods (or virtual methods ). An abstract class may provide implementations of some methods, and may also specify virtual methods via signatures that are to be implemented by direct or indirect descendants of the abstract class. Before a class derived from an abstract class can be instantiated, all abstract methods of its parent classes must be implemented by some class in

682-504: A member function), but an attempt to use it by referring to the name of the member from the client code will be prevented by the type checker. The various object-oriented programming languages enforce member accessibility and visibility to various degrees, and depending on the language's type system and compilation policies, enforced at either compile time or runtime . For example, the Java language does not allow client code that accesses

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744-412: A need for a better way to describe the heterogeneity and the operation of a system. To go further with his ideas on a formal computer language for describing a system, Nygaard realized that he needed someone with more computer programming skills than he had. Ole-Johan Dahl joined him on his work January 1962. The decision of linking the language up to ALGOL 60 was made shortly after. By May 1962,

806-421: A superclass of Rectangle and Ellipse , while Square would be a subclass of Rectangle . These are all subset relations in set theory as well, i.e., all squares are rectangles but not all rectangles are squares. A common conceptual error is to mistake a part of relation with a subclass. For example, a car and truck are both kinds of vehicles and it would be appropriate to model them as subclasses of

868-601: A type is an interface . Different (concrete) classes can produce objects of the same (abstract) type (depending on type system). For example, the type (interface) Stack might be implemented by SmallStack that is fast for small stacks but scales poorly and ScalableStack that scales well but has high overhead for small stacks. A class contains data field descriptions (or properties , fields , data members , or attributes ). These are usually field types and names that will be associated with state variables at program run time; these state variables either belong to

930-491: A vehicle class. However, it would be an error to model the parts of the car as subclass relations. For example, a car is composed of an engine and body, but it would not be appropriate to model an engine or body as a subclass of a car. In object-oriented modeling these kinds of relations are typically modeled as object properties. In this example, the Car class would have a property called parts . parts would be typed to hold

992-460: A wide range of platforms. The TOPS-10 implemented the concept of public, protected, and private member variables and procedures, that later was integrated into Simula Standard in 1986. Simula Standard 1986 is the latest standard and is ported to a wide range of platforms. There are mainly four implementations: In November 2001, Dahl and Nygaard were awarded the IEEE John von Neumann Medal by

1054-407: Is a common set of access specifiers : Although many object-oriented languages support the above access specifiers,their semantics may differ. Object-oriented design uses the access specifiers in conjunction with careful design of public method implementations to enforce class invariants—constraints on the state of the objects. A common usage of access specifiers is to separate the internal data of

1116-417: Is also commonly known as a has-a relationship. For example, a class "Car" could be composed of and contain a class "Engine". Therefore, a Car has an Engine. One aspect of composition is containment, which is the enclosure of component instances by the instance that has them. If an enclosing object contains component instances by value, the components and their enclosing object have a similar lifetime . If

1178-427: Is as follows: The main block is prefixed with Simulation for enabling simulation. The simulation package can be used on any block and simulations can even be nested when simulating someone doing simulations. The fitting room object uses a queue ( door ) for getting access to the fitting room. When someone requests the fitting room and it's in use they must wait in this queue ( Wait (door) ). When someone leaves

1240-545: Is called a pure abstract base class (or pure ABC ) in C++ and is also known as an interface by users of the language. Other languages, notably Java and C#, support a variant of abstract classes called an interface via a keyword in the language. In these languages, multiple inheritance is not allowed, but a class can implement multiple interfaces. Such a class can only contain abstract publicly accessible methods. In some languages, classes can be declared in scopes other than

1302-405: Is given end of an association and describes the role of the corresponding class. For example, a "subscriber" role describes the way instances of the class "Person" participate in a "subscribes-to" association with the class "Magazine". Also, a "Magazine" has the "subscribed magazine" role in the same association. Association role multiplicity describes how many instances correspond to each instance of

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1364-411: Is not an intrinsic aspect of classes. An object-based language (i.e. Classic Visual Basic ) supports classes yet does not support inheritance. A programming language may support various class relationship features. Classes can be composed of other classes, thereby establishing a compositional relationship between the enclosing class and its embedded classes. Compositional relationship between classes

