Casting Operators
Static casting operator (as)
The static casting operator as can be used to statically type cast a value to a type.
If the static type of the value is a subtype of the given type (the target type), the operator returns the value as the given type.
The cast is performed statically (i.e., when the program is type-checked). Only the static type of the value is considered — not the run-time type of the value.
This means it is not possible to downcast using this operator. Consider using the conditional downcasting operator as? instead.
_22// Declare a constant named `integer` which has type `Int`._22//_22let integer: Int = 1_22_22// Statically cast the value of `integer` to the supertype `Number`._22// The cast succeeds, because the type of the variable `integer`,_22// the type `Int`, is a subtype of type `Number`._22// This is an upcast._22//_22let number = integer as Number_22// `number` is `1` and has type `Number`_22_22// Declare a constant named `something` which has type `AnyStruct`,_22// with an initial value which has type `Int`._22//_22let something: AnyStruct = 1_22_22// Statically cast the value of `something` to `Int`._22// This is invalid, the cast fails, because the static type of the value is type `AnyStruct`,_22// which is not a subtype of type `Int`._22//_22let result = something as Int
Conditional downcasting operator (as?)
The conditional downcasting operator as? can be used to dynamically type cast a value to a type. The operator returns an optional. If the value has a run-time type that is a subtype of the target type the operator returns the value as the target type; otherwise, the result is nil.
The cast is performed at run-time (when the program is executed) and not statically (when the program is checked).
_17// Declare a constant named `something` which has type `AnyStruct`,_17// with an initial value which has type `Int`._17//_17let something: AnyStruct = 1_17_17// Conditionally downcast the value of `something` to `Int`._17// The cast succeeds, because the value has type `Int`._17//_17let number = something as? Int_17// `number` is `1` and has type `Int?`_17_17// Conditionally downcast the value of `something` to `Bool`._17// The cast fails, because the value has type `Int`,_17// and `Bool` is not a subtype of `Int`._17//_17let boolean = something as? Bool_17// `boolean` is `nil` and has type `Bool?`
Downcasting works for concrete types, but also works for nested types (e.g., arrays), interfaces, optionals, and so on.
_10// Declare a constant named `values` which has type `[AnyStruct]`,_10// i.e. an array of arbitrarily typed values._10//_10let values: [AnyStruct] = [1, true]_10_10let first = values[0] as? Int_10// `first` is `1` and has type `Int?`_10_10let second = values[1] as? Bool_10// `second` is `true` and has type `Bool?`
Force-downcasting operator (as!)
The force-downcasting operator as! behaves like the conditional downcasting operator as?. However, if the cast succeeds, it returns a value of the given type instead of an optional; if the cast fails, it aborts the program instead of returning nil:
_17// Declare a constant named `something` which has type `AnyStruct`,_17// with an initial value which has type `Int`._17//_17let something: AnyStruct = 1_17_17// Force-downcast the value of `something` to `Int`._17// The cast succeeds, because the value has type `Int`._17//_17let number = something as! Int_17// `number` is `1` and has type `Int`_17_17// Force-downcast the value of `something` to `Bool`._17// The cast fails, because the value has type `Int`,_17// and `Bool` is not a subtype of `Int`._17//_17let boolean = something as! Bool_17// Run-time error
Implicit casting
Cadence does not allow implicit casting (coercion).
_10let value: UInt8 = 1_10_10// invalid: implicit cast_10let intValue: Int = value
Instead, conversion must be explicitly performed by calling a conversion function. Cadence provides a conversion function for each number type. The functions have the same name as the type, accept any number, and return the number type. If the value cannot be converted, the function panics.
For example, for the type Int, the conversion function fun Int(_ number: Number): Int is provided.
For example, to convert a UInt8 value to an Int value:
_10let value: UInt8 = 1 _10let intValue = Int(value)
See Number type casting for more information.