> You probably don't understand std::list and std::deque forget std::rope std::map std::unorderd_map, std::set etc. etc.
These were our general interview questions at the last place I worked (until we moved almost completely to Javascript development). Understanding big O performance characteristics is a very important skill, I don't consider it optional; at least in the world of C++ development. We asked about them because they use different, well-known, algorithms to get various results and we wanted to know if people had the skill to figure out appropriate trade offs when solving a problem.
> I know what they do, I know how they're probably implemented. I know their big O characteristics etc.
Then iterator invalidity should be easy to figure out for each of the containers. You don't even need a table.
> But they're optimised for high level generality not for the thing I'm doing on the hardware I'm targeting.
They're designed to be as efficient as they can be while also being general, yes.
> I don't know how they perform on particular hardware, how cache sizes affect them etc etc etc.
That's hardly a surprise, and that knowledge will go stale pretty quickly. Which is why Stroustrup says everybody gets it wrong. Many programmers have no idea how badly cache hostility will hurt them.
> You have to find that out for yourself, hopefully by benchmarking before you use them, rather than finding out they suck very, very hard in production.
I don't see how it could be different. Again, hardware architecture changes over time.
> Note that the benchmark he shows is not available, and it's not available in the STL.
I think you're looking for something like http://www.stepanovpapers.com/container_benchmark.cpp . I can imagine a compiler vendor or library vendor shipping something like this, but I wouldn't expect it to be in the STL.
I believe the first programming textbook I read mentioned the use of a profiler. If you're trying to improve performance, it's probably better to start with a profiler than with benchmarks. Then again, Douglas Crockford mentions how silly it is for programmers to make every choice based on performance instead of things like maintainability. Only worry about performance if you know it actually matters (and a profiler can help you decide if it matters).
> I have never seen an exception safe C++ codebase, I don't believe such a beast exists.
We may be talking about different things. I've worked on moderate sized code bases that wouldn't leak memory or leave files open, etc. if exceptions were thrown (and used ScopeGuard to do that), but our error handling code was as simple as possible. We had a large loop processing requests, and we'd throw out the request if there was any exception. We followed what Anders Hejlsberg describes as the expected C# pattern ( http://www.artima.com/intv/handcuffs.html ): "Surely in any kind of event-driven application like any kind of modern UI, you typically put an exception handler around your main message pump, and you just handle exceptions as they fall out that way. But you make sure you protect yourself all the way out by deallocating any resources you've grabbed, and so forth. You clean up after yourself, so you're always in a consistent state. You don't want a program where in 100 different places you handle exceptions and pop up error dialogs. What if you want to change the way you put up that dialog box? That's just terrible. The exception handling should be centralized, and you should just protect yourself as the exceptions propagate out to the handler."
> How does the silly contrived example of opening a file work with multiple threads?
There isn't enough detail to answer this question. Are the threads going to share a single std::fstream? Or do they each get their own? Do we have access to some kind of file locking mechanism?
Clearly there are solutions to the problem. I have databases running on my system that were written in C and C++ and they manage to manipulate files from multiple threads.
> Look at his example, push_back of a pointer onto a vector.
Is the vector responsible for cleaning up the pointer? Will the pointer outlive the vector? Can you use shared_ptr? There are three, simple, answers to the question (std::vector<T*> if the pointer will outlive the vector and something else promises to clean up; std::vector<std::unique_ptr<T>> if the vector takes over responsibility to clean up; and std::vector<shared_ptr<T>> if the responsibility has to be shared). You don't need ScopeGuard for any of those cases. vec.push_back(ptr) will do the right thing for raw pointers (if cleanup is guaranteed elsewhere) and shared_ptrs (i.e., if "ptr" is a shared_ptr<T>), while vec.push_back(std::move(ptr)) will do the right thing for a std::vector<std::unique_ptr>> (if "ptr" is a unique_ptr<T>). In fact, vec.push_back(std::unique_ptr<T>(new T(...))) will also do the right thing for a std::vector<std::unique_ptr<T>>.
> maybe I'm missed some and it's only 99% of c++ code that is by the Alexandrecou definition "exception unsafe"
Not all code has to be exception safe (e.g., if the policy is to abort an any exception, or if memory leaks are deemed acceptable). And Google, somewhat famously bans them ( http://google-styleguide.googlecode.com/svn/trunk/cppguide.h... ). But the numbers are better than you believe. Mozilla seems to do a decent job with exception safety. The biggest source of exception unsafe code appears to be programmers who believe everything must be allocated with new in all cases.
> If you use C or C++ and genuinely don't care about performance you performed a premature optimisation in the choice of these languages. Any JVM language would be suit your needs better in some way.
That is a surprising statement, especially since you don't necessarily have any idea what my needs are. There are times that I choose to write code in C# or Perl (never Java, but that's my personal preference), so I don't use C or C++ if it doesn't make sense. But there are times that I choose C++ for reasons other than performance. It can actually be a good fit for what I'm doing (e.g., I don't like using neutered generics when I really need templates, and I don't like putting things into classes if there's no need to ( https://news.ycombinator.com/item?id=3717715 )). Or (more often) I know a C++ library that's a great fit and that doesn't exist in C# or Perl.
