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The Best C++ Interview Question – Ever

• pingswept 17y ago
  As an outsider looking in at the world of C++ development (I work on hardware), my first thought is "Why would you want to use a tool that requires an understanding at that level of complexity for mastery?" Perhaps C++ has some extraordinary benefits that I don't understand, which makes deleting the v-table before the method returns the stack to the heap worth the trouble [1]? [1] I might have mixed up some of the terminology a little here. Feel free to correct me vigorously.
  • tedunangst 17y ago
    You have mixed up the terminology, vigorously. :) And that is the exact purpose of the question. To distinguish c++ programmers from pretenders. If you don't know C++, the question is nonsense, but if you are going to be writing code, you should understand the execution model sufficiently to answer it. "Yeah, this one time I double allocated a vtable on the stack of a doubly-linked binary hash."
    • pingswept 17y ago
      A good point. And by the way, thanks so much for your work on OpenBSD (assuming you are the Ted Unangst of legend). I've been using it for years, and I appreciate the dedication of the folks who created it.
  • kscaldef 17y ago
    Do you find it useful, in your field, to understand how your tools work? If so, perhaps you might allow that if someone is interviewing for a job using another tool (C++, in this case) that they might be expected to know how that tool works. Also, with respect, it's fairly clear from your second paragraph that you don't understand the terminology being used at all. Which is okay, it's not your field. But, it makes your assessment of how complex this discussion is somewhat suspect.
    • pingswept 17y ago
      Totally agreed about understanding how tools work being useful. I'd just be wary of using an extremely complex tool. From what I can tell, this "C++" looks like it could self-destruct at any minute, unless handled by an expert. Perhaps that makes the case even more strongly that you ought to filter your hires most strictly, but I wonder whether there might exist a simpler tool that could do the job with fewer risks.
  • DenisM 17y ago
    C++ is a toolkit for constructing problem-specific languages. You do this by creating new types (classes), using templates, template metaprogramming, preprocessor, multiple inheritance, and all kinds of other stuff. Once you develop a language, teach it to all your developers and establish a policy to punish deviations you can write very efficient programs - at near or exceeding asm performance (modern compilers are smarter than developers when it comes to CPU-specific optimizations). Programming in raw C++ using all its features is probably not productive - the code will not be readable by anyone except Stroustroup himself. Other languages such as Lisp also allow you to create your own language for specific problems, however they are focusing on expressive power and not on performance.
• tptacek 17y ago
  This is a completely broken C++ interview question. It asks the candidate to reason through a worst-practice, which exposes you to their knowledge of trivia and tells you nothing about whether they can actually get stuff done in C++. My old Sonicity C++ code, which was built on ACE, is littered with "delete this" calls for Reactor callbacks. Obviously, "delete this" is OK if you're on the heap and you can guarantee that the code isn't going to touch the object again. But what I remember most about that ACE Reactor code was how fragile the event handling was; "delete this" is a recipe for horrible memory corruption flaws. This is what you get when your C++ interviewers decide that interviews are more about proving how smart they are than about qualifying candidates.
  • bmm6o 17y ago
    Why do posts about interview questions always attract such vehement reactions? The politer replies on the blog itself suggest that the author should work for McDonald's instead of programming. Reddit's discussion isn't much better (predictably). HN can't even have a reasoned discussion about it?
    • hristov 17y ago
      Because people really do not like being unfairly or capriciously judged on interviews. For many engineers, doing well on an interview is very important for their life, and it is extremely frustrating to have a silly or unfair question.
      • kscaldef 17y ago
        > For many engineers, doing well on an interview is very important for their life, and it is extremely frustrating to have a question they cannot answer. FTFY
      • bmm6o 17y ago
        I get that part. But then in order to justify the level of intensity, doesn't that require someone to concede that they wouldn't have gotten the answer to this one? There seems to be more to it; like "I knew the answer to that one, but he could have easily asked a question that would exclude me!" Is there a question he could have posted that would not have elicited this reaction?
• deleted 17y ago
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• wglb 17y ago
  At the level of this question, checking out Scott Meyer's 50 ways will show you some that he got wrong. An interesting one is how to write a smart pointer class, and how many ways it can go wrong.
• InclinedPlane 17y ago
  I'm torn on whether this is a good question or not. The important part of the question is definitely the "why?" step. Finding out whether a developer will question their own assumptions, fall back to solid knowledge and ask questions to acquire the knowledge needed to proceed or if they will simply bumble down the garden path of ignorance, making things up as they go with no basis in fact other than a gut feeling is very important.
  • roc 17y ago
    It's just a language-specific variant of a 'Microsoft problem': you don't want the answer, you want to watch the process. It's similarly useful in theory. But in practice I think it has the same key failing: Performance is too skewed in interview situations for it to give you consistently useful insight into candidate fitness. If you want to thin out a hundred otherwise-equivalent candidates - knock yourself out. But at that point you might as well be flipping coins.
