COM技术中的VARIANT and VARIANTARG

Posted 2020-11-28 johnyang

VARIANT and VARIANTARG

Use VARIANTARG to describe arguments passed within DISPPARAMS, and VARIANT to specify variant data that cannot be passed by reference. When a variant refers to another variant by using the VT_VARIANT | VT_BYREF vartype, the variant being referred to cannot also be of type VT_VARIANT | VT_BYREF. VARIANTs can be passed by value, even if VARIANTARGs cannot. The following definition of VARIANT is described in OAIDL.H automation header file:

typedef struct FARSTRUCT tagVARIANT VARIANT;
typedef struct FARSTRUCT tagVARIANT VARIANTARG;

typedef struct tagVARIANT  {
   VARTYPE vt;
   unsigned short wReserved1;
   unsigned short wReserved2;
   unsigned short wReserved3;
   union {
      Byte                    bVal;                 // VT_UI1.
      Short                   iVal;                 // VT_I2.
      long                    lVal;                 // VT_I4.
      float                   fltVal;               // VT_R4.
      double                  dblVal;               // VT_R8.
      VARIANT_BOOL            boolVal;              // VT_BOOL.
      SCODE                   scode;                // VT_ERROR.
      CY                      cyVal;                // VT_CY.
      DATE                    date;                 // VT_DATE.
      BSTR                    bstrVal;              // VT_BSTR.
      DECIMAL                 FAR* pdecVal          // VT_BYREF|VT_DECIMAL.
      IUnknown                FAR* punkVal;         // VT_UNKNOWN.
      IDispatch               FAR* pdispVal;        // VT_DISPATCH.
      SAFEARRAY               FAR* parray;          // VT_ARRAY|*.
      Byte                    FAR* pbVal;           // VT_BYREF|VT_UI1.
      short                   FAR* piVal;           // VT_BYREF|VT_I2.
      long                    FAR* plVal;           // VT_BYREF|VT_I4.
      float                   FAR* pfltVal;         // VT_BYREF|VT_R4.
      double                  FAR* pdblVal;         // VT_BYREF|VT_R8.
      VARIANT_BOOL            FAR* pboolVal;        // VT_BYREF|VT_BOOL.
      SCODE                   FAR* pscode;          // VT_BYREF|VT_ERROR.
      CY                      FAR* pcyVal;          // VT_BYREF|VT_CY.
      DATE                    FAR* pdate;           // VT_BYREF|VT_DATE.
      BSTR                    FAR* pbstrVal;        // VT_BYREF|VT_BSTR.
      IUnknown                FAR* FAR* ppunkVal;   // VT_BYREF|VT_UNKNOWN.
      IDispatch               FAR* FAR* ppdispVal;  // VT_BYREF|VT_DISPATCH.
      SAFEARRAY               FAR* FAR* pparray;    // VT_ARRAY|*.
      VARIANT                 FAR* pvarVal;         // VT_BYREF|VT_VARIANT.
      void                    FAR* byref;           // Generic ByRef.
      char                    cVal;                 // VT_I1.
      unsigned short          uiVal;                // VT_UI2.
      unsigned long           ulVal;                // VT_UI4.
      int                     intVal;               // VT_INT.
      unsigned int            uintVal;              // VT_UINT.
      char FAR *              pcVal;                // VT_BYREF|VT_I1.
      unsigned short FAR *    puiVal;               // VT_BYREF|VT_UI2.
      unsigned long FAR *     pulVal;               // VT_BYREF|VT_UI4.
      int FAR *               pintVal;              // VT_BYREF|VT_INT.
      unsigned int FAR *      puintVal;             //VT_BYREF|VT_UINT.
   };
};
 

To simplify extracting values from VARIANTARGs, Automation provides a set of functions for manipulating this type. Use of these functions is strongly recommended to ensure that applications apply consistent coercion rules.

