Struct and class

[taylorswift]

@macnib

If the struct is small and contains value types it will be faster. (as suggested earlier) From what I’ve read/studied that appears to be the case. If the struct has a reference type within it, then you might want to box that reference in a struct and avoid a allocation if the struct its a member of is copied.

false. my post in this thread explains more

@macnib

Its my understanding the justification to use structs is to avoid heap allocations – structs use stack memory. This memory is already allocated. So, the position of the top of the stack is just incremented. This is extremely fast. This is the main argument to use structs.

not the main, but a significant argument

@macnib

But ( here is the catch and confusion for me ) if the Swift struct is too big, then Swift is going to put part of it into the heap anyways – well thats locking and slow down.

okay let’s clear things up, you can totally put structs in the heap. just allocate(capacity:) space for it in memory and initialize it. swift native Arrays are also heap buffers (actually they use a weird tail-allocation thing where the array’s own metadata is stored in the same buffer) and if Element is a struct, it will be stored in the heap

@macnib

( Swift makes tables internally that reference the heap and is transparent )

you’re probably thinking of protocol type containers. completely different animal

@macnib

I recollect it might have to do with the size of a stack frame which limits what can be stuffed into the stack?

??

@macnib

For instance, if I have a struct with 100,000 value type Doubles in it and I change just one of those Doubles, then the entire struct will get copied (Value Type) not only on the stack but the heap too because its a “big” struct?

i’m sure Swift is smart enough to do lvalue assigments in place, and internal structs are transparent to the compiler so i’d be shocked it it wasn’t smart enough to put the relevant pieces of structs into registers and only write to them in memory when necessary

the important word in the sentence “mutation is modeled as copyback assignment” is “modeled”.

@macnib

Also, I’ve heard Arrays are value types but internally they are really referenced.

this is correct.

@macnib

Like if I have a buffer pointer I’m passing to c that is an Array in Swift.

you’re confusing swift’s c interop with withUnsafeMutableBufferPointer (although they do similar things)