Appendix K The Variant Subtypes

The reference material in this appendix is a companion to the detailed explanation of data typesand the VBScript Variant in Chapter 3. However, a brief description of these concepts is as follows .

VBScript is what is known as a weakly typed programming language, which is the opposite of a ³strongly typed ³ language. A weakly typed language does not allow you to declare variableswith specific data types such as String , Date , or Boolean . Instead, in VBScript all variablesare automatically and implicitly assigned a special data type called Variant . The Variant data type is actually many data types in one. VBScript still has the concept of data types suchas String , Date , and Boolean , but these data types are somewhat hidden inside the Variant type.

The mechanism by which the Variant type encompasses many data types in one is called the subtype . A Variant variable can have one of many different subtypes-but only one subtypeat a time. The subtype of a Variant variable can change in one of two ways: implicitly and explicitly.

An implicit change in subtype occurs when a new value is assigned to a Variant variable and VBScript uses some intelligence to examine the new value and automatically decides what the subtype should be. If the new value fits within the bounds of the already assigned subtype, VBScript will not change it. This automatic subtype change process is called implicit type coercion . As a VBScript programmer, it is very important to understand how implicit type coercion works and when and how it occurs.

A programmer can also explicitly force a change in subtype. The primary way of doing this is with one of VBScript's conversion functions . For example, the CLng() function will change the subtype to Long . Similarly, the CStr() function will change it to String . Sometimes it is necessary to use a conversion function to force a subtype change so that a particular operation or function call will work properly.

The Variant data type, the subtypes, and the ins and outs of implicit and explicit type coercion are covered in great detail in Chapter 3. Even an experienced programmer coming from another language would do well to read this chapter. Management of the Variant subtypes in your scripts is a key to your success as a VBScript programmer.

The following table lays out the particulars of all of the Variant subtypes. This is intended as a reference for those times when you're not sure which subtype you might need or when you're not sure if a particular value is too big for a certain subtype.

Following the Variant Subtypes table is a table of the Visual Basic data types. Visual Basic is a strongly typed language. Visual Basic also supports the Variant data type, but many others besides. The list of Visual Basic data types lines up pretty closely with the list of Variant subtypes. The reason we include this information on the Visual Basic data types is that often your VBScript code will be calling components that use specific data types as parameters or return values. When communicating with external components that expose these standard data types, it is important to make sure that your Variant subtype corresponds to the Visual Basic data type required by the external component.

The Variant Subtypes

*Visual Basic supports conversion functions for the Error and Decimal subtypes called CVErr() and CDec() ,respectively. VBScript, however, does not support these conversion functions. See the sidebar later in this section for more information.

$ Because of a bug in VBScript, the TypeName() function does not support the Decimal 1 subtype (although VarType() does). See the sidebar later in this section for more information.

$ This value is actually returned from the VarType() function in combination with the value for Variant (12) . See the section on arrays at the end of this chapter.

The Visual Basic Data Types