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Learning Perl on Win32 Systems

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OLE Automation
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19.4 Variants

In order for automation controllers and servers to cooperate, they have to have some way to agree on the type of data that they're passing. Automation accomplishes this through a data type called a VARIANT. The VARIANT data type is built on a C-language union. It contains a type field that identifies that type of data in the union (things such as strings, numbers, automation objects, etc.) and a field that contains the data.

Usually, Perl handles data-type conversion for you. If you need more control, though, you can create a Variant object and specify the type yourself. Perl provides access to the types listed in Table 19.3.


Table 19.3: Variant Types

Variant Type

Description

VT_UI1

Unsigned character (1 byte)

VT_I2

Signed integer (2 bytes)

VT_I4

Signed integer (4 bytes)

VT_R4

Floating point (4 bytes)

VT_R8

Floating point (8 bytes)

VT_DATE

OLE Date (floating-point value measuring days since midnight, Dec. 30, 1899)

VT_BSTR

OLE String

VT_CY

OLE Currency

VT_BOOL

OLE Boolean

By default, Perl converts integer data to the VT_I4 type, string data to the VT_BSTR type, and floating-point data to the VT_R8 type. Usually, these conversions are what you'd expect, but let's look at how you might specify your own type:

$vt = new OLE::Variant(OLE::VT_DATE, "May 31, 1997" );
$Message->{TimeSent} = $vt;

This example first creates a Variant object, setting the type to VT_DATE and the date to "May 31, 1997." It then assigns the date to the Message object TimeSent property (something you might do if you were posting a message to a folder, for example).


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19.3 Using Automation ObjectsBook Index19.5 Tips and Techniques