Cast Any to Float always fails in swift4.1

Question

In the former version, to get a float value from a [String: Any] dictionary, I can use let float = dict[\"somekey\"] as? Float, but in swift4.1, it

Answer 1

You need to distinguish two cases (in Swift 3, it was three cases):

• Any containing a Swift native Double
• Any containing an NSNumber

(In Swift 3, there was type preserving NSNumber case other than the normal NSNumber.)

---

When you create a native Swift Dictionary such as [String: Any], and set Double value in a normal way like this in your update:

let dict: [String : Any] = ["key": 0.1]

In this case, Any holds the metadata representing Double and the raw value 0.1 as Double.

And casting Any holding Double to Float always fails. As Double to Float cannot be converted with as-castings.

let dblValue: Double = 0.1
if let fltValue = dblValue as? Float { //<-Cast from 'Double' to unrelated type 'Float' always fails
    print(fltValue)
} else {
    print("Cannot convert to Float")
}
//->Cannot convert to Float

---

But, in case of Any holding NSNumber, as always happens when the Array or Dictionary is bridged from NSArray or NSDictionary, the behaviors are different between former Swifts and Swift 4.1.

The result of JSONSerialization matches this case.

In former Swifts, (normal) NSNumber to Float was an always-success operation.

And in Swift 4.1, the behavior has changed with the new feature which I have shown in my comment:

SE-0170 NSNumber bridging and Numeric types

---

I omit the third case once found in Swift 3, it's past.

But it is very important how you get your Dictionary or Array, and how you set the value to them, to solve the issue Any to Float.

---

Finally again, is this a new feature or a bug? If someone knows, can you guys show up the announcement link?

This is a new feature, not a bug. See the link above, and related threads below.

Unable to bridge NSNumber to Float Swift 3.3

Unexpected behavior when casting an NSNumber to Float