3.11 Error Recovery

The error handling mechanisms of the Algorithm Library implies three different types of errors:

syntax errors in MDL files are resolved by the MDL parser which is implemented by using the Bison [13,30] LALR(1) parser generation tool. A verbose description of the error is written to an ANSI C++ stream provided by the application which defaults to the Posix stderr stream, including the line number range of the incorrect statements. Any syntax error recognized within an MDL statement invalidates the complete statement and the parsing process is continued right after the faulty definitions. In case errors occurred during the parsing process, the application is notified by the return status of the function call to the MDL parser. Subsequent parser evaluations are not affected by such errors despite the unavailability of the incorrect definitions.

initialization errors: MDL class instances are created dynamically at runtime of the application implying rigorous type checks in all involved MDL expressions. Since Parameter types and operators are added by user supplied MDL extension libraries, also the availability of operators in the involved MDL expressions is checked during the instantiation process. Similar to the procedure in case of syntax errors, initialization errors result in a verbose error message forwarded to an error stream and an invalid handle (NULL Pointer) to the requested MDL class.

runtime errors resulting from incorrect parameter values supplied to methods of an MDL object should be detected within the MDL definitions of the affected MDL classes and signaled to the application with a return false; statement. Since the Algorithm Library delegates the actual execution of MDL expressions consequently to binary instances of user supplied C++ classes and data types, remaining errors will result in either Posix signals or C++ exceptions if they are available for the underlying C++ compiler system. Both are ignored by the Algorithm Library and forwarded to the application.