[my-next-account]

1) In practice, this is not true, especially if you implement unwinding in your arena.

You probably don't want to have an Arena in main, and you do all of your allocations from there, for example. That "just" leaks everything.

Here's a classic Arena-with-rewind bug:

  {
    Arena a;
    avec<int> v(a);
    {
      RewindMark rm(a);
      v.push(1); v.push(1); // Trigger resize
    }
    v[2] // Oh no! The underlying data array has been deallocated
  }

[tardedmeme]

Thanks. This is a good illustration of how data structures aren't cleanly separable from memory management strategies.

[eska]

I’m sorry, but I look at this code and I can immediately see the bug? Instead of a rewind mark I personally create a local arena from the outer arena. If any allocations leak to the parent scope I put the code in a local function that takes the parent arena as a parameter. Again, if you’re so inclined you can use Rust or C++ to create smart pointers that catch this at runtime or even compile time, it’s just not a real issue for me at this point, reducing the value of complicated compiler machinery leading to bad build times.

[my-next-account]

That's right! I made the example in such a way that it is obvious what the bug is, for pedagogical reasons :-). This particular bug is a bit trickier to spot when you have more complex programs, and is something that I have seen in real (large) codebases. Your inner/outer arena sounds exactly like the same thing, though?

Generally, it's best to avoid the rewind trick, IMO, it makes it difficult to create composable programs.