7 ms·

I have three standard "screener" questions: (a) what's a virtual method, (b) why would you want to have a destructor be virtual, and (c) what's a virtual base c

• Eliezer 17y ago
  Long as you don't need any honest 9s or 10s for whatever it is you're doing, I guess that's fine.
  • kabdib 17y ago
    Snarky answer: You can easily tell the 9s or 10s because they answer "eight." Less snarky: They give you evidence, really quick, that they're a 9 or 10. It's not hard to tell.
• d0m 17y ago
  A better question is: Why do we need to put the archaic virtual keyword in front of a method? Shouldn't non-virtual functions used in an inheritence way be declared as private while the other being automatically virtual ? Then if he agrees, he's ok. If he gives you more examples of why C++ is archaic, he's great. If he begins talking about non-virtual methods being more optimized than virtual one, thanks him for his time and try to find someone else.
  • killedbydeath 17y ago
    Public non-virtual functions may be useful. For example, to provide alternative API functions. Something like: virtual public setOrigin(const Point &p); public setOrigin(float x, float y) { setOrigin(Point(x,y)); } Someone inheriting from this class will know that all the customization is in one function.
    • d0m 17y ago
      Still, I find it more logical to say virtual by default and using another keyword for rare case when you want to specify a non-virtual function.
      • alnayyir 17y ago
        Sure, if you learned to code in dynamic langs like Python and Ruby. I didn't, I learned in C. I'd rather take the safe route and non-virtual by default.
  • haberman 17y ago
    > If he begins talking about non-virtual methods being more optimized than virtual one, thanks him for his time and try to find someone else. What's your problem with this answer? Non-virtual methods can be easily inlined, whereas virtual methods (almost always) require: 1. a load (though a hoistable one) from the vtable 2. an indirect branch 3. register re-allocation 4. function call overhead (standard prologue/epilogue)
    • VBprogrammer 17y ago
      That is only a problem if and only if that method is being called in a very tight loop and its body is not particularly time consuming. A person making optimisations like this without first timing the code or at the very least spending some time introspecting on the pros and cons of doing so would not be someone I wanted to work with!
      • pmjordan 17y ago
        Tell that to someone writing a PlayStation 2 game. Using virtual functions without thinking will kill performance on that machine.
  • sorbits 17y ago
    What if he makes a case for keeping types as POD when possible? :) Anyway, it is clear from your comment that you dislike C++ and you prefer to hire people who agree with you that C++ is archaic, that is fine and all, but it doesn’t show in-depth understanding of the language. Explaining why things are as they are does show deep understanding of the language and a desire to learn the dirty details, IMHO a valuable trait.
    • d0m 17y ago
      It's a way to avoid a 100% only (and closed mind) C++ programmer. Understanding the cost of a virtual function is something. Not understanding that in most situation this cost is trivial, is something else. And every C++ great programmer should know that C++ is archaic and that higher level languages exist. If someone only knows C++, he is a C++ programmer, not a programmer.
      • tspiteri 17y ago
        Virtual functions are helpful only when you have derivation and need some common interface. When your class is not meant to be derived from, declaring functions as virtual is just plain misleading. And I am strongly against fitting everything artificially into a class hierarchy, that is, I dislike using the OOP sledgehammer for every simple class. Also, great C++ programmers are those who use C++ in a problem domain matched to it, not those who use C++ where Perl or Python or something else is so much better. So they know that C++ is not archaic at all, it is just not suitable for every job, but then, what language is?
      • huherto 17y ago
        IMO, you shouldn't dismiss a candidate as close minded with one question. If you have fears that he only knows C++ and doesn't care about higher level languages, you should digg into that with other questions.
  • pmjordan 17y ago
    C++ is used in places other than bleeding edge PCs, you know. The impact of a virtual function call is non-trivial on some systems, the most prominent one that springs to mind is the PlayStation 2. Knowing how they work and when to use them is always a good thing. With that sort of attitude you'll only hire people less competent than yourself; you're trying to find more competent people.
    • d0m 17y ago
      Thanks for saying I'm not competent. First, I know what a virtual function is and how performance might be important. However, it's just illogical to use inheritences and OO without using virtual function. Just think about the virtual destructor.. why isn't it automatically virtual? I find the inverse more interesting: Virtual by default and declaring non-virtual when the rare need arrives.
