Writing an OS in Rust: Advanced Paging

Re:Writing an OS in Rust: Advanced Paging by mehrdadn on Monday January 28, 2019 @05:37PM (#19021742)

Huh? No. You can perfectly fine have a constructor that just leaves the object in an error state if it fails to acquire its resources. And a destructor that doesn't destroy unless acquisition is successful. No need for init(), though you can have that too. And your "arguably" is not arguable, it's just plain wrong. You don't have to successfully acquire something on every single invocation in order to have RAII. Otherwise e.g. initializing a smart pointer with null would suddenly imply smart pointers are not RAII...

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Re:Writing an OS in Rust: Advanced Paging by Retra on Monday January 28, 2019 @07:26PM (#19022576)

This is the first paragraph on wikipedia:

>In RAII, holding a resource is a class invariant, and is tied to object lifetime: resource allocation (or acquisition) is done during object creation (specifically initialization), by the constructor, while resource deallocation (release) is done during object destruction (specifically finalization), by the destructor. Thus the resource is guaranteed to be held between when initialization finishes and finalization starts (holding the resources is a class invariant), and to be held only when the object is alive. Thus if there are no object leaks, there are no resource leaks.

All of this is violated if you are constructing objects in an 'invalid' state. The whole point of RAII is that you don't do this. If you have to check if the object is in an error state at the start of every method call, then you're not benefiting from RAII.

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Re:Writing an OS in Rust: Advanced Paging by mehrdadn on Monday January 28, 2019 @07:34PM (#19022612)

No, you're just arguing and misinterpreting things for the sake of arguing. RAII doesn't mean you absolutely have to hold a resource all the time. Like I said, nobody (other than you) says e.g. shared_ptr isn't RAII just because it can be in a null state that holds no resources. If you understand RAII you know that no benefit of RAII goes out any window if a smart pointer or some other object is initialized in a state that happens to hold nothing.

If you want to redefine things to be different just to win an argument, you can, but it's not something I'm going to continue entertaining as I'm pretty tired of it now.

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Re:Writing an OS in Rust: Advanced Paging by Retra on Tuesday January 29, 2019 @09:04AM (#19026313)

Look, I said it was arguably not RAII, so I can see how one could make the point that it still works if you do it that way. But you saying "I'm just plain wrong" to say that RAII actually means resources must be allocated on initialization... I don't know what to tell you. That's literally what the acronym means.

And just because some low-level pointer primitive allows itself to have a null state doesn't mean your Files and DBConnections, etc., should have free reins to do the same.

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