Search code examples
swiftvariablesweak-references

Why can the keyword "weak" only be applied to class and class-bound protocol types

When I'm declaring variables as weak in Swift, I sometimes get the error message from Xcode:

'weak' may only be applied to class and class-bound protocol types

or

'weak' must not be applied to non-class-bound 'SomeProtocol'; consider adding a protocol conformance that has a class bound

I'm wondering why the keyword weak can only applied to class and class-bound protocol types? What is the reason behind this requirement?

Solution

  • weak is a qualifier for reference types (as opposed to value types, such as structs and built-in value types).

    Reference types let you have multiple references to the same object. The object gets deallocated when the last strong reference stops referencing it (weak references do not count).

    Value types, on the other hand, are assigned by copy. Reference counting does not apply, so weak modifier does not make sense with them.