Cannot convert value of type 'Measurement<UnitLength>' to type 'Measurement<-CodePudding

I have the struct Vector:

struct Vector{
var X : Measurement<Dimension>
var Y : Measurement<Dimension>
var Z : Measurement<Dimension>
...

And I create new object, for example:

let test = Measurement(value: 1.5, unit: UnitLength.inches) as Measurement<Dimension>
var lVector = Vector(x: Measurement(value: 10, unit: UnitLength.meters), y: test, z: Measurement(value: 10, unit: UnitLength.meters))

Everything works fine. But if try to use variable from other class, I got the error: "Cannot convert value of type 'Measurement' to type 'Measurement' in coercion"

final class SettingsManager{
  ...
 var test  = Measurement(value: 1.5, unit: UnitLength.inches)
  ...  }
class Calculator {
 ...
 let test = SettingsManager.shared.test as Measurement<Dimension>
 var lVector = Vector(x: Measurement(value: 10, unit: UnitLength.meters), y: test, z: Measurement(value: 10, unit: UnitLength.meters))

I tried "as!" and got

Cast from 'Measurement<UnitLength>' 
to unrelated type 'Measurement<Dimension>' 
always fails

The same variable declared in this class works fine as I showed above. What I did wrong?

CodePudding user response：

Use

var test : Measurement<Dimension>  = Measurement(value: 1.5, unit: UnitLength.inches)

CodePudding user response：

A Measurement<UnitLength> is not a kind of Measurement<Dimension>. These are unrelated types. It only works here:

let test = Measurement(value: 1.5, unit: UnitLength.inches) as Measurement<Dimension>
var lVector = Vector(x: Measurement(value: 10, unit: UnitLength.meters), y: test, z: Measurement(value: 10, unit: UnitLength.meters))

because the Measurement(...) calls here are actually creating Measurement<Dimension>s, since that is what is being expected at the callsite. as Measurement<Dimension> tells it that you want a Measurement<Dimension>, and the parameter type of x and z are also Measurement<Dimension>. The type inference algorithm is smart enough to see that you must also mean Measurement<Dimension>(...).

Measurement<Dimension>.init takes a Dimension as its second parameter, and UnitLength is a subtype of that, so no problems there.

On the other hand, in SettingsManager, you declared test like this:

var test = Measurement(value: 1.5, unit: UnitLength.inches)

Nowhere here did you mention Measurement<Dimension>, so the type inference algorithm just uses the second parameter to infer that you must mean Measurement<UnitLength>, and so the type of test is Measurement<UnitLength>.

If you just add as Measurement<Dimension>

var test = Measurement(value: 1.5, unit: UnitLength.inches) as Measurement<Dimension>

it should work.

However, wouldn't it make more sense to for all three components of the vector to have the same type of unit?

struct Vector<T: Dimension>{
    var X : Measurement<T>
    var Y : Measurement<T>
    var Z : Measurement<T>
}