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Oh wow. So it sounds like what you got out of virtual inheritance here is avoiding some kind of ABI breakage? That's interesting, I've never come across this us

Oh wow. So it sounds like what you got out of virtual inheritance here is avoiding some kind of ABI breakage? That's interesting, I've never come across this use case before, but I'll be thinking more about it and looking out for it in the future. Thanks!

gumby 6y ago
No, it was about encapsulation. Consider the save dialogue: if wants to have an array of the json saver, sql saver etc and perhaps display them in a pull down list. Virtual inheritance let’s you make the std::vector of the base class. Otherwise what could you use? A pointer, and manypually reimplement the vtable? Also this means changes to the underlying representation don’t always require changes to the dependent classes, which I suppose you could consider an ABI change. Maybe an easier way to characterize it is to consider the virtual classes a way to implement plug ins.
- mehrdadn 6y ago
  Okay then I'm lost. Why can't you have JsonSaver, SqlSaver, etc. inherit from a Saver class the normal way, and store instances of them in a std::vector<std::unique_ptr<Saver>>? Then you'd just need struct Saver { virtual ~Saver() { } virtual void save(...) { ... } } along with struct JsonSaver : Saver { void save(...) override { ... } } and it should work, right? I'm confused why you need a virtual base class on top of that. Are you by any chance confusing "base classes with virtual methods" with "virtual base classes" (which may or may not contain virtual methods)?
  - gumby 6y ago
    Yes, I mean literally base classes with vtables not the "virtual" specifier of a superclass ("base class" in C++). Apologies for the confusion! I do think it's a useful feature, except as I think most people finally agree, MI is more often than not a cause for confusion. It's funny that of all the different method combinators (another source of confusion) only that one was chosen. BTW I don't think a unique_ptr is needed in the case I described.
- maxlybbert 6y ago
  One annoying thing about C++ is that certain terms and keywords are overloaded. So, while having virtual functions in a base class is a very common design approach, it isn’t “virtual inheritance” ( https://en.wikipedia.org/wiki/Virtual_inheritance https://en.wikipedia.org/wiki/Virtual_inheritance ). Many people consider virtual inheritance a complexity of the language. Why is it possible for a class to inherit from a base class multiple times through multiple paths (the standard case) or to have the compiler coalesce those paths so that the child class only inherits once from the parent/grandparent? The thing is, once you have multiple inheritance, somebody is going to try to inherit from the same base class multiple times. The language can either support it or declare it an error. It’s not clear to me why it’s wrong to for the language to allow it, even though everywhere I’ve worked with a C++ coding standard has said “if your design requires virtual inheritance, discuss your design with other experienced programmers to see if that can be changed.”
  - gumby 6y ago
    > One annoying thing about C++ is that certain terms and keywords are overloaded. It's a no-win choice, though I agree C++ chose the wrong path: trying as much as possible to avoid adding new keywords. It leads to the problem you describe, as well as some confusion in this very thread (which could be my fault, but honestly I don't know -- that's the problem!). The committee seems to have moved away from this over the years which has lead to complaints of too many keywords and identifiers. Which is why I call it no win. The function virtual specifier in the base clause is reasonable: if you store something into parent_class.instannce_var and there are two copies of parent_class in your structure which one is stored to and is the choice consistent in all compilation units? This specifier guarantees that the answer is "yes". I don't know why it isn't the default though. More generally, back in the day I used to use a bunch of different method combinators with flavors (later CLOS). Apart from the useful :before and :after, they generally lead to undebuggable code.
    - maxlybbert 6y ago
      I understand why they try to reuse keywords and symbols, and why they intentionally pick really ugly keywords when they add new ones. I can’t say they’re wrong, but I still find it annoying.

gumby 6y ago

No, it was about encapsulation. Consider the save dialogue: if wants to have an array of the json saver, sql saver etc and perhaps display them in a pull down list. Virtual inheritance let’s you make the std::vector of the base class. Otherwise what could you use? A pointer, and manypually reimplement the vtable? Also this means changes to the underlying representation don’t always require changes to the dependent classes, which I suppose you could consider an ABI change. Maybe an easier way to characterize it is to consider the virtual classes a way to implement plug ins.

mehrdadn 6y ago
Okay then I'm lost. Why can't you have JsonSaver, SqlSaver, etc. inherit from a Saver class the normal way, and store instances of them in a std::vector<std::unique_ptr<Saver>>? Then you'd just need struct Saver { virtual ~Saver() { } virtual void save(...) { ... } } along with struct JsonSaver : Saver { void save(...) override { ... } } and it should work, right? I'm confused why you need a virtual base class on top of that. Are you by any chance confusing "base classes with virtual methods" with "virtual base classes" (which may or may not contain virtual methods)?
- gumby 6y ago
  Yes, I mean literally base classes with vtables not the "virtual" specifier of a superclass ("base class" in C++). Apologies for the confusion! I do think it's a useful feature, except as I think most people finally agree, MI is more often than not a cause for confusion. It's funny that of all the different method combinators (another source of confusion) only that one was chosen. BTW I don't think a unique_ptr is needed in the case I described.
maxlybbert 6y ago
One annoying thing about C++ is that certain terms and keywords are overloaded. So, while having virtual functions in a base class is a very common design approach, it isn’t “virtual inheritance” ( https://en.wikipedia.org/wiki/Virtual_inheritance https://en.wikipedia.org/wiki/Virtual_inheritance ). Many people consider virtual inheritance a complexity of the language. Why is it possible for a class to inherit from a base class multiple times through multiple paths (the standard case) or to have the compiler coalesce those paths so that the child class only inherits once from the parent/grandparent? The thing is, once you have multiple inheritance, somebody is going to try to inherit from the same base class multiple times. The language can either support it or declare it an error. It’s not clear to me why it’s wrong to for the language to allow it, even though everywhere I’ve worked with a C++ coding standard has said “if your design requires virtual inheritance, discuss your design with other experienced programmers to see if that can be changed.”
- gumby 6y ago
  > One annoying thing about C++ is that certain terms and keywords are overloaded. It's a no-win choice, though I agree C++ chose the wrong path: trying as much as possible to avoid adding new keywords. It leads to the problem you describe, as well as some confusion in this very thread (which could be my fault, but honestly I don't know -- that's the problem!). The committee seems to have moved away from this over the years which has lead to complaints of too many keywords and identifiers. Which is why I call it no win. The function virtual specifier in the base clause is reasonable: if you store something into parent_class.instannce_var and there are two copies of parent_class in your structure which one is stored to and is the choice consistent in all compilation units? This specifier guarantees that the answer is "yes". I don't know why it isn't the default though. More generally, back in the day I used to use a bunch of different method combinators with flavors (later CLOS). Apart from the useful :before and :after, they generally lead to undebuggable code.
  - maxlybbert 6y ago
    I understand why they try to reuse keywords and symbols, and why they intentionally pick really ugly keywords when they add new ones. I can’t say they’re wrong, but I still find it annoying.