>> If you use C or C++ and genuinely don't care about performance you performed a premature optimisation in the choice of these languages. Any JVM language would be suit your needs better in some way.
>That is a surprising statement, especially since you don't necessarily have any idea what my needs are.
Look we've probably exhausted this discussion and are talking past eachother, but I do want to point out that this is a simple misunderstanding.
I meant "you" in the sense of the archaic english "one"
"If one uses C or C++ and one does not care about performance one has performed a premature optimisation." It sounds terribly old-fashioned expressed like this but I guess the meaning is clearer given the way "you" is overloaded to mean you singular (you yourself), you plural (you and your admirers) and you in the sense of 2. here: http://dictionary.reference.com/browse/you "one; anyone; people in general"
I obviously have no knowledge or opinion about choices made specifically by yourself, and we are in total agreement that holding an opinion on such would be ridiculous in the extreme.
You might find this interesting, it highlights the problem I'm talking about with exceptions by a guy who has done it in c++ with amazing success, likes the language and appreciates its defects as well as its strengths:
http://250bpm.com/blog:4
Where you are writing those pieces of code in your program that caused you to rationally choose C or C++ as your language (ie has to perform & caching is important), the STL is a bad option because it wasn't designed for your use, it was designed for everybody's use and you pay for everybody else's use and their architecture choices and so on none of which you are using! So uou pay when you don't use! This is kind of supposed to be what doesn't happen in low level languages. Good C++ code in such a case bears more than a passing resemblance to C and the fact that this is possible is one of the greatest strengths of C++.
If you're writing pieces of code that were largely irrelevant in your choice of C++ as a language and if that was the only code you were writing in this project you could have happily chosen a higer level language, the STL is just dandy, use it, write your code fast and forget about its performance as totally uninteresting to your problem.
More interesting is where you're not sure, "a deque could be good enough here and will save me time to write." How do you know if it's good enough. Big O don't help you. You should be able to run a canned benchmark you understand on the hardware you're targeting so you have an idea of "good enough" or "won't cut it, need to implement a ring buffer" or whatever. Instead you have to write that yourself or just ouija board. So we duplicate masses of effort from conscientious c++ hackers and simultaneously encourage ouija board decisions. C++ could and should do better. Stepanov's container benchmarks? Is a sorted vector with unique run on it better than a set, ok. Very limited in its general usefulness, for what are supposed to be general solutions. But yeah, about 200 more tests like it for different access patterns, insertion patterns, payload sizes and so on is what is required.
Here is Mike Acton (knowingly) wildly overstating the case for audience effect at cppcon last year. On the standards of these lectures this one is an absolute tour de force. Definitely worth your and any C++ hacker's time. Look up Alexandrescou's lecture on optimisation at the same conference as a benchmark to see just how good it is.
https://www.youtube.com/watch?v=rX0ItVEVjHc
>>> If you use C or C++ and genuinely don't care about performance you performed a premature optimisation in the choice of these languages. Any JVM language would be suit your needs better in some way.
>> That is a surprising statement, especially since you don't necessarily have any idea what my needs are.
> Look we've probably exhausted this discussion and are talking past each other,
I agree, it's petered out.
> but I do want to point out that this is a simple misunderstanding.
> I meant "you" in the sense of the archaic english "one" "If one uses C or C++ and one does not care about performance one has performed a premature optimisation."
I prefer "you" over "one." And, for the record, I didn't take it as an insult. But I find it odd to believe that the only reason anybody would use C++ is for performance. That there couldn't possibly be another use case (or if there were, it would be incredibly rare). I guess the only reason somebody would pick Python would be a strange obsession with whitespace? And Java's only for people who like checked exceptions?
Second, if you're going to publish a library, it's certainly important to have an error handling policy. But I find it very strange to assume that your clients will screw up error handling and blame you for it. The blog post doesn't say this explicitly, but I think it's the source of the "no exceptions, ever" policy ("no exceptions, because the clients will screw it up").
Third, you shouldn't throw exceptions for every error. You throw exceptions when something went wrong and you don't think your immediate caller can do anything intelligent about it. If the immediate caller can do something, you return an error code.
Fourth, did you know that printf() can fail? What should you do if it does? You can't print a warning to the user. Did you know that closing a file can fail? What should you do? Close it again? If a destructor is unable to clean something up, it's basically impossible to do anything about it. You might log the error, but I don't see what you'd do about it.
These were our general interview questions at the last place I worked (until we moved almost completely to Javascript development). Understanding big O performance characteristics is a very important skill, I don't consider it optional; at least in the world of C++ development. We asked about them because they use different, well-known, algorithms to get various results and we wanted to know if people had the skill to figure out appropriate trade offs when solving a problem.
Minor quibble: unless something has changed, rope ( https://www.sgi.com/tech/stl/Rope.html ) isn't in the Standard ( http://en.cppreference.com/w/cpp/container ).