The vt value governs the interpretation of the union as follows:

 

Value	Description
VT_EMPTY	No value was specified. If an optional argument to an Automation method is left blank, do not pass a VARIANT of type VT_EMPTY. Instead, pass a VARIANT of type VT_ERROR with a value of DISP_E_PARAMNOTFOUND.
VT_EMPTY | VT_BYREF	Not valid.
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 inpbVal.
VT_UI2	An unsigned 2-byte integer value is stored in uiVal.
VT_UI2 | VT_BYREF	A reference to an unsigned 2-byte integer was passed. A pointer to the value is inpuiVal.
VT_UI4	An unsigned 4-byte integer value is stored in ulVal.
VT_UI4 | VT_BYREF	A reference to an unsigend 4-byte integer was passed. A pointer to the value is inpulVal.
VT_UINT	An unsigned integer value is stored in uintVal.
VT_UINT | VT_BYREF	A reference to an unsigned integer value was passed. A pointer to the value is inpuintVal.
VT_INT	An integer value is stored in intVal.
VT_INT | VT_BYREF	A reference to an integer value was passed. A pointer to the value is inpintVal.
VT_I1	A 1-byte character value is stored in cVal.
VT_I1 | VT_BYREF	A reference to a 1-byte character was passed. A pointer the value is inpcVal.
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 inpiVal.
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 inplVal.
VT_R4	An IEEE 4-byte real value is stored in fltVal.
VT_R4 | VT_BYREF	A reference to an IEEE 4-byte real value was passed. A pointer to the value is inpfltVal.
VT_R8	An 8-byte IEEE real value is stored in dblVal.
VT_R8 | VT_BYREF	A reference to an 8-byte IEEE real value was passed. A pointer to its value is inpdblVal.
VT_CY	A currency value was specified. A currency number is stored as 64-bit (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 incyVal.
VT_CY | VT_BYREF	A reference to a currency value was passed. A pointer to the value is inpcyVal.
VT_BSTR	A string was passed; it is stored in bstrVal. This pointer must be obtained and freed by the BSTR functions, which are described inConversion and Manipulation Functions.
VT_BSTR | VT_BYREF	A reference to a string was passed.A BSTR* that points to a BSTR is inpbstrVal.The referenced pointer must be obtained or freed by the BSTR functions.
VT_DECIMAL	Decimal variables are stored as 96-bit (12-byte) unsigned integers scaled by a variable power of 10. VT_DECIMAL uses the entire 16 bytes of the Variant.
VT_DECIMAL | VT_BYREF	A reference to a decimal value was passed. A pointer to the value is inpdecVal.
VT_NULL	A propagating null value was specified. (This should not be confused with the null pointer.) The null value is used for tri-state logic, as with SQL.
VT_NULL | VT_BYREF	Not valid.
VT_ERROR	An SCODE was specified. The type of the error is specified in scodee. 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 inpscode.
VT_BOOL	A 16 bit Boolean (True/False) value was specified. A value of 0xFFFF (all bits 1) indicates True; a value of 0 (all bits 0) indicates False. No other values are valid.
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 indate. This is the same numbering system used by most spreadsheet programs, although some specify incorrectly 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 usingVariantTimeToDosDateTime, which is discussed in Conversion and Manipulation Functions.
VT_DATE | VT_BYREF	A reference to a date was passed.A pointer to the value is inpdate.
VT_DISPATCH	A pointer to an object was specified.The pointer is inpdispVal.This object is known only to implement IDispatch. The object can be queried as to whether it supports any other desired interface by callingQueryInterface on the object.Objects that do not implementIDispatch 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 byppdispVal.
VT_VARIANT	Invalid. VARIANTARGs must be passed by reference.
VT_VARIANT | VT_BYREF	A pointer to another VARIANTARG is passed in pvarVal. This referenced VARIANTARG,pvarVal, cannot be another VT_VARIANT|VT_BYREF. 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 the IUnknown interface is passed inppunkVal. The pointer to the interface is stored in the location referred to byppunkVal.
VT_ARRAY | <anything>	An array of data type <anything> was passed. (VT_EMPTY and VT_NULL are invalid types to combine with VT_ARRAY.) The pointer inpparray 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 the functions described inConversion and Manipulation Functions.