1426-486: Is often understated, and Simula-type objects are reimplemented in C++ , Object Pascal , Java , C# , and many other languages. Computer scientists such as Bjarne Stroustrup , creator of C++, and James Gosling , creator of Java, have acknowledged Simula as a major influence. The following account is based on Jan Rune Holmevik's historical essay. Kristen Nygaard started writing computer simulation programs in 1957. Nygaard saw

1488-414: Is shared by all of them. The object methods include access to the object state (via an implicit or explicit parameter that references the object) whereas class methods do not. If the language supports inheritance , a class can be defined based on another class with all of its state and behavior plus additional state and behavior that further specializes the class. The specialized class is a sub-class , and

1550-526: The Institute of Electrical and Electronics Engineers "For the introduction of the concepts underlying object-oriented programming through the design and implementation of SIMULA 67". In April 2002, they received the 2001 A. M. Turing Award by the Association for Computing Machinery (ACM), with the citation: "For ideas fundamental to the emergence of object oriented programming, through their design of

1612-560: The University of Oslo is named Ole Johan Dahl's House, in Dahl's honour, and the main auditorium is named Simula. The empty computer file is the minimal program in Simula, measured by the size of the source code . It consists of one thing only; a dummy statement . However, the minimal program is more conveniently represented as an empty block: It begins executing and immediately terminates. The language lacks any return value from

1674-539: The IFIP Working Conference on simulation languages in Oslo , May 1967. This paper became the first formal definition of Simula 67. In June 1967, a conference was held to standardize the language and initiate a number of implementations. Dahl proposed to unify the type and the class concept. This led to serious discussions, and the proposal was rejected by the board. Simula 67 was formally standardized on

1736-601: The Internet requires this level of flexibility and the technology standards such as the Web Ontology Language (OWL) are designed to support it. A similar issue is whether or not the class hierarchy can be modified at run time. Languages such as Flavors, CLOS, and Smalltalk all support this feature as part of their meta-object protocols . Since classes are themselves first-class objects, it is possible to have them dynamically alter their structure by sending them

1798-464: The Jensen's Device must therefore specify call by name for the parameters when compiled by a Simula compiler. Another much simpler example is the summation function ∑ {\displaystyle \sum } which can be implemented as follows: The above code uses call by name for the controlling variable (k) and the expression (u). This allows the controlling variable to be used in

1860-507: The appropriate messages. Other languages that focus more on strong typing such as Java and C++ do not allow the class hierarchy to be modified at run time. Semantic web objects have the capability for run time changes to classes. The rationale is similar to the justification for allowing multiple superclasses, that the Internet is so dynamic and flexible that dynamic changes to the hierarchy are required to manage this volatility. Although many class-based languages support inheritance, inheritance

1922-410: The class from outside the enclosing class. A related concept is inner types , also known as inner data type or nested type , which is a generalization of the concept of inner classes. C++ is an example of a language that supports both inner classes and inner types (via typedef declarations). A local class is a class defined within a procedure or function. Such structure limits references to

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1984-418: The class it is based on is its superclass . As an instance of a class, an object is constructed from a class via instantiation . Memory is allocated and initialized for the object state and a reference to the object is provided to consuming code. The object is usable until it is destroyed – its state memory is de-allocated. Most languages allow for custom logic at lifecycle events via a constructor and

2046-431: The class name to within the scope where the class is declared. Depending on the semantic rules of the language, there may be additional restrictions on local classes compared to non-local ones. One common restriction is to disallow local class methods to access local variables of the enclosing function. For example, in C++, a local class may refer to static variables declared within its enclosing function, but may not access

2108-465: The class or specific instances of the class. In most languages, the structure defined by the class determines the layout of the memory used by its instances. Other implementations are possible: for example, objects in Python use associative key-value containers. Some programming languages such as Eiffel support specification of invariants as part of the definition of the class, and enforce them through

2170-597: The classes that they are derived from. For example, if "class A" inherits from "class B" and if "class B" implements the interface "interface B" then "class A" also inherits the functionality(constants and methods declaration) provided by "interface B". In languages that support access specifiers , the interface of a class is considered to be the set of public members of the class, including both methods and attributes (via implicit getter and setter methods ); any private members or internal data structures are not intended to be depended on by external code and thus are not part of

2232-502: The components are contained by reference, they may not have a similar lifetime. For example, in Objective-C 2.0: This Car class has an instance of NSString (a string object), Engine , and NSArray (an array object). 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 superclasses ) and