> I know what they do, I know how they're probably implemented. I know their big O characteristics etc.
Then iterator invalidity should be easy to figure out for each of the containers. You don't even need a table.
> But they're optimised for high level generality not for the thing I'm doing on the hardware I'm targeting.
They're designed to be as efficient as they can be while also being general, yes.
> I don't know how they perform on particular hardware, how cache sizes affect them etc etc etc.
That's hardly a surprise, and that knowledge will go stale pretty quickly. Which is why Stroustrup says everybody gets it wrong. Many programmers have no idea how badly cache hostility will hurt them.
> You have to find that out for yourself, hopefully by benchmarking before you use them, rather than finding out they suck very, very hard in production.
I don't see how it could be different. Again, hardware architecture changes over time.
> Note that the benchmark he shows is not available, and it's not available in the STL.
I think you're looking for something like http://www.stepanovpapers.com/container_benchmark.cpp . I can imagine a compiler vendor or library vendor shipping something like this, but I wouldn't expect it to be in the STL.
https://github.com/rjernst/stepanov-components-course/tree/m... is also useful.
> Seriously. WTF is up with that?
I believe the first programming textbook I read mentioned the use of a profiler. If you're trying to improve performance, it's probably better to start with a profiler than with benchmarks. Then again, Douglas Crockford mentions how silly it is for programmers to make every choice based on performance instead of things like maintainability. Only worry about performance if you know it actually matters (and a profiler can help you decide if it matters).
> I have never seen an exception safe C++ codebase, I don't believe such a beast exists.
We may be talking about different things. I've worked on moderate sized code bases that wouldn't leak memory or leave files open, etc. if exceptions were thrown (and used ScopeGuard to do that), but our error handling code was as simple as possible. We had a large loop processing requests, and we'd throw out the request if there was any exception. We followed what Anders Hejlsberg describes as the expected C# pattern ( http://www.artima.com/intv/handcuffs.html ): "Surely in any kind of event-driven application like any kind of modern UI, you typically put an exception handler around your main message pump, and you just handle exceptions as they fall out that way. But you make sure you protect yourself all the way out by deallocating any resources you've grabbed, and so forth. You clean up after yourself, so you're always in a consistent state. You don't want a program where in 100 different places you handle exceptions and pop up error dialogs. What if you want to change the way you put up that dialog box? That's just terrible. The exception handling should be centralized, and you should just protect yourself as the exceptions propagate out to the handler."
But using C++ meant we didn't have to try to get the try ... finally's right ( http://mail.openjdk.java.net/pipermail/coin-dev/2009-Februar... , for the record, they accepted the proposal).
> How does the silly contrived example of opening a file work with multiple threads?
There isn't enough detail to answer this question. Are the threads going to share a single std::fstream? Or do they each get their own? Do we have access to some kind of file locking mechanism?
Clearly there are solutions to the problem. I have databases running on my system that were written in C and C++ and they manage to manipulate files from multiple threads.
> Look at his example, push_back of a pointer onto a vector.
Is the vector responsible for cleaning up the pointer? Will the pointer outlive the vector? Can you use shared_ptr? There are three, simple, answers to the question (std::vector<T*> if the pointer will outlive the vector and something else promises to clean up; std::vector<std::unique_ptr<T>> if the vector takes over responsibility to clean up; and std::vector<shared_ptr<T>> if the responsibility has to be shared). You don't need ScopeGuard for any of those cases. vec.push_back(ptr) will do the right thing for raw pointers (if cleanup is guaranteed elsewhere) and shared_ptrs (i.e., if "ptr" is a shared_ptr<T>), while vec.push_back(std::move(ptr)) will do the right thing for a std::vector<std::unique_ptr>> (if "ptr" is a unique_ptr<T>). In fact, vec.push_back(std::unique_ptr<T>(new T(...))) will also do the right thing for a std::vector<std::unique_ptr<T>>.
> maybe I'm missed some and it's only 99% of c++ code that is by the Alexandrecou definition "exception unsafe"
Not all code has to be exception safe (e.g., if the policy is to abort an any exception, or if memory leaks are deemed acceptable). And Google, somewhat famously bans them ( http://google-styleguide.googlecode.com/svn/trunk/cppguide.h... ). But the numbers are better than you believe. Mozilla seems to do a decent job with exception safety. The biggest source of exception unsafe code appears to be programmers who believe everything must be allocated with new in all cases.
> If you use C or C++ and genuinely don't care about performance you performed a premature optimisation in the choice of these languages. Any JVM language would be suit your needs better in some way.
That is a surprising statement, especially since you don't necessarily have any idea what my needs are. There are times that I choose to write code in C# or Perl (never Java, but that's my personal preference), so I don't use C or C++ if it doesn't make sense. But there are times that I choose C++ for reasons other than performance. It can actually be a good fit for what I'm doing (e.g., I don't like using neutered generics when I really need templates, and I don't like putting things into classes if there's no need to ( https://news.ycombinator.com/item?id=3717715 )). Or (more often) I know a C++ library that's a great fit and that doesn't exist in C# or Perl.