• RiderOfGiraffes 17y ago
  You know, the more I read suggested interview practices and the responses they elicit, the more I think that the best interview question would be to get the candidate to critique a few interview questions. The point here is not whether this is good practice, or whether the candidate knows the answer, or whether they would write it themselves, or whether they get the right answer immediately, or whether they automatically know the darkest corners of the C++ spec (of which this isn't one). The point is whether they can reason through, with the interviewer, and then potentially change their mind if they find that they're wrong. Part of being a good programmer is finding the good ideas among the dross, finding the right implementation technique among the many at their fingertips, and finding the right design among the myriad plausible constructions. If you can't find the good ideas in these sorts of interview questions, if you can't read carefully enough to see that the whole point was to uncover whether of not the candidate can reason through then potentially change their mind, or if you think that to do so isn't an important characteristic to test for, then that tells me something. Yes, it's important to see why things don't work, or are broken, or are a bad idea, but the ability to find good ideas is infinitely more important. I like this interview question, and if you don't, then we're not a good fit. It doesn't mean you're a bad programmer, it doesn't mean (necessarily (I hope)) that I'm a bad employer/manager/programmer/interviewer - but it doesn mean we probably won't work well together. But I could change my mind if you provide better arguments than the ones I've seen so far.
• raganwald 17y ago
  This question and the associated discussion remind me very strongly of something I learned very early in own professional C++ development: C++ the language is an extremely leaky abstraction. There are so many places where you really do need to think about what the compiler is doing and what is happening behind the scenes rather than thinking at a consistent abstract level. Is that a bad thing? I suspect that C++ programmers would say no. But then again, there would be a lot of bias in that answer...
• ryanwaggoner 17y ago
  These are people you do not want to hire under any circumstances because they cannot think outside their initial preconceptions. I know nothing about C++, but I know that this is bullshit. I also think that it's pretty ironic if you think about it for a minute.
• nearestneighbor 17y ago
  C++ is my main language these days (and my guess at the answer happened to be correct), but I think this interview question misses the point. The key to using C++ successfully, like it or not, is not to understand what happens in every possible circumstance (this is pretty much impossible). The key is to carve out a subset of the language (and some usage patterns) that you know how to use and stick with it. I wouldn't use delete this without consulting the documentation first, even if I suspect it's legal (hell, I use "idiomatic" delete maybe once in 5000 LOC)
• DanielBMarkham 17y ago
  I'm cold on C++ (haven't coded in several years) but let me try this out: deleting "this" releases memory held by the instance data of the object. The programmer can't be sure that any subsequent references to that data will return valid data (although the compiler will allow it). Static data and type-level methods should be fine to call -- although every time the program counter ticks you can't be sure that you're continuing to walk through uncontaminated memory. I think that's it. Sounds like a BS question to me because you're going way, way under the hood with exactly how the compiler runs the code. It's like those coding questions where you have to start byte-counting to get the answer. Neat trivia perhaps, but I'd much rather have somebody who delivered clean, organized, easily-maintainable code and got the answer wrong. EDIT: Seems like I've seen delete this on several occasions (although I can't remember where), but hell if I'd use it unless I had no choice. Even then, it'd be the last statement in the method.
  • tptacek 17y ago
    It also invokes the dtors for the object, and, more importantly, does all this before the caller method returns, so that when "delete this" finishes, the program still has to execute code in the method context of a dead object --- which usually works.
  • machrider 17y ago
    The function's body is not part of the object itself. When you delete the object, you can keep executing the function just fine, as long as you don't access any member variables.
    • haseman 17y ago
      You keep executing the function because it's on the stack. The program counter keeps moving through the method until it hits a return statement. God help you if you try to reference one of the class' internal data objects as their heap space could have been allocated to something else running on another thread.
    • DanielBMarkham 17y ago
      The function's body is not part of the object itself Most all of the time, yes. But the function could have been dynamically allocated to an instance of the object. You don't know from the definition of the question. A dynamic function, presumably, would be picked up by the dtor.
• Batsu 17y ago
  I always find it curious when the number of comments on an article exceed that of the point value. It leads me to suspect that users get angry over the article for some reason. To add to the discussion, however, I knew the answer because I had a run in with doing this sort of thing. I had built a simple game with OpenGL - stuff moved around on the screen and you could shoot it. Due to the way I constructed the objects, it was trivial to "Delete this" when the object should disappear - a bullet collides with an enemy, perhaps. I did it, it worked exactly as expected. The follow up question - "What can you do after delete this; ?" - also reared it's head, and that's when I found the other edge to the sword and shied away from it's usage. It felt so much more elegant as "delete this;" :(
  • tptacek 17y ago
    The fact that the comments are often better than the articles they attach to is the thing a lot of us like about HN.
    • deleted 17y ago
      [deleted]
• nomoresecrets 17y ago
  When I was interviewing C++ programmers, we had a C++ test, and some stock questions I asked face to face. However, I found I could pretty much weed the good C++ programmers from the mediocre/just read about C++ before the interview by asking one question: "What is a virtual function?" If they have trouble starting to answer it, give them a hint with "What would cause you to type the keyword 'virtual' in your code?" You would be amazed how many C++ coders don't really know the answer to this, or what is going on (vtables etc, or even just the behaviour). One candidate answered "I'd make a function virtual if there were any other virtual functions in the class." It seems like there is no shortage of bad answers to this question. All the people who ever gave me a correct answer (hit rate was <10% of candidates) were good C++ developers. It's a good question :-).
• brg 17y ago
  Looking over the responses, it is really hard to believe many of the people here have worked in C++. This is basic style for anyone who has ref counted, such as implementing IUknown.