swift - When is my struct too large? -

we're encouraged use struct on class in swift.

this because

1. the compiler can lot of optimizations
2. instances created on stack lot more performant malloc/free calls

the downside struct variables copied each time when returning or assigned function. obviously, can become bottleneck too.

e.g. imagine 4x4 matrix. 16 float values have copied on every assign/return 1'024 bits on 64 bit system.

one way can avoid using inout when passing variables functions, swifts way of creating pointer. we're discouraged using inout.

so question:
how should handle large, immutable data structures in swift?
have worry creating large struct many members?
if yes, when crossing line?

this accepted answer not entirely answering question had: swift copies structs. trick array/dictionary/string/etc wrappers around classes (which contain actual stored properties). way sizeof(array) size of pointer class (memorylayout<array<string>>.stride == memorylayout<unsaferawpointer>.stride)

if have big struct, might want consider wrapping stored properties in class efficient passing around arguments, , checking isuniquelyreferenced before mutating give cow semantics.

structs have other efficiency benefits: don't need reference-counting , can decomposed optimiser.

in swift, values keep unique copy of data. there several advantages using value-types, ensuring values have independent state. when copy values (the effect of assignment, initialization, , argument passing) program create new copy of value. large values these copies time consuming , hurt performance of program.

https://github.com/apple/swift/blob/master/docs/optimizationtips.rst#the-cost-of-large-swift-values

also section on container types:

keep in mind there trade-off between using large value types , using reference types. in cases, overhead of copying , moving around large value types outweigh cost of removing bridging , retain/release overhead.