2294-533: The concept of abstract classes , since classes with pure virtual procedures can be instantiated . This means that in the above example, all classes can be instantiated. Calling a pure virtual procedure will however produce a run-time error . Simula supports call by name so the Jensen's Device can easily be implemented. However, the default transmission mode for simple parameter is call by value , contrary to ALGOL which used call by name . The source code for

2356-474: The derivation chain. Most object-oriented programming languages allow the programmer to specify which classes are considered abstract and will not allow these to be instantiated. For example, in Java , C# and PHP , the keyword abstract is used. In C++ , an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). A class consisting of only pure virtual methods

2418-624: 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. Structural and behavioral members of the parent classes are inherited by the child class. Derived classes can define additional structural members (data fields) and behavioral members (methods) in addition to those that they inherit and are therefore specializations of their superclasses. Also, derived classes can override inherited methods if

2480-457: The expression. Note that the Simula standard allows for certain restrictions on the controlling variable in a for loop . The above code therefore uses a while loop for maximum portability. The following: Z = ∑ i = 1 100 1 ( i + a ) 2 {\displaystyle Z=\sum _{i=1}^{100}{1 \over (i+a)^{2}}} can then be implemented as follows: Simula includes

2542-584: The first meeting of the Simula Standards Group (SSG) in February 1968. Simula was influential in the development of Smalltalk and later object-oriented programming languages. It also helped inspire the actor model of concurrent computation although Simula only supports coroutines and not true concurrency . In the late sixties and the early seventies, there were four main implementations of Simula: These implementations were ported to

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2604-428: The fitting room the first one (if any) is released from the queue ( Activate door.first ) and accordingly removed from the door queue ( door.First.Out ). Person is a subclass of Process and its activity is described using hold (time for browsing the store and time spent in the fitting room) and calls procedures in the fitting room object for requesting and leaving the fitting room. The main program creates all

2666-431: The full description of a television, including its attributes (structure) and buttons (interface). Getting the total number of televisions manufactured could be a static method of the television class. This method is associated with the class, yet is outside the domain of each instance of the class. A static method that finds a particular instance out of the set of all television objects is another example. The following

2728-446: The global scope. There are various types of such classes. An inner class is a class defined within another class. The relationship between an inner class and its containing class can also be treated as another type of class association. An inner class is typically neither associated with instances of the enclosing class nor instantiated along with its enclosing class. Depending on the language, it may or may not be possible to refer to

2790-432: The interface. Object-oriented programming methodology dictates that the operations of any interface of a class are to be independent of each other. It results in a layered design where clients of an interface use the methods declared in the interface. An interface places no requirements for clients to invoke the operations of one interface in any particular order. This approach has the benefit that client code can assume that

2852-623: The language allows. Not all languages support multiple inheritance. For example, Java allows a class to implement multiple interfaces, but only inherit from one class. If multiple inheritance is allowed, the hierarchy is a directed acyclic graph (or DAG for short), otherwise it is a tree . The hierarchy has classes as nodes and inheritance relationships as links. Classes in the same level are more likely to be associated than classes in different levels. The levels of this hierarchy are called layers or levels of abstraction. Example (Simplified Objective-C 2.0 code, from iPhone SDK): In this example,

2914-442: The language also provides polymorphism and inheritance it is classified as object-oriented . A language that supports creating an object from a class is classified as class-based . A language that supports object creation via a template object is classified as prototype-based . The concept of object is used in many different software contexts, including: Class (computer programming) In object-oriented programming ,

2976-525: The main concepts for a simulation language were set. SIMULA I was born, a special purpose programming language for simulating discrete event systems. Kristen Nygaard was invited to visit the Eckert–Mauchly Computer Corporation late May 1962 in connection with the marketing of their new UNIVAC 1107 computer. At that visit, Nygaard presented the ideas of Simula to Robert Bemer , the director of systems programming at Univac . Bemer

3038-420: The mechanisms that provide some higher-level behavior. Put another way, an object represents an individual, identifiable item, unit, or entity, either real or abstract, with a well-defined role in the problem domain. A programming language can be classified based on its support for objects. A language that provides an encapsulation construct for state, behavior, and identity is classified as object-based . If

