Overview of Variants

A variety of functions convert to these subtypes and test for these subtypes. Table 4-1 shows the development of the Variant data type through the versions of Visual Basic, along with the matching functions.

Table 4-1 The Evolution of Variants

Type	Visual Basic Name	Visual Basic Version	Convert Function	Test Function
0	Empty	2	= Empty	IsEmpty
1	Null	2	= Null	IsNull
2	Integer	2	CInt	IsNumeric*
3	Long	2	CLng	IsNumeric
4	Single	2	CSng	IsNumeric
5	Double	2	CDbl	IsNumeric
6	Currency	2	CCur	IsNumeric
7	Date	2	CVDate/CDate	IsDate
8	String	2	CStr
9	Object	4		IsObject
10	Error	4	CVErr	IsError
11	Boolean	4	CBool
12	Variant	4	CVar
13	Data Object	4
14	Decimal	5	CDec	IsNumeric
17	Byte	4	CByte
36	UDT	6
8192	Array	4	Array	IsArray
16384	ByRef	Never?

*Strictly speaking, IsNumeric tests to see if a variable can be converted to a numeric value, and is not simply reporting on a Variant's subtype.

Internal Structure

A Variant always takes up at least 16 bytes of memory and is structured as shown in Figure 4-1.

Figure 4-1 The structure of a Variant

The first two bytes correspond to the value returned by the VarType function. (The VarType return values are defined as constants in the VbVarType enumeration.) For example, if the VarType is 2 (the value of the constant vbInteger), the Variant has a subtype of Integer. You cannot change this value directly, but the conversion functions (such as CInt) will do this for you.

The Reserved bytes have no documented function yet; their principal purpose is to pad the structure out to 16 bytes. The Data area holds the value of the variable, if the value fits into 8 bytes; otherwise, the Data area holds a pointer to the data (as with strings and so on). The type indicates how the Data portion of the Variant is to be understood or interpreted.

In this way, Variants are self-describing, meaning they contain within them all the information necessary to use them.