3. The Model Definition Language

The Model Definition Language (MDL) is an object-oriented language which has been designed to integrate seamlessly into the Algorithm Library. It supports the full set of conditional and loop expressions of the C++ language, class definitions with data and method encapsulation, multiple inheritance and polymorphic method calls. The major design goals for the implementation of MDL were:

• The syntax of MDL is compatible to C++ wherever the meaning of the statements is comparable. Therefore in many cases the statements of MDL methods can be interchanged with existing C or C++ functions in ``Cut and Paste'' manner. This is especially true for the syntax and precedence of all operators (Section 3.1.8), functions (Section 3.1.9), conditional, and loop expressions (Section 3.1.10).
• The developer of MDL models should be relieved from any responsibility for the memory management of the models. Therefore MDL has no built-in statements to allocate, free or access any kind of memory and does not provide derived parameter types like pointers or vectors of parameters. Whenever such facilities are necessary the simulator developer has to provide the appropriate MDL parameter types and operators. Good examples for such parameter types are container classes of the C++ Standard Template Library.
• For the definition of new MDL classes the inheritance of parameters and methods from MDL classes defined either in C++ model libraries, or in previously parsed MDL files is supported . Likewise it is possible to aggregate several MDL classes to enhance the functionality of the newly defined one. Within the MDL syntax no difference is visible between native C++ and interpreted MDL classes.
• A major design goal was to support an orthogonal set of MDL expressions, which can efficiently be executed by the MDL interpreter and compiled into an accordingly efficient C++ code, compatible with C++ compilers on all supported platforms. The compatibility requirements resulted in the exclusion of the C++ exception mechanism, while the most visible result of the orthogonality issue is the restriction of the MDL class method definition syntax in comparison with the C++ method definition possibilities.
• MDL methods take no argument list, and their return type is always MdlBool. The parameter exchange with MDL methods has to be done by using local, static, or interface parameters of the particular MDL class.
• The syntax of MDL expressions is independent of the underlying parameter types. The Algorithm Library comes with a minimal set of built-in parameter types and MDL functions. This set has to be supplemented by MDL functions and types according to the specific application in the particular simulator program to deploy the full strength of the MDL concept.