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Real cost of C++ exception vs error-code checking
- McP 16y agoI prefer Raymond Chen's take on exceptions: http://blogs.msdn.com/b/oldnewthing/archive/2005/01/14/352949.aspx http://blogs.msdn.com/b/oldnewthing/archive/2005/01/14/35294...
- bostonpete 16y ago"since you have to check every single line of code (indeed, every sub-expression) and think about what exceptions it might raise and how your code will react to it" Yes, that's my concern with exceptions as well. It seems like the Java model (if I remember it right -- it's been a decade), which requires a method to either handle an exception that a sub-method throws or explicitly allow it to be thrown, would be preferable and help people avoid accidentally ignoring an exception. I'd like to see support for exceptions like this in C++0X, but I haven't bothered to check to see if it's there...
- snprbob86 16y agoYou're thinking of "checked exceptions" and C++ already has them (pre 0X) via the "throws" clause on method declarations. Checked exceptions is a hotly debatted topic and I think the world has kinda finally come around to deciding that the are a bad idea overall. Google it and see for yourself.
- supersillyus 16y agoI don't know that it's accurate to compare C++ exception specifications with Java-style checked exceptions. Exception specifications aren't checked at compile time and typically don't do what you want at runtime.
- snprbob86 16y agoshrug You're probably right. I've never really used them in C++, especially because I tend to avoid C++ in favor of the smallest possible amount of C to bootstrap primary use of a much higher level language.
- cpeterso 16y agoAs mentioned, C++ exception specifications are checked at run-time, not compile-time. And if your function's exception specification declares `throws(FooException)` and you call some third-party library function (which may or may not have its own exception specification!) that throws a `BarException`, C++'s runtime checks will `terminate()` your program! C++ exceptions and exception specifications are pretty much all the worst possible design decisions. :(
- SoftwareMaven 16y agoRequiring all exceptions to be checked is evil. It leads to horrendous abstraction leakage (I need to let every caller know I use FooBarWidget in the core of my application??), improper error handling (just catch whatever comes out and move on so I don't have to declare it), or ubiquitous error wrapping (every class has a try...catch that turns the called exceptions into new exception types to throw). Ironic that C++ is moving towards that as the Java community is moving away (by relying more on RuntimeException, which isn't checked).
- ekiru 16y ago> It leads to horrendous abstraction leakage (I need to let every caller know I use FooBarWidget in the core of my application??), If your code catches any exceptions that might be thrown by its use of FooBarWidget, then you wouldn't need to specify it as an exception that your code might throw, right? If it doesn't, then your code exposes to callers that it uses FooBarWidget every time FooBarWidget throws an exception.
- Xurinos 16y agoI didn't see any timings in his code.
- bediger 16y agoYes. How can he possibly say that "exceptions are faster" without such timings? C++ style, unwind-the-stack-and-call-destructor exceptions must really have a high run-time cost when an exception occurs. Also, his instruction count misses the instructions that occur at run-time handling an exception. Even when an exception doesn't occur, it isn't obvious that some run-time code doesn't get excuted.
- SoftwareMaven 16y agoBut you have to unwind the stack either way. With exceptions, the exception handling does it. With error checking, you do it every time you type "if (something() == -1) return -1;"
- AshleysBrain 16y agoThis reads to me like 'writing good code is hard, and writing even better code is even harder'. With modern C++0x smart pointers (like unique_ptr) and RAII, you can get very high performance and (more) easily exception-safe code. Maybe a lot of C++ exception criticism comes from people still trying to code like C in C++?
- marshray 16y agoExactly.
- svlla 16y ago"modern C++" has changed meanings so many times in the past 10 years that it's not funny anymore. there's always some "modern" solution in C++ that ends up causing more problems, leading to further "modern" solutions that end up... you get my point. some folks have decided to get off the C++ feature treadmill and go back to, well... getting things done with solid languages (e.g. C) instead of learning about the latest C++ non-solutions to non-problems.
- AshleysBrain 16y agoHmm, well, I'm relatively young so I guess I missed all those broken promises :P Still, I think the "trying to code like C in C++" point still stands.
- bad_user 16y agoWell, if you're ever bored, read this book: http://www.amazon.com/Modern-Design-Generic-Programming-Patterns/dp/0201704315 http://www.amazon.com/Modern-Design-Generic-Programming-Patt...
- jerf 16y agoA compressed version can be obtained simply by comparing the initial release of C++ to modern C++, and recalling that the initial version of C++ itself shipped with, well, pretty much the same set of promises that modern C++ ships with.
- AshleysBrain 16y agoToday with terabyte harddrives, gigabytes of RAM and broadband connections, when is the binary size a more important factor than both execution speed and ease of development? Especially when the binary size difference is probably not huge? Shouldn't the advice of this article just be "use exceptions"?
- scott_s 16y agoNot all development targets desktops or laptops.
- svlla 16y agoCache sizes have not seen gains proportional to RAM or HDs.
- T-hawk 16y agoSure they have. My 486 built in 1993 had 8 KB cache, 4 MB RAM, and a 120 MB HD. My desktop built in 2009 has 2 MB cache, 2 GB RAM, and a 250 GB HD. Okay, the cache has lagged behind by one or three doublings compared to the other storage types. But that's still pretty close to proportional in a world of exponential gains.
