FAQ 27.21 Are there any hidden costs for type-safe downcasts?

Yes, type-safe downcasts have five hidden costs.

Although type-safe downcasts never cast a pointer to an incorrect type, they have five hidden costs. They increase coding cost, maintenance cost, testing cost, runtime CPU cost, and extensibility cost.

1. Coding cost: Type-safe downcasts move complexity from the server code into the user code, from the few to the many.

2. Maintenance cost: Moving code from the server code to the user code increases the overall software bulk.

3. Testing cost: A test harness must be devised to exercise every if, including the ifs used to test the type safety of the downcasts.

4. Runtime CPU cost: Additional code must be executed to test the type safety of the downcasts. This is not a constant time cost, by the way, since it may be necessary to search an entire inheritance hierarchy.

5. Extensibility cost: The additional control flow code needs to be modified when new derived classes are added.

The underlying cause for these costs lies with the style of programming implied by type-safe downcasts rather than with the downcasts themselves. Embracing the more extensible style of programming that does not use unnecessary downcasts is part of using C++ properly.