Does anyone know why the STL containers don't have virtual destructors?
As far as I can tell, the only benefits are:
- it reduces the size of an instance by one pointer (to the virtual method table) and
- it makes destruction and construction a tiny bit faster.
The downside is that it's unsafe to subclass the containers in the usual way.
EDIT: Perhaps my question could be rephrased "Why weren't STL containers designed to allow for inheritance?"
Because they don't support inheritance, one is stuck with the following choices when one wants to have a new container that needs the STL functionality plus a small number of additional features (say a specialized constructor or new accessors with default values for a map, or whatever):
- Composition and interface replication: Make a new template or class that owns the STL container as a private member and has one pass-through inline method for each STL method. This is just as performant as inheritance, avoids the cost of a virtual method table (in the cases where that matters). Unfortunately, the STL containers have fairly broad interfaces so this requires many lines of code for something that should seemingly be easy to do.
- Just make functions: Use bare (possibly templated) file-scoped functions instead of trying to add member functions. In some ways this can be a good approach, but the benefits of encapsulation are lost.
- Composition with public STL access: Have the owner of the STL container let users access the STL container itself (perhaps guarded through accessors). This requires the least coding for the library writer, but it's much less convenient for users. One of the big selling points for composition is that you reduce coupling in your code, but this solution fully couples the STL container with the owner container (because the owner returns a true STL container).
- Compile-time polymorphism: Can be somewhat tricky to do write, requires some code gymnastics, and isn't appropriate for all situations.
As a side question: is there a standards-safe way of subclassing with non-virtual destructors (let's assume that I don't want to override any methods, just that I want to add new ones)? My impression is that there is no generic and safe way of doing this if one does not have the power to change the code defining the non-virtual class.
EDIT 2:
As @doc points out, C++ 11's fancier using declarations lower the cost of composition somewhat.
