Understanding data providers

While working on this chapter's examples, you might have noticed that we cannot even start an edit session on our GeoNames layer. Why do different vector data types act differently? The answer is simple and it can be found in the implementation details. Every GIS software has to decide at one point how to handle vector data. They can build some kind of internal structure, read to and write from this structure, and handle every kind of vector data consistently during the workflow. This is one popular option. GRASS GIS does exactly this, additionally materializing this internal structure to make other processes more consistent and efficient.

QGIS, on the other hand, strives for extensibility and modularity before consistency. It utilizes one of the greatest features of object-oriented programming, polymorphism, to achieve this goal. It has a template class called qgsVectorDataProvider on which different data providers can implement their format-specific functionality...