Swift and mutating struct

Question

There is something that I don\'t entirely understand when it comes to mutating value types in Swift.

As the \"The Swift Programming Language\" iBook states:

Answer 1

A structure is an aggregation of fields; if a particular structure instance is mutable, its fields will be mutable; if an instance is immutable, its fields will be immutable. A structure type must thus be prepared for the possibility that the fields of any particular instance may be mutable or immutable.

In order for a structure method to mutate the fields of the underlying struct, those fields have to be mutable. If a method that mutates fields of the underlying struct is invoked upon an immutable structure, it would try to mutate immutable fields. Since nothing good could come of that, such invocation needs to be forbidden.

To achieve that, Swift divides structure methods into two categories: those that modify the underlying structure, and thus may only be invoked on mutable structure instances, and those that do not modify the underlying structure and should thus be invokable on both mutable and immutable instances. The latter usage is probably the more frequent, and is thus the default.

By comparison, .NET presently (still!) offers no means of distinguishing structure methods that modify the structure from those that don't. Instead, invoking a structure method on an immutable structure instance will cause the compiler to make a mutable copy of the structure instance, let the method do whatever it wants with it, and discard the copy when the method is done. This has the effect of forcing the compiler to waste time copying the structure whether or not the method modifies it, even though adding the copy operation will almost never turn what would be semantically-incorrect code into semantically-correct code; it will merely cause code that is semantically wrong in one way (modifying an "immutable" value) to be wrong in a different way (allowing code to think it's modifying a structure, but discarding the attempted changes). Allowing struct methods to indicate whether they will modify the underlying structure can eliminate the need for a useless copy operation, and also ensures that attempted erroneous usage will get flagged.