      • pmjordan 17y ago
        Thanks for saying I'm not competent. This isn't what I said, but you know that anyway. Just think about the virtual destructor.. why isn't it automatically virtual? Basically, because not every class is used polymorphically, many are designed to be used primarily on the stack or as members. The merits of what should be the default are debatable, but as with anything in C++, the reason is probably historical. For example, it would break all C code that used structs. One of the C++ design principles is to not impose features upon you that you don't need, so that might be why they decided not to e.g. make classes have virtual functions by default, but not structs, a bit like classes have private members and bases by default. In any case, these are straw man arguments. The point I was trying to make is that you suggested turning down people because they know the low-level details of the platform and their implications. I counter that this is a phenomenally bad idea; knowing what lies beneath the abstraction is always a good idea, especially if that reveals dragons. You haven't said anything to counter that, instead pointing out aspects of C++ you don't like. If you dislike C++ enough not to use it, why are you interviewing people for C++ programming positions anyway?
        • 17y ago
  • tbrownaw 17y ago
    Why C# also makes methods non-virtual by default: http://www.artima.com/intv/nonvirtual.html http://www.artima.com/intv/nonvirtual.html Other (shorter) commentary: http://stackoverflow.com/questions/814934/why-c-implements-methods-as-non-virtual-by-default http://stackoverflow.com/questions/814934/why-c-implements-m...
  • scott_s 17y ago
    One of the main design principles behind C++ was "you don't pay for what you don't use." Hence, if you don't want to use polymorhpic member functions, then you don't pay for the overhead.
• gridspy 17y ago
  I've been using C++ as my primary language for the last 10 years and know what virtual base classes are. I would never use them, Instead I'd avoid diamond inheritance at all costs. I'd rate myself as a 9 or 10 for language proficiency but perhaps not for certain niches. For example I know comparably little about Win32 or COM (ugh). However, there are tons of programmers out there who can't program. Faced with a clean slate they would either choke or create an atrocity. If they rated themselves 5/10 then for me to rate myself only 8/10 would be a disservice to myself. The same is true of perhaps 90% of the programmers on HN. Of course, I can look back at my former self and know that he thought he was awesome but clearly I have surpassed his ability. It only makes sense that there are far greater programmers out there and I have met a few.
  • kabdib 17y ago
    Right. The bit about virtual base classes is to simultaneously find the folks who know what it is, and to get a good feel for their design sense. "Don't know" is okay, if they're otherwise a strong engineer. "Love them!" is only good if they can justify it. "Yeah, diamond inheritance, avoid that" is pretty much what I'm looking for, though I'm willing to have a serious conversation about their merit. These are just questions I can get through in a minute or two, to gauge the person quickly.
    • haberman 17y ago
      I think that LinkedListItem or RefCounted are good candidates for sane virtual base classes.
    • scott_s 17y ago
      I guess you'd never hire me: http://github.com/scotts/cellgen/blob/offstencil/src/xformers.h#L1175 http://github.com/scotts/cellgen/blob/offstencil/src/xformer...
      • gridspy 17y ago
        "OpenMP like support for the Cell Processor"? Where do I sign?
• _sh 17y ago
  I used to think a good C++ screener question might be: "When would you use the 'nothrow' constant, and when would you have empty 'throws' specifications in method declarations?" But then I realized this is just getting tricky with C++ arcana, and is unfair. I once read all of Herb Sutter's 'Exceptional C++' series, and they scared me so much that I still get the frights, years on, when contemplating approaching a C++ compiler. And I used to think I was OK with C++ too. But after Sutter's books I had to accept that, nah, at best I was mediocre. I admire people who can boldly cast out and rapidly prototype a library or something. Or who have the patience to iteratively engineer a Boost library. I will never achieve this, despite once desperately wanting too.
• ntoshev 17y ago
  So you're asking for OOP, no generic programming / templates? I know template metaprogramming magic becomes madness real fast :-) I tend not to ask direct language questions anymore, everything is related to the code written on the interview. Like, you asked for a function doing something with an array of ints, and when you get the code you say "now, can we have this function work with arbitrary types instead of just int", "what are the constraints about this types", "what happens if it has a copy constructor but no assignment operator" etc. And I accept everyone knows and uses some subset of the language.