Exception Safety and Garbage Collection and Some Other Stuff

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

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!

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

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

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

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!

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

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

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

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

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

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

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

One More Time (D)
voidexample() {
scopemyFile = newFile(filename);
scopedbConn= newDBConnection(credentials);
scopemyLock = newLock(g_mutex);
}

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

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

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

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?

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Exception Safety and Garbage Collection and Some Other Stuff

by eplawless on May 06, 2011

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