Using ARC in tweaks
What is ARC?
In Objective-C and Swift programming, Automatic Reference Counting (ARC) is a memory management enhancement where the burden of keeping track of an object's reference count is lifted from the programmer to the compiler. In traditional Objective-C, the programmer would send retain and release messages to objects in order to mark objects for deallocation or to prevent deallocation. Under ARC, the compiler does this automatically by examining the source code and then adding the retain and release messages in the compiled code.
How should I use ARC in tweaks?
It is not recommended that you use ARC in any "hooks", unless if you absolutely know what you are doing. Hook methods are slightly different from your typical Objective-C class declarations, and as such, the compiler has a more limited concept of ownership due to the nature of these hooks. In most cases it would be totally fine, but there *are* certain cases where you would get unexpected behavior, so in general it ends up being less of a headache if you avoid ARC altogether. Also, typically with hook methods, the compiler sees you passing objective-C objects to and from vanilla C methods which requires (__bridge) casts which ends up being more annoying than manually managing memory.
For class declarations
If you are declaring new classes, or subclassing existing ones, then ARC will behave exactly as one would expect it to behave and it would be totally fine to use.
However, even though using ARC for class declarations is A-OK, people still consider it to be a bad idea to use it *at all*. Generally, you *never* use ARC in your hook methods, so there ends up being this disconnect between your different implementation files, and the programmer ends up forgetting which files use ARC and which files don't. This inevitably leads to memory leaks. One example of this is when you write some code in a file that you thought uses ARC (but actually doesn't).
So the gist of this is, for most cases: it's more trouble than it's worth.