18.3.8 Mapping for VARIANT

The COM VARIANT provides semantically similar functionality to the CORBA any. However, its allowable set of data types are currently limited to the data types supported by Automation. VARTYPE is an enumeration type used in the VARIANT structure. The structure member vt is defined using the data type VARTYPE. Its value acts as the discriminator for the embedded union and governs the interpretation of the union. The list of valid values for the data type VARTYPE are listed in Table 18-9, along with a description of how to use them to represent the OMG IDL any data type.

Table 18-9 Valid OLE VARIANT Data Types


Value	Description
VT_EMPTY	No value was specified. If an argument is left blank, you should not return VT_EMPTY for the argument. Instead, you should return the VT_ERROR value: DISP_E_MEMBERNOTFOUND.
VT_EMPTY \| VT_BYREF	Illegal.
VT_UI1	An unsigned 1-byte character is stored in bVal.
VT_UI1 \| VT_BYREF	A reference to an unsigned 1-byte character was passed; a pointer to the value is in pbVal.
VT_I2	A 2-byte integer value is stored in iVal.
VT_I2 \| VT_BYREF	A reference to a 2-byte integer was passed; a pointer to the value is in piVal.
VT_I4	A 4-byte integer value is stored in lVal.
VT_I4 \| VT_BYREF	A reference to a 4-byte integer was passed; a pointer to the value is in plVal.
VT_R4	An IEEE 4-byte real value is stored in fltVal.
VT_R4 \| VT_BYREF	A reference to an IEEE 4-byte real was passed; a pointer to the value is in pfltVal.
VT_R8	An 8-byte IEEE real value is stored in dblVal.
VT_R8 \| VT_BYREF	A reference to an 8-byte IEEE real was passed; a pointer to its value is in pdblVal.

Table 18-9 Valid OLE VARIANT Data Types (Continued) Table 18-9 Valid OLE VARIANT Data Types (Continued)


VT_CY	A currency value was specified. A currency number is stored as an 8-byte, two’s complement integer, scaled by 10,000 to give a fixed-point number with 15 digits to the left of the decimal point and 4 digits to the right. The value is in cyVal.
VT_CY \| VT_BYREF	A reference to a currency value was passed; a pointer to the value is in pcyVal.
VT_BSTR	A string was passed; it is stored in bstrVal. This pointer must be obtained and freed via the BSTR functions.
VT_BSTR \| VT_BYREF	A reference to a string was passed. A BSTR*, which points to a BSTR, is in pbstrVal. The referenced pointer must be obtained or freed via the BSTR functions.
VT_NULL	A propagating NULL value was specified. This should not be confused with the NULL pointer. The NULL value is used for tristate logic as with SQL.
VT_NULL \| VT_BYREF	Illegal.
VT_ERROR	An SCODE was specified. The type of error is specified in code. Generally, operations on error values should raise an exception or propagate the error to the return value, as appropriate.
VT_ERROR \| VT_BYREF	A reference to an SCODE was passed. A pointer to the value is in pscode.
VT_BOOL	A Boolean (True/False) value was specified. A value of 0xFFFF (all bits one) indicates True; a value of 0 (all bits zero) indicates False. No other values are legal.
VT_BOOL \| VT_BYREF	A reference to a Boolean value. A pointer to the Boolean value is in pbool.
VT_DATE	A value denoting a date and time was specified. Dates are represented as double-precision numbers, where midnight, January 1, 1900 is 2.0, January 2, 1900 is 3.0, and so on. The value is passed in date. This is the same numbering system used by most spreadsheet programs, although some incorrectly believe that February 29, 1900 existed, and thus set January 1, 1900 to 1.0. The date can be converted to and from an MS-DOS representation using VariantTimeToDosDateTime.
VT_DATE \| VT_BYREF	A reference to a date was passed. A pointer to the value is in pdate.


VT_DISPATCH	A pointer to an object was specified. The pointer is in pdispVal. This object is only known to implement IDispatch; the object can be queried as to whether it supports any other desired interface by calling QueryInterface on the object. Objects that do not implement IDispatch should be passed using VT_UNKNOWN.
VT_DISPATCH \| VT_BYREF	A pointer to a pointer to an object was specified. The pointer to the object is stored in the location referred to by ppdispVal.
VT_VARIANT	Illegal. VARIANTARGs must be passed by reference.
VT_VARIANT \| VT_BYREF	A pointer to another VARIANTARG is passed in pvarVal. This referenced VARIANTARG will never have the VT_BYREF bit set in vt, so only one level of indirection can ever be present. This value can be used to support languages that allow functions to change the types of variables passed by reference.
VT_UNKNOWN	A pointer to an object that implements the IUnknown interface is passed in punkVal.
VT_UNKNOWN \| VT_BYREF	A pointer to a pointer to the IUnknown interface is passed in ppunkVal. The pointer to the interface is stored in the location referred to by ppunkVal.
VT_ARRAY \| <anything>	An array of data type <anything> was passed. (VT_EMPTY and VT_NULL are illegal types to combine with VT_ARRAY.) The pointer in pByrefVal points to an array descriptor, which describes the dimensions, size, and in-memory location of the array. The array descriptor is never accessed directly, but instead is read and modified using functions.

A COM VARIANT is mapped to the CORBA any without loss. If at run-time a CORBA client passes an inconvertible any to a COM server, a DATA_CONVERSION exception is raised.