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It's a cool anecdote. It's also a case study in heavyweight copies being something that shouldn't happen by default, and should require explicit annotation indi

It's a cool anecdote. It's also a case study in heavyweight copies being something that shouldn't happen by default, and should require explicit annotation indicating that the engineer expects a heavyweight copy of the entire structure.

mhlakhani 2y ago
I don’t know if that would have helped here, if memory serves me right: 1. The copy was needed initially 2. This structure wasn’t as heavy back then … over time the code evolved so it became heavy and the copy became unnecessary. That’s harder to find without profiling to guide things
ehsankia 2y ago
If it's safety/correctness versus performance, I think the default should be the former. Copying, while inefficient is generally more correct and avoids hard-to-debug errors. It's the whole discussion about premature optimization. I'd rather make a copy than make sure the array is not mutated anywhere ever.
- ltbarcly3 2y ago
  Yes, everyone agrees with you. The claim you responded to was that you should have to be explicit, because it is very easy to unintentionally copy. For example, it is easy to copy when there is never more than one live pointer to a datastructure. It's easy to copy when you allocate a resource in a function and return it, which makes the original an orphan which is then immediately freed. It's extremely easy to make a mistake which prevents move from working and you have to go back and carefully check if you want to be sure. It should be trivial to just say "move this" and if something isn't right it's an error at compile time, rather than just falling back to silently being wasteful.
- School-Cotton 2y ago
  This exact problem is basically why Rust exists.
- JoshTriplett 2y ago
  I'm not saying it should silently alias any more than it should silently copy. It should give an error, and require the developer to explicitly copy or explicitly alias.

mhlakhani 2y ago

I don’t know if that would have helped here, if memory serves me right: 1. The copy was needed initially 2. This structure wasn’t as heavy back then … over time the code evolved so it became heavy and the copy became unnecessary. That’s harder to find without profiling to guide things

ehsankia 2y ago

If it's safety/correctness versus performance, I think the default should be the former. Copying, while inefficient is generally more correct and avoids hard-to-debug errors. It's the whole discussion about premature optimization. I'd rather make a copy than make sure the array is not mutated anywhere ever.

ltbarcly3 2y ago
Yes, everyone agrees with you. The claim you responded to was that you should have to be explicit, because it is very easy to unintentionally copy. For example, it is easy to copy when there is never more than one live pointer to a datastructure. It's easy to copy when you allocate a resource in a function and return it, which makes the original an orphan which is then immediately freed. It's extremely easy to make a mistake which prevents move from working and you have to go back and carefully check if you want to be sure. It should be trivial to just say "move this" and if something isn't right it's an error at compile time, rather than just falling back to silently being wasteful.
School-Cotton 2y ago
This exact problem is basically why Rust exists.
JoshTriplett 2y ago
I'm not saying it should silently alias any more than it should silently copy. It should give an error, and require the developer to explicitly copy or explicitly alias.