C++: doubts about visitor pattern

Question

I know what Visitor Pattern is and how to use it; this question is not a duplicate of this one.

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I\'ve got a library where I put most of the reusable code I write,

Answer 1

First: "Visitor Pattern is a way to simulate Double Dispatching in C++." This is, erm, not fully right. Actually, double dispatch is one form of multiple dispatch, which is a way to simulate (the missing) multi-methods in C++.

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Whether operations on a class hierarchy should be implemented by adding virtual functions or by adding visitors is determined by the probabilities of adding classes vs. adding operations:

• If the number of classes keeps changing more rapidly than the number of operations, use virtual functions. That is because adding a class requires modifying all visitors.
• If the number of classes is relatively stable compared to the number of operations, use visitors. That is because adding a virtual function requires changing all classes in the hierarchy.

Yes, many libraries do not come with a visitor interface.
When we are just looking at the reasoning above, this would be right if the number of classes changes often. That is, if a library is released often, with new classes being added constantly, then providing a visitor interface wouldn't make much sense, because each time a new release brings new classes, everybody using the library need to adapt all of their visitors. So if we only looked at the above reasoning, a visitor interface would seem to only be helpful if the number of classes in a lib's class hierarchy seldom or never changes.

However, with 3rd-party libraries there's another aspect to that: Usually, users cannot change the classes in the library. That is, if they need to add an operation, the only way they can do this is by adding a visitor - if the library provides the hooks for them to plug into it.
So if you are writing a library and feel like users should be able to add operations to it, then you need to provide a way for them to plug their visitors into your lib.