undefined

points

[-]

This is a general problem with destructors, you can't "batch delete" objects. To free a lot of stuff you're required to go pointer by pointer through the tree to clean up each object. To get real performance gains from pools you can't have per-object/subobject custom cleanup code.

by aabhay22 hours ago|

parent|

[-]

Not necessarily. Drop semantics are just syntactic sugar, and can thus be aggressively inlined or auto vectorized by the compiler.

by jstimpfle1 days ago|

prev|

[-]

I've argued elsewhere some things that are wrong with RAII and C++ objects in general.

Here I would just like to mention that if you have to rely on "de-virtualization" passes, you're in a miserable situation architecturally. If you have code where the overhead of virtual function calls might be too much to pay, don't do virtual functions then. End of story.

To deconstruct a pool of objects, I don't see what should ever be wrong with a function pointer. The overhead of loading the function pointer will get divided by the number of objects being deconstructed. Care to explain what's the issue here?

by hugmynutus1 days ago|

parent|

[-]

> I don't see what should ever be wrong with a function pointer. [...]Care to explain what's the issue here?

1. You're writing code you don't have to

2. That adds runtime overhead

3. That when you screw up has non-trivial security & resource management side effects

This is objectively indefeasible in nearly any vaguely professional context.

by jstimpfle1 days ago|

parent|

[-]

1. No, you're not writing code you don't have to. It's not different to implementing this as non-virtual methods, in fact I'd argue doing simple functions is more straightforward.

2. And the code being compiled is abstract & generic, it won't be instantiated for every type and bloat the executable or instruction cache.

3. Security concerns: With C++ virtual methods every object carries a mutable pointer too (to a vtable containing function pointers). What resource management side effects please?

by kllrnohj1 days ago|

parent|

[-]

Re #3: vtable pointers aren't mutable...?

by jstimpfle21 hours ago|

parent|

[-]

Of course they are. The pointers to the vtable are part of the object. They aren't mutable fields as per the language, but for security concerns it doesn't matter what the language thinks. Being part of the object, the vtable pointer has to live in a writeable memory mapping (like stack / heap).

by Conscat18 hours ago|

parent|

[-]

Clang pointer authentication makes any type of vtable attack impossible in C++.

by jstimpfle17 hours ago|

parent|

[-]

Fair enough, this is an extension though and I suppose you could use it with manually constructed vtables as well?

by kllrnohj16 hours ago|

parent|

prev|

[-]

Then I don't understand your argument. If you're just saying what could go wrong with heap corruption, then your vtable complaint also applies to storing function pointers in arena allocators in Zig? Zig doesn't have anything special here?

by jstimpfle13 hours ago|

parent|

[-]

I asked you what's wrong with pool destructor function pointers. You gave a reason what's wrong and I refuted it. So no, I'm not saying that storing a function pointer is special, just that nothing's wrong with it. (And implying that since there's nothing wrong and it's probably the most straightforward thing to do, it's also probably the right thing).