Can a Swift struct be allocated on the Heap when it's a stored property?

Question

One of the great things about structs is that they are allocated on the stack which make them really fast.

However, an instance of a class and its properties are heap allocated. So wouldn't a property that's a struct be allocated on the heap as well?

Answer 1

Short answer, yes, structs that are declared as stored instance properties are allocated in the heap, because the object storage is already in the heap.

In Swift any value type (structs included) is allocated on the memory location where it's declared:

• local variables end up on the stack^[1]
• variables captured by escaping closures need to live on the heap, otherwise by the time the closure executes the stack might be used for something else
• instance properties are allocated on heap, along with the rest of the properties
• global variables and class/static members, well they get a different treatment because they're lazy: https://railsware.com/blog/2014/06/11/global-variables-in-swift-are-not-variables/
• value types passed trough existential containers (aka as protocols) and occupy more than 3 words, get copied on the heap, even if they start on the stack
• value types assigned to other locations will change their storage from stack to heap, or heap to stack, if the destination is stored on a different memory type.