Modelica — A unified objectoriented language for system modeling and simulation
 Peter Fritzson,
 Vadim Engelson
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Abstract
A new language called Modelica for hierarchical physical modeling is developed through an international effort. Modelica 1.0 [http:// www.Dynasim.se/Modelica] was announced in September 1997. It is an objectoriented language for modeling of physical systems for the purpose of efficient simulation. The language unifies and generalizes previous objectoriented modeling languages. Compared with the widespread simulation languages available today this language offers three important advances: 1) noncausal modeling based on differential and algebraic equations; 2) multidomain modeling capability, i.e. it is possible to combine electrical, mechanical, thermodynamic, hydraulic etc. model components within the same application model; 3) a general type system that unifies objectorientation, multiple inheritance, and templates within a single class construct.
A class in Modelica may contain variables (i.e. instances of other classes), equations and local class definitions. A function (method) can be regarded as a special case of local class without equations, but including an algorithm section.
The equationbased noncausal modeling makes Modelica classes more reusable than classes in ordinary objectoriented languages. The reason is that the class adapts itself to the data flow context where it is instantiated and connected. The multidomain capability is partly based on a notion of connectors, i.e. certain class members that can act as interfaces (ports) when connecting instantiated objects. Connectors themselves are classes just like any other entity in Modelica. Simulation models can be developed using a graphical editor for connection diagrams. Connections are established just by drawing lines between objects picked from a class library.
The Modelica semantics is defined via translation of classes, instances and connections into a flat set of constants, variables and equations. Equations are sorted and converted to assignment statements when possible. Strongly connected sets of equations are solved by calling a symbolic and/or numeric solver. The generated C/C++ code is quite efficient.
In this paper we present the Modelica language with emphasis on its class construct and type system. A few short examples are given for illustration and compared with similar constructs in C++ and Java when this is relevant.
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 Title
 Modelica — A unified objectoriented language for system modeling and simulation
 Book Title
 ECOOP’98 — ObjectOriented Programming
 Book Subtitle
 12th European Conference Brussels, Belgium, July 20–24, 1998 Proceedings
 Pages
 pp 6790
 Copyright
 1998
 DOI
 10.1007/BFb0054087
 Print ISBN
 9783540647379
 Online ISBN
 9783540690641
 Series Title
 Lecture Notes in Computer Science
 Series Volume
 1445
 Series ISSN
 03029743
 Publisher
 Springer Berlin Heidelberg
 Copyright Holder
 SpringerVerlag
 Additional Links
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 Industry Sectors
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 Editors
 Authors

 Peter Fritzson ^{(1)}
 Vadim Engelson ^{(1)}
 Author Affiliations

 1. PELAB, Dept. of Computer and Information Science, Linköping University, S58183, Linköping, Sweden
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