Question

I was looking at a few sources for STL implementations (SGI, STLport, libc++) and saw a few design patterns that seemd common to all or most implementations, but I could find no reason for. I assume there must be a good reson, and want to know what it is:

1. Many classes, including vector and list_iterator among others, were implemented as 2 classes, e.g. list_iterator_base with part of the functionality, and then list_iterator which inherits list_iterator_base with the rest of the interface. What is the point? It seems it could be done jut as easily in one class.

2. The iterators seem to not make use of the iterator class. Is there some performance penalty to using it?

Those are 2 questions I found in just a quick skim. If anyone knows of a good resource explaining the implementation rationale of a STL implementation, I will be happy to hear of it.

Answer 1

The answers are fairly straight forward:

1. STL is all about generic programming. The key idea is not to have duplicate code. The immediate goal is to not have duplicate source code but as it turns out it also makes sense to not duplicate binary code. Thus, it is quite common that STL components factor commonly used parts out and use them. The links for a list class or the type independent attributes of a vector are just two examples. For vectors there are even multiple layers: some parts are entirely independent of the type (e.g., the size), others only need the type itself (e.g., all the accessors, the iterators, etc.), and some parts need to know how to deal with resource allocation (e.g., insertion and destruction needs to know about the allocator being used).
2. It turns out that std::iterator<...> doesn’t really work: The types defined in base classes depending on template parameters are not directly accessible in class template deriving from such a base. That is, the types need to be qualified with the base class and need to be marked as types using typename. To make matters worse, users could in theory allocate objects of the derived class and release them through a pointer to std::iterator<...> (yes, that would be a silly thing to do). That is, there is no benefit but a potential drawback, i.e., it is best avoided.

That said, I’m not aware of any good resource covering the techniques of implementing generic libraries. Most of the details applied in STL implementations were independently invented by multiple people but the literature on Generic Programming is still relatively scarce. I don’t think that any of the papers describing STL actually discuss implementation techniques: They normally concentrate on design details. Given that only very few people seem to understand what STL is about, it isn’t a big surprise that authors tend to concentrate on describing what STL is rather than how to implement it.