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Found 2 results

  1. Hi, Here comes a rather basic question but which I'm always struggling with since you can often argue both ways. Hopefully some of you have a few nuggets to share here... 1) A parent class should have no knowledge of any child class' concrete type, i.e. only data and methods that is inherently common to all decendents of that parent should be defined by it. 2) For dynamic dispatching to work the parent class must define virtual methods for dynamic dispatchees to work from. Those two rules often contradict each other. Methods vetted against 1) will often eliminate the possibility of 2). A specific example: Currently at GPower we're working on a set of classes that enables us to do a lot of stuff on all the different LabVIEW files. So, we have a parent file.lvclass and then a collection of specific children that inherits from "file", for instance vi.lvclass, lvproj.lvclass, ctl.lvclass, facadevi.lvclass etc. A) Generic stuff that has to do with any file obviously goes into file.lvclass data (together with data member access methods for each). For instance IsWriteprotected?, FileSize, FilePath and such. A FileRefnum should probably go into each child class to enable us to have it most specific. B) Then there are data elements that definetely has to do with each concrete file type and thus should be in each child class' private data, such as AllowsDebugging?, IsFPStyleOK?, Icon etc. - as those are only valid for a subset of file types. C) Now it gets a bit more murky; If we define data elements in file that each child can use to flag if it has this or that feature (like HasIcon? or SupportsProtection?), then you can ask each and every file type if it support that, before calling a method to work on that. For the subset of file types that supports a given feature (those that has an icon for instance) it'd be great to have dynamic dispatching though. But for that to work file.lvclass would have to implement virtual methods like GetIcon() and SetIcon() (where the latter maybe recalculates the IsIconStyleOK? data field for instance). But implementing these (numerous it turns out) virtual methods in the parent class, suddenly riddles the parent class with specific knowledge or nomenclature that is very specific to different subsets of descendent file types. I always vet the parent against each piece of information I'd like to put in there by asking myself something like this: Does my generic file know anything about disk write protection? Yes, all files deal with that, so it can safely go into file.lvclass. Does my generic file know anything about block diagrams? No, only a handful LabVIEW file types knows about block diagrams, so I can't put it into file.lvclass. But the latter forces me to case out the different file types in my code, so I can call the different specific methods to work on block diagrams for those file type objects that supports this. Hence, if I want to be able to just wire any file type object into a "block diagram worker" method I need dynamic dispatching, and that only works if my parent has a virtual method for me to override with meaningful code in a few child classes... How do you go about selecting this balance, on one hand abstracting the parent from the concrete types, and on the other hand implementing the necessary dynamic dispatch virtual methods in the parent? Regards, Steen I must add that I'm currently leaning towards the more pure approach of not mixing abstract and concrete. Thus I tend to omit dynamic dispatching except for the very generic cases. I only allow the "do I support this feature" flags into the parent data, and then I implement static dispatch methods for each concrete type - even though these static dispatch methods then will often be quite similar, but that is solved with traditional LabVIEW modularization and code reuse (like subVIs, typedefs and such). It works well, but I feel I don't get the full benefit of dynamic dispatching. But that is what I'd do in C++... /Steen
  2. I have a class that defines a message. I inherit from this class for all my messages. The class contains a must override method called 'Execute'. Every time I create a child of this class and create the override method for Execute then go to save it, it wants to save the parent as well. This only occurs after I create the override method for Execute. If I just create the class, save it, change its inheritance and save it again the parent class is not affected. I have no idea why it is doing this but it is really annoying. Especially since everything is in version control, I end up having to check the parent out just to save the project. The reason given for the need to save the parent is 'Item moved within library'. The parent class is in a lvlib. I am running LV 12.0f3. Any ideas? Is there a CAR for this or am I just doing something wrong? thanks, -John
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