3100-667: The object-oriented community consider antithetical to the goals of using object classes in the first place. Understanding which class will be responsible for handling a message can get complex when dealing with more than one superclass. If used carelessly this feature can introduce some of the same system complexity and ambiguity classes were designed to avoid. Most modern object-oriented languages such as Smalltalk and Java require single inheritance at run time. For these languages, multiple inheritance may be useful for modeling but not for an implementation. However, semantic web application objects do have multiple superclasses. The volatility of

3162-458: The objects and activates all the person objects to put them into the event queue. The main program holds for 100 minutes of simulated time before the program terminates. Object (computer science) In software development , an object is an entity that has state , behavior , and identity . An object can model some part of reality or can be an invention of the design process whose collaborations with other such objects serve as

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3224-473: The objects, such as error checking on get and set methods . One important question when modeling and implementing a system of object classes is whether a class can have one or more superclasses. In the real world with actual sets, it would be rare to find sets that did not intersect with more than one other set. However, while some systems such as Flavors and CLOS provide a capability for more than one parent to do so at run time introduces complexity that many in

3286-405: The operations of an interface are available for use whenever the client has access to the object. The buttons on the front of your television set are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to toggle the television on and off. In this example, your particular television is the instance, each method is represented by

3348-507: The other class of the association. Common multiplicities are "0..1", "1..1", "1..*" and "0..*", where the "*" specifies any number of instances. There are many categories of classes, some of which overlap. In a language that supports inheritance, an abstract class , or abstract base class ( ABC ), is a class that cannot be directly instantiated. By contrast, a concrete class is a class that can be directly instantiated. Instantiation of an abstract class can occur only indirectly, via

3410-415: The private data of a class to compile. In the C++ language, private methods are visible, but not accessible in the interface; however, they may be made invisible by explicitly declaring fully abstract classes that represent the interfaces of the class. Some languages feature other accessibility schemes: Conceptually, a superclass is a superset of its subclasses. For example, GraphicObject could be

3472-465: The program. An example of a Hello world program in Simula: Simula is case-insensitive . A more realistic example with use of classes, subclasses and virtual procedures: The above example has one super class (Glyph) with two subclasses ( Char and Line ). There is one virtual procedure with two implementations . The execution starts by executing the main program. Simula lacks

3534-766: The programmer to define and call these special methods. Every class implements (or realizes ) an interface by providing structure and behavior. Structure consists of data and state, and behavior consists of code that specifies how methods are implemented. There is a distinction between the definition of an interface and the implementation of that interface; however, this line is blurred in many programming languages because class declarations both define and implement an interface. Some languages, however, provide features that separate interface and implementation. For example, an abstract class can define an interface without providing an implementation. Languages that support class inheritance also allow classes to inherit interfaces from

3596-680: The programming languages Simula I and Simula 67." Dahl and Nygaard died in June and August of that year, respectively, before the ACM Turing Award Lecture that was scheduled to be delivered at the November 2002 OOPSLA conference in Seattle. Simula Research Laboratory is a research institute named after the Simula language, and Nygaard held a part-time position there from the opening in 2001. The new Computer Science building at

3658-653: The type system. Encapsulation of state is necessary for being able to enforce the invariants of the class. The behavior of a class or its instances is defined using methods . Methods are subroutines with the ability to operate on objects or classes. These operations may alter the state of an object or simply provide ways of accessing it. Many kinds of methods exist, but support for them varies across languages. Some types of methods are created and called by programmer code, while other special methods—such as constructors, destructors, and conversion operators—are created and called by compiler-generated code. A language may also allow

3720-466: Was a great ALGOL fan and found the Simula project compelling. Bemer was also chairperson of a session at the second international conference on information processing hosted by International Federation for Information Processing (IFIP). He invited Nygaard, who presented the paper "SIMULA – An Extension of ALGOL to the Description of Discrete-Event Networks". The Norwegian Computing Center got

3782-490: Was designed for doing simulations ; Simula 67 though was designed to be a general-purpose programming language and provided the framework for many of the features of object-oriented languages today. Simula has been used in a wide range of applications such as simulating very-large-scale integration (VLSI) designs, process modeling , communication protocols , algorithms , and other applications such as typesetting , computer graphics , and education . The influence of Simula

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3844-529: Was later implemented on other computers including the Burroughs B5500 and the Russian Ural-16 . In 1966 C. A. R. Hoare introduced the concept of record class construct, which Dahl and Nygaard extended with the concept of prefixing and other features to meet their requirements for a generalized process concept. Dahl and Nygaard presented their paper on Class and Subclass declarations at

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