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    <p>If I understand your goal, there is a very simple way to accomplish this. Consider this alternative:</p> <pre><code>package Pkg partial class Equations Real x; end Equations; class DefaultEquations extends Equations(x=0); end DefaultEquations; class Base replaceable class T = DefaultEquations; replaceable T equations extends Equations; end Base; end Pkg; model TestEquations import Pkg.*; class CustomEquation extends Equations(x=3); end CustomEquation; class CustomizedClass extends Base(redeclare class T = CustomEquation); end CustomizedClass; CustomizedClass customized; end TestEquations; </code></pre> <p>But you can simplify it ever further if you are willing to eliminate all the redeclarations:</p> <pre><code>package Pkg class Equations Real x; end Equations; class Base Equations equations; end Base; end Pkg; model TestEquations import Pkg.*; class CustomizedClass extends Base(equations(x=3)); end CustomizedClass; CustomizedClass customized; end TestEquations; </code></pre> <p>...and yet further if you don't need all the layers of inheritance:</p> <pre><code>package Pkg class Equations Real x; end Equations; end Pkg; model TestEquations import Pkg.*; Equations equations(x=3); end TestEquations; </code></pre> <p>These modifications (the <code>x=3</code> parts) work the same for variables as they do for parameters. So you can take the "initialization equation" for any variable (the part that appears after the variable declaration) and override. The limitation of this approach is that it doesn't let you override general equations, just equations that solve for a single variable explicitly.</p> <p><strong>Update (May 2nd, 2012)</strong></p> <p>If you need to handle general equations (including the possibility to introduce internal state), then you also have to deal with the issue of "balancing" the equations. That is to say that Modelica (v 3+) includes certain semantics on what is considered a partial (non-instantiable) model vs. a non-partial model. The point is, you need to give the compiler enough information about what will be computed internally and what will be provided externally in order for it to figure out if the definition is really partial or not.</p> <p>In general, you should define a partial model/block to describe the potential external interactions and then use replaceable components to substitute the equations in. Based on your original example, it might look something like this:</p> <pre><code>package Pkg partial block Equations input Real y; // Input for any information it will need in the calculation output Real x; // Output to indicate something the model will compute end Equations; block DefaultEquations extends Equations; equation x = 0; end DefaultEquations; class Base replaceable DefaultEquations equations constrainedby Equations; end Base; end Pkg; model TestEquations import Pkg.*; block CustomEquation extends Equations equation x*x + y*y = 25.0; // example of non explicit equation end CustomEquation; class CustomizedClass extends Base(redeclare CustomEquation equations); end CustomizedClass; CustomizedClass customized; end TestEquations; </code></pre> <p>I should add that (at least in your example above), you could completely eliminate <code>Base</code> and <code>CustomizedClass</code>. But I left them in because I assume they somehow map to your actual problem. Finally, I never use <code>class</code> in actual code (I try to be more specific) so I changed some of your definitions to <code>block</code> definitions.</p>
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    1. POHow to override default equations?
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    1. USMichael Tiller
    1. USMichael Tiller
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    1. POHow to override default equations?
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    1. COApparently my example was too simple, hence misleading. The default equations are always of type x=0. However, the user usually wants to override them with very complicated equations involving x. Not only one equation has to be overridden, as in the example, but a system of equations. How would this change your answer? Either way, I upvoted your answer.
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    2. COI was wondering why I did not get an answer... Apperantly I have missed your update, very sorry for that. Thanks for your answer!
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