Upcast vs downcast6/16/2023 Upcasting is applied automatically when you pass arguments to methods on an object type. In many object-oriented languages, upcasting is implicit in F#, the rules are slightly different. The :> operator casts up the hierarchy, and the :?> operator casts down the hierarchy. Casting down a hierarchy, from a base object reference to a derived object reference, succeeds only if the object actually is an instance of the correct destination (derived) type or a type derived from the destination type.į# provides operators for these types of conversions. Such a cast is guaranteed to work as long as the base class is in the inheritance hierarchy of the derived class. Casting up a hierarchy means casting from a derived object reference to a base object reference. There are two basic types of conversions: casting up (upcasting) and casting down (downcasting). Casting Object TypesĬonversion between types in an object hierarchy is fundamental to object-oriented programming. Error: types are incompatibleįor more information, see Enumerations. In the following code, the conversion fails to compile because of the mismatch between int32 and uint32. Note that the enumeration casts work only if the underlying type of the enumeration is compatible with the type being converted. You can also specify the target enumeration type explicitly as a type parameter, as in the following code: let col3 = enum 3 The target type is supplied by a type annotation. The target type of the conversion cannot be determined by type inference, so the type parameter must be explicit. Alternatively, you can supply a type annotation, as with col2 in the following example. Therefore, the compiler can deduce that you are converting to a Color enumeration. In the following example, the variable col1 is not explicitly annotated, but its type is inferred from the later equality test. When it converts to an enumerated type, type inference attempts to determine the type of the enum that you want to convert to. The enum operator is a generic operator that takes one type parameter that represents the type of the enum to convert to. As a special exception to the general rule that methods cannot be overloaded by return type, you can do this for op_Explicit and op_Implicit. For example, the int conversion operator works with any type that provides a static method op_Explicit that takes the type as a parameter and returns int. In addition to built-in primitive types, you can use these operators with types that implement op_Explicit or op_Implicit methods with appropriate signatures. OperatorĬonvert to a 64-bit double-precision IEEE floating point number.Ĭonvert to a 32-bit single-precision IEEE floating point number.Ĭonvert to System.Char, a Unicode character. The following table shows conversion operators defined in F#. The first occurrence is the type and the second is the conversion operator. For example, in the following code, in which the types are explicitly annotated, byte appears with two different meanings. The checked forms check for overflow and generate a runtime exception if the resulting value exceeds the limits of the target type.Įach of these operators has the same name as the name of the destination type. The unchecked forms are defined in and the checked forms are defined in. The integral and char conversion operators have checked and unchecked forms the floating point operators and the enum conversion operator do not. Arithmetic Typesį# provides conversion operators for arithmetic conversions between various primitive types, such as between integer and floating point types. I had a function that returned a trait object and I needed a trait object for one of its supertraits.This article describes support for type conversions in F#. I recently hit a limitation of Rust when working with trait objects. Traits, dynamic dispatch and upcasting Traits, dynamic dispatch and upcasting
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