Talk about VARIANT

    VARIANT specify variant data that can not be passed by reference.When a variant refers to another variant by using the VT_VARIANT | VT_BYREF vartype,the variant being reffered to cannot also be of type VT_VARIANT | VT_BYREF,VARIANTs can be passed by value,even if VARIANTARGSs can not.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 in pbVal.
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 in puiVal.
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 in pulVal.
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 in puintVal.
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 in pintVal.
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 in pcVal.
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 value 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 value was passed. A pointer to its value is in pdblVal.
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 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 by the BSTR functions, which are described in Conversion and Manipulation Functions.
VT_BSTR | VT_BYREF	A reference to a string was passed. A BSTR* that points to a BSTR is in pbstrVal. 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 in pdecVal.
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 in pscode.
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 in date. 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 using VariantTimeToDosDateTime, 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 in pdate.
VT_DISPATCH	A pointer to an object was specified. The pointer is in pdispVal. This object is known only 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	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 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 invalid types to combine with VT_ARRAY.) The pointer in pparray 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 in Conversion and Manipulation Functions.

 

 

 

Here,I just give you a simple instance as an application of VARIANT as follows:

  I write a simple com interface method like this(using ATL designs COM):

 

 

  STDMETHODIMP CMyMath::add(VARIANT a,VARIANT b,VARIANT* c)
{
    if(a.vt==VT_I4 && b.vt==VT_I4)        //judge whether it is integer type.
 {
      c->vt=VT_I4;                                 // assign a integer type to c point to vt.
      c->lVal=a.lVal+b.lVal;
 }
 else if(a.vt==VT_R4 && b.vt==VT_R4)   //judge whether it is float type.
 {
       c->vt=VT_R4;                               //assign a float type to c point to vt
       c->fltVal=a.fltVal+b.fltVal;
 }
 return S_OK;                                      // return HRESULT.
}

     If you wanna apply to ATL to create a COM,you must add macro like STDMETHODIMP or you will get  nothing about that. The second row of this code above like (if (a.vt == VT_I4)),here ,"==",you can also use '&' instead of "==".

 Do you know why? I just here give you a latent question~~!But how to call this interface? First of all,you must add some containing file like :XXXX.h,XXXX.c,atlbase.h and so on like :

    #include <atlbase.h>
    #include "../DemoVariant.h"
    #include "../DemoVariant_i.c"

mthod one:

   void TestDlg::Button1()

   {

      CComPtr<IMath> pTools;

      HRESULT hr = pTools.CreateInstance(CLSID_Math);

      if (hr == S_OK)

      {

         VARIANT a;

         VARIANT b;

         VARIANT c;

   

         a.vt = VT_I4;

         b.vt = VT_I4;

         c.vt = VT_I4;

        

         a.lVal = 10;

         b.lVal = 20;

         pTools->Add(a,b,&c);

         CString strTemp;

         strTemp.Format(_T("%s"),c.lVal);

             MessageBox(strTemp);

      }

      else

      {

          Exception something .......

      }

   }

 Method two:

   using CComVariant class.

   void CTestDlg::Button2()

   {

       TODO: Add your control notification handler code here
       CComPtr<IMyMath> tools;
       HRESULT hr=tools.CoCreateInstance(CLSID_MyMath);
      if(hr==S_OK)
     {
        CComVariant v1(12),v2(13),v3;

        tools->add(v1,v2,&v3);
        CString strTemp;
        strTemp.Format("%d",v3.lVal);
        MessageBox(strTemp);
      }

   }

Mthod three:

if you wanna use this method ,you must contain header file comdef.h!

   #include <comdef.h>
void CTestDlg::OnButton3()
{
     // TODO: Add your control notification handler code here
     CComPtr<IMyMath> tools;
     HRESULT hr=tools.CoCreateInstance(CLSID_MyMath);
     if(hr==S_OK)
    {
         _variant_t v1;
         v1.vt=VT_I4;
         v1.lVal=12;
         _variant_t v2;
         v2.vt=VT_I4;
         v2.lVal=13;
         _variant_t v3;

        tools->add(v1,v2,&v3);
       CString strTemp;
       strTemp.Format("%d",v3.lVal);
      MessageBox(strTemp);
   }
}