> On Jul 6, 2016, at 9:54 AM, Alexey Komnin via swift-users
> <[email protected]> wrote:
>
> Here is the code:
>
> let a: String = "abc"
> let b: NSString = "abc"
>
> assert(a == b)
> assert(a.hashValue == b.hashValue, "a.hashValue(\(a.hashValue)) !=
> b.hashValue(\(b.hashValue))”)
Try comparing
(a as NSString).hashValue == b.hashValue
or
a.hashValue == (b as String).hashValue
It’s true that equal objects should have equal hash codes, but that really only
applies when the objects have the same type so there’s no implicit coercion
going on. For example, in scripting languages where “1” == 1 (pretty sure this
is true of JavaScript and Perl, for example) it’s probably not true that
“1”.hashCode == 1.hashCode.
In a Swift Dictionary, if you give the item type as String or NSString then
everything’s converted to the same type when added, so hashCode will work as
you expect. And if you use a Dictionary<AnyObject>, there won’t be any implicit
String<->NSString conversions going on, so if you added both your ‘a’ and ‘b’
objects I believe they would be stored separately as distinct keys.
(The moral is that the distinction between String and NSString is kind of
messy, unfortunately. But the implicit conversion has been pretty important for
Swift adoption given how ubiquitous NSString is in Cocoa APIs and existing
apps.)
—Jens
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