- svlla 16y agoGood point :) Here's a different take, then, and probably harder to verify, but I am guessing is true: Cache utilization has increased much more than RAM or HD, not just because programs are handling more data but also because of increases in program size and number of programs being run simultaneously. Your hard drive is probably not full... RAM could be, depends on your workload... but I bet most caches are churning like mad, more than they used to be.
- silentbicycle 16y agoMemory speeds have increased much more slowly than processors have, so the cost of page faults, bad locality, etc. have grown proportionally worse over time. http://seven-degrees-of-freedom.blogspot.com/2009/10/latency-elephant.html http://seven-degrees-of-freedom.blogspot.com/2009/10/latency...
- Quarrelsome 16y agoNice article but as far as I'm concerned you don't need to _prove_ this as it is a logical fallacy to start with. If an exception is being thrown then something is wrong, if something isn't wrong then you implemented your exceptions incorrectly as exceptions shouldn't exist in normal program flow. So to recap, you're writing a crap ton of more code just so you can return your error code _slightly_ faster than it would take an exception. You're optimising your failure cases, which (in the _vast_ majority of cases) is UTTERLY ABSURD.
- deleted 16y ago[deleted]
- shin_lao 16y agoIt's not slightly faster, it can be an order of magnitude faster. Example: a listen loop which handles disconnections through exceptions. This isn't stupid but it's not very efficient.
- Quarrelsome 16y agoWhy is it not stupid? If disconnections are part of normal application flow then why would you use an exception? You are correct, I was somewhat disingenuous with _slightly_ faster. It is lots faster but lots faster in error cases, which from a philosophical angle is still absurd. As long as you use your exceptions for "bad shit" (uncommon error conditions or completely unexpected failures or returns) then I still strongly believe that the performance comparison is silly.
- anonymous246 16y agoMaybe I'm missing something. Are you saying that in your application, handling rarely-occuring unanticipated disconnections via exceptions has such high overhead that its results in unacceptable performance?
- aplusbi 16y agoI think you partially missed the point - it's not that throwing exceptions is slow, is that even having them in your code is [allegedly] slow. According to the article there are two methods used to implement exceptions in C++ - one that has higher overhead when you throw an exception (zero-cost) and one that has higher overhead when you call a function that might throw an exception (setjmp/longjmp). Unfortunately the author didn't go over the latter method, which would have been more interesting.
- pilif 16y agoI assume he was proving a point where main() of his full program using error code checking was not in fact checking the return value of foo() Even in simple example code like this you can forget a check. In this case that result would be undefined if any call to devide failed. I'd much rather have my program blow up with a readable stack trace pointing to where it happened than it working with a basically random value and then maybe blowing up somewhere totally unrelated or worse, destroying user data.
- davidsiems 16y agoYou can accomplish this by using asserts in your code. You don't need exceptions to get a callstack and you can assert that values are valid and force a crash / callstack dump when they're not. On top of that, you can compile the asserts out for release builds if you're confident they won't be hit.
- pilif 16y agoSure I can use asserts. But I'm as likely to forget the assert() as I am to forget to check the return value. And even if I did: If you consider the faulty main() in the linked article: How would you use assert() there to make sure that result as used after the call to foo() is actually usable? If foo() returns -1 (because any of the calls to divide returned -1) then result is undefined.
- davidsiems 16y agoYou would put an assert inside of the divide function like so: int divide (int x, int y) { defend (y != 0); return x / y; } Now divide is guaranteed to produce a correct result if it's called with correct data. It's up to the caller to make sure the data is correct, or an assert will happen. Just to finish out the example to show how much cleaner asserting is compared to error handling: int foo(void) { volatile int x = 4, y = 28; return divide(x, y) + divide(y, x); } int main () { return foo(); } That's not to say that error handling doesn't have its place, but it should only be used for data that you can't anticipate.
- JoeAltmaier 16y agoPerhaps the worst problem with checking-vs-exceptions is, either solution dominates your code structure, obscuring the algorithm logic. The holy grail would be some method of ensuring the code cannot fail e.g. weirdly constrained argument semantics. Thus separating algorithm from constraints instead of shuffling them together on the page like a deck of cards.
- johnny531 16y agoIf only c++ had some sort of static type system which could be leveraged to provide compile-time checks... But seriously, this is a large part of the power of c++'s type system. Taking the article's example, if the argument types were of (user class) 'non_zero_float', there's no possibility for error. You still have to check that your input is non-zero at some point, but you've now focused it into one place (the 'non_zero_float' class ctor), and other chunks of your program depending on those type semantics no longer need to worry about it.
- JoeAltmaier 16y agoYou can really make that type do a compile-time check on runtime values? It would be better to have some way of getting the compiler to optimize constraints, perhaps by proving at compile time that the error is impossible.
- adrianN 16y agoYou can't prevent exceptions when you do IO or dynamically allocate memory.
- gersh 16y agoA few issues. Does he actually benchmark? Things don't always work in practice the way you would think. If you are going for ultra-high performance, do you even have error-checking? Do you write it in assembler?