It’s very likely that you’ve been writing totally incorrect code without realizing it. While not a full solution, this is an extremely informal attempt at an overview of the problem.

1. Exception Safety and Garbage Collection and Some Other Stuff A Moderately Directed Rant

2. Why should I care about this? It’s very likely that you’ve been writing totally incorrect code without realizing it Once you do realize it, it’s usually not too hard to fix the problem, depending on the language This is information that isn’t all that widely known, for whatever reason You can use it to show off at interviews! You can use it to start arguments about which programming language is the best!

3. Boring Definitions We need to define a couple basic things at the beginning so everybody’s on the same page It’ll be quick, I promise Garbage collection is a method of managing dynamic (heap-allocated) memory It’s non-deterministic, and there is usually no guarantee that memory is cleaned up at all Most modern languages use garbage collection

4. More Boring Definitions A resource is anything you need to return to the system once you’re done using it File handles, dynamic memory, locks, etc. Exception safety means that you can throw an exception in the middle of your function without bad things happening There’s a complicated formal definition with degrees of exception safety but this is good enough for our purposes

5. Reasonable Code Example (C) Let’s look at some C code so we can figure out what this talk is even about voidexample() { lock(&g_mutex); int* my_int_pointer = (int*)malloc(sizeof(int)); do_something_with(my_int_pointer);free(my_int_pointer); unlock(&g_mutex); } This is fairly reasonable, safe C code. It executes deterministically and everyone is happy

6. Terrible Code Example (C++) Let’s see that exact same code, but now we’ll pretend that it was compiled as C++ voidexample() { lock(&g_mutex); int* my_int_pointer = (int*)malloc(sizeof(int)); do_something_with(my_int_pointer);free(my_int_pointer); unlock(&g_mutex); } Catastrophe! This is going to compile and run without warnings, but be completely and totally unsafe! Why? Exceptions!

7. That Sucks! Yes, it does suck! It’s such a problem that people were motivated to go try to solve it BjarneStroustrup (C++ language creator) came up with a solution which he named Resource Acquisition Is Initialization (RAII) Incidentally, in addition to providing exception safety, RAII made C++ way easier to use Let’s look at a correct C++ version of our code example, using RAII

8. Modern Code Example (C++) voidexample() { Lockmy_lock(&g_mutex); auto_ptr<int> my_int_pointer(new int()); do_something_with(my_int_pointer); } Thanks to RAII this example is exception safe, and we don’t have to worry about cleanup. auto_ptr<T> is part of the C++ standard library, but we’ve just made up Lock Let’s look at the code for our made-up Lock class so we can see how RAII actually works

9. How RAII Actually Works classLock { private: mutex* m_pMutex; public: Lock(mutex* pMutex) : m_pMutex(pMutex) { lock(m_pMutex); } ~Lock() { unlock(m_pMutex); } }; In C++ a stack-allocated object’s destructor is always called once it goes out of scope, whether due to a function returning, due to normal code execution, or due to stack unwinding caused by a thrown exception

10. Sure, but I don’t use C++ That’s understandable. We are (for better or worse) a Java school, so let’s see if we can’t make RAII work in Java Immediately we run into some problems Java doesn’t have destructors Java doesn’t have stack allocation for objects So RAII won’t work with Java, then. What else have we got? For dynamic memory we have garbage collection, but that’s a special case of the problem that doesn’t really need (or provide) determinism The best we can do is the Dispose pattern

11. The Dispose Pattern (Java) voidexample() { IntegermyInteger = newInteger(0); Locklock = newLock(g_mutex); try { doSomethingWith(myInteger); } finally { lock.dispose(); } } While rewriting this every time gives you exception safety, it’s really easy to forget it If you forget to do this, your program will still compile and run with no warnings, despite being wrong. Awesome! This is more verbose than even the C example, yet is the minimum amount of code required for Java

12. More Dispose Loveliness (Java) voidexample() { FilemyFile = newFile(filename); try { DBConnectiondbConn = newDBConnection(credentials); try { LockmyLock = newLock(g_mutex); try { doSomething(myFile, dbConn); } finally { myLock.dispose(); } } finally { dbConn.dispose(); } } finally { myFile.dispose(); } } This is again the minimum code required to be correct

13. Again, but with RAII (C++) voidexample() { FilemyFile(filename); DBConnectiondbConn(credentials); LockmyLock(&g_mutex); doSomething(myFile, dbConn); }

14. One More Time (D) voidexample() { scopemyFile = newFile(filename); scopedbConn= newDBConnection(credentials); scopemyLock = newLock(g_mutex); doSomething(myFile, dbConn); }

15. “You can take my deterministic resource management when my cold dead hand goes out of scope.” -- Anon

16. Why does Java suck so bad? By choice. You can have deterministic resource management alongside garbage collection, but the Java guys specifically chose not to The D programming language supports RAII and has a garbage collector, so it’s definitely possible Java, C#, Python, Ruby all screw this up to varying degrees The latter three have some syntactic sugar for resource management, but the onus is still on you to remember to use it Java 7 catches up with C# and adds the same syntactic sugar, but still doesn’t solve the problem Perl, PHP, C++ and D all get it right to varying degrees If PHP gets something right before your language does, you should reassess your life goals

17. So what should I do? My (unpopular) answer? Use C++ and Perl/PHP for everything until the D ecosystem matures a bit, then switch over to D entirely C++ has its own set of problems, but it’s my opinion that they’re exaggerated and the benefits far outweigh them If you’re stuck using a broken language like Java, I really don’t know what to tell you I guess you could cry a little bit, but I don’t think it would solve the problem Learn the Dispose pattern, always remember to use it

18. Okay, done ranting I wish I had more helpful advice for Java, C#, Python, Ruby users, but this is the unfortunate state we find ourselves in If you want more information about anything mentioned: The D Programming Language by Andrei Alexandrescu is an excellent D introduction If you want to learn how to code modern C++, you should read Effective C++ by Scott Meyers The Boost website has good information about exception safety and reference-counted smart pointers, which I didn’t really talk about (Scott Meyers does in Effective C++) Google knows all Questions?