A Problem of Neo.VM.Types.Struct

[nan01ab]

There are several builtin types in Neo-VM, and each type has its own characteristics(about Mutability, Reference or Value). For examples:

• Boolean: Immutable
• Integer: Immutable
• ByteString: Immutable
• InteropInterface: Immutable
• Pointer: Immutable
• Buffer: Reference Type and Mutable
• Array: Reference Type and Mutable

But characteristics of Struct are somewhat ambiguous(or unclear).

According to doc from https://developers.neo.org/docs/n3/Advances/Neo%20VM%20instructions, the semantics of Neo-VM builtin types are affected by some types in C#.
Struct Inherited from Array, a Clone method is added, and Equals is not supported.

From some running logic of the Neo-VM, it seems that Struct should be a value type. For examples:

• When a Struct add to an Array or a Map(by Append, SetItem), it will be cloned;
• When a Struct picked from an Array or a Map(by Values), it will be cloned too.
• When a Struct is deep copied , it's sub Struct will deep copied too.

But from other logic, struct seems to be a reference type, For examples:

• Dup a Struct does not get a copy of the original Struct.
• PickItem is similar to Dup operation.

so if a dupped or picked Struct is mutated by SetItem, ClearItems or PopItem, the original Struct is mutated too(because Struct Inherited from Array, swtich (x) { case Array _: ... } or if (x is Array) will be true, a struct can be mutated by SetItem, ClearItems or PopItem).

So, is Struct value type or reference type?
If Struct is value type, why not copied when Dup, PickItem or ...?
If Struct is reference type, why needs to copy when Append, Values or ..?

[Jim8y]

I think DUP for all types are like ref type..

[Jim8y]

@shargon @roman-khimov is struct be of any real use that can not be replaced with Array? isn't array already sufficient?

[roman-khimov]

It's special and mostly follows its primary use cases. Technically it can be emulated by compilers with arrays (although deep copying is not really fun and will burn some GAS), but at the same time this type is being currently used (at least by NeoGo, likely other compilers as well, @gsmachado, @ixje) and I just wouldn't touch it unless there is some huge problem (implicit copies can be problematic, but we hope most of them were solved previously).

[shargon]

I always thought that we don't need the structs, but, now it's hard to remove

[gsmachado]

I always thought that we don't need the structs, but, now it's hard to remove

Same here: I agree ☝️ .
And yes, it'd be a pain to remove.

[nan01ab]

May NEO can re-explainDup and PickItem:

• Dup creates a reference of the stack top;
• PickItem returns the reference of the item;