When can classes be stored in stack memory?

In Swift, classes, being reference types, are always stored in heap memory. The stack is mainly used for storing value types (structures, enumerations, basic types), as well as references to objects stored in the heap.

Although classes themselves are not stored on the stack, a reference to an instance of a class can be stored on the stack, for example, as a local variable inside a function.

There are specific cases where the optimizer might place an instance of a class on the stack (so-called Stack Allocation), but this is not standard behavior and depends on complex compiler heuristics. This occurs only with very simple classes whose lifetime is fully known at compile time and that are not used outside of the local scope.

class MySimpleClass {  
    var value: Int = 0  
  
    init(value: Int) {  
        self.value = value  
    }  
}  
  
func exampleFunction() {  
    // 'instance' is a reference to MySimpleClass,  
    // stored on the stack.  
    // The actual MySimpleClass instance is stored in the heap.  
    let instance = MySimpleClass(value: 42)  
    print(instance.value)  
}