Object-oriented design of complex systems is hard

• Several OO class hierarchies:

  • Geo-features in repository (OODBMS)
  • Application copies of geo-features in memory
  • Visible representations of geo-features on screen
  • Operating system & GUI frameworks, e.g. MFC, AWT
  • Language toolkits, e.g. STL, ATL, Java library, JavaBeans

• Two event architectures:

  • WIMP events, programmed events
  • OO GIS reflex methods

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3 solutions: 1. OO Methodology, 2. Meyer 'design by contract', 3. Design patterns.

OO GIS rarely/never provide a seamless interface to persistent programming (as is the aim of generic OODBMSs such as POET or ObjectStore) for good reasons.

STL: C++ Standard Template Library, ATL: Microsoft Active Template Library makes it easier for Visual C++ programmers to write ActiveX components. Java comes with a class library and AWT (abstract windowing kit) + JavaBeans components

A further complication arise because Windows applications in

Visual C++ are constructed by Microsoft Wizards which provide a

great deal of default complexity. The benefit of this complexity

is standardisation: all Visual C++ applications of a particular

type are structured in the same way: this greatly aids

maintenance and extensibility, but makes for a steep learning

curve at the beginning.

A very good principle is to design class hierarchies as if you are going to do a series of related GIS projects.

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