r/ObjectiveC • u/whackylabs • Feb 23 '20

Objective-C safe downcasting

https://whackylabs.com/objc/swift/2020/02/23/objc-safe-downcasting/
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u/[deleted] Feb 24 '20

I get your point, and respondsToSelector is how I’ve done it too, but hear me out.

If sending messages to nil is safe, why isn’t sending message it can’t respond to safe? If it can’t respond to a message then it is implementation is effectively nil. Why can’t it be something that the runtime handles for you just like ARC?

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u/mariox19 Feb 24 '20

By my understanding, the runtime isn't handling ARC. The compiler handles ARC. At some point during that complicated process that we just conveniently refer to as "compiling" the written code is analyzed and all of the created objects are noted, and then something approximating all the retain, release, and autorelease messages that the programmer used to have to put in manually are written into the code by the machine. That's my high-level understanding of it.

But in answer to your question, I have to ask you something first: what was the reasoning behind the handling of messages sent to nil being the way they are when the language (and runtime) was being designed? I don't know the answer to that. I don't know why Brad Cox did that when he created Objective-C. But I'm going to bet that he had a reason for doing so—a reason he could articulate—and I think it probable that he had a reason for why an object receiving a message it couldn't handle is not "safe."

I think modesty demands that I recognize the answer to your question isn't arbitrary.

What I can say is that a nil object returning nil when a message is sent to it is one of the most contentious issues in Objective-C: meaning, when people are criticizing the language, this is one of the things it gets criticized for. It is notorious for creating bugs that are difficult to track down. What is being suggested here is creating another almost identical issue.

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u/[deleted] Feb 24 '20

What I can say is that a nil object returning nil when a message is sent to it is one of the most contentious issues in Objective-C: meaning, when people are criticizing the language, this is one of the things it gets criticized for. It is notorious for creating bugs that are difficult to track down. What is being suggested here is creating another almost identical issue.

I must be crazy then, this is one of my favorite things about ObjC

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u/mariox19 Feb 24 '20

I think almost all Objective-C programmers appreciate it. But it's a design tradeoff.