COleVariant 本质上是一个枚举，用同一种类型来表达不同的子类型。如同boost中的variant。

例子

COleVariant var(3.6f);

float v = var.fltVal;

CString str("testCOleVariant");

COleVariant var2(str);

CString cpStr(var2.bstrVal);

例子

CString CMFCPropertyGridProperty::m_strFormatChar = _T("%c");

CString CMFCPropertyGridProperty::m_strFormatShort = _T("%d");

CString CMFCPropertyGridProperty::m_strFormatLong = _T("%ld");

CString CMFCPropertyGridProperty::m_strFormatUShort = _T("%u");

CString CMFCPropertyGridProperty::m_strFormatULong = _T("%u");

CString CMFCPropertyGridProperty::m_strFormatFloat = _T("%f");

CString CMFCPropertyGridProperty::m_strFormatDouble = _T("%lf");



CString CMFCPropertyGridProperty::FormatProperty()

{
ASSERT_VALID(this);
ASSERT_VALID(m_pWndList);


COleVariant& var = m_varValue;


CString strVal;


.............


switch (var.vt)
{
case VT_BSTR:
	strVal = var.bstrVal;
	break;


case VT_I2:
	strVal.Format(m_strFormatShort, (short)var.iVal);
	break;


case VT_I4:
case VT_INT:
	strVal.Format(m_strFormatLong, (long)var.lVal);
	break;


case VT_UI1:
	if ((BYTE)var.bVal != 0)
	{
		strVal.Format(m_strFormatChar, (TCHAR)(BYTE)var.bVal);
	}
	break;


case VT_UI2:
	strVal.Format( m_strFormatUShort, var.uiVal);
	break;


case VT_UINT:
case VT_UI4:
	strVal.Format(m_strFormatULong, var.ulVal);
	break;


case VT_R4:
	strVal.Format(m_strFormatFloat, (float)var.fltVal);
	break;


case VT_R8:
	strVal.Format(m_strFormatDouble, (double)var.dblVal);
	break;


case VT_BOOL:
	strVal = var.boolVal == VARIANT_TRUE ? m_pWndList->m_strTrue : m_pWndList->m_strFalse;
	break;


default:
	// Unsupported type
	strVal = _T("*** error ***");
}


return strVal;

}

COleVariant框架内容

class COleVariant : public tagVARIANT

{
COleVariant(LPCTSTR lpszSrc);
COleVariant(LPCTSTR lpszSrc, VARTYPE vtSrc); // used to set to ANSI string
COleVariant(CString& strSrc);


COleVariant(BYTE nSrc);
COleVariant(short nSrc, VARTYPE vtSrc = VT_I2);
COleVariant(long lSrc, VARTYPE vtSrc = VT_I4);
COleVariant(const COleCurrency& curSrc);


COleVariant(float fltSrc);
COleVariant(double dblSrc);
COleVariant(const COleDateTime& timeSrc);

....................................
const COleVariant& operator=(float fltSrc);
const COleVariant& operator=(double dblSrc);
const COleVariant& operator=(const COleDateTime& dateSrc);


const COleVariant& operator=(const CByteArray& arrSrc);
const COleVariant& operator=(const CLongBinary& lbSrc);

};


void Clear();
void ChangeType(VARTYPE vartype, LPVARIANT pSrc = NULL);
void Attach(VARIANT& varSrc);
VARIANT Detach();

   void GetByteArrayFromVariantArray(CByteArray& bytes);

void SetString(LPCTSTR lpszSrc, VARTYPE vtSrc); // used to set ANSI string
operator LPVARIANT();
operator LPCVARIANT() const

/

struct tagVARIANT

    {    union 

        {

        struct __tagVARIANT

            {
VARTYPE vt;
.......................

            union 

                {

                LONGLONG llVal;

                LONG lVal;

                BYTE bVal;

                SHORT iVal;

                FLOAT fltVal;

                DOUBLE dblVal;

                ...........

                DATE date;

                BSTR bstrVal;

                IUnknown *punkVal;
 IDispatch *pdispVal;
..............
}


COleVariant::COleVariant(float fltSrc)
	{ vt = VT_R4; fltVal = fltSrc; }

_AFXDISP_INLINE COleVariant::COleVariant(CString& strSrc)
	{ vt = VT_EMPTY; *this = strSrc; }


const COleVariant& COleVariant::operator=(const CString& strSrc)

{
// Free up previous VARIANT
Clear();


vt = VT_BSTR;
bstrVal = strSrc.AllocSysString();


return *this;
}


typedef unsigned short VARTYPE;



/*

 * VARENUM usage key,

 *

 * * [V] - may appear in a VARIANT

 * * [T] - may appear in a TYPEDESC

 * * [P] - may appear in an OLE property set

 * * [S] - may appear in a Safe Array

 *

 *

 *  VT_EMPTY            [V]   [P]     nothing

 *  VT_NULL             [V]   [P]     SQL style Null

 *  VT_I2               [V][T][P][S]  2 byte signed int

 *  VT_I4               [V][T][P][S]  4 byte signed int

 *  VT_R4               [V][T][P][S]  4 byte real

 *  VT_R8               [V][T][P][S]  8 byte real

 *  VT_CY               [V][T][P][S]  currency

 *  VT_DATE             [V][T][P][S]  date

 *  VT_BSTR             [V][T][P][S]  OLE Automation string

 *  VT_DISPATCH         [V][T]   [S]  IDispatch *

 *  VT_ERROR            [V][T][P][S]  SCODE

 *  VT_BOOL             [V][T][P][S]  True=-1, False=0

 *  VT_VARIANT          [V][T][P][S]  VARIANT *

 *  VT_UNKNOWN          [V][T]   [S]  IUnknown *

 *  VT_DECIMAL          [V][T]   [S]  16 byte fixed point

 .......................................



enum VARENUM

    {	VT_EMPTY	= 0,
VT_NULL	= 1,
VT_I2	= 2,
VT_I4	= 3,
VT_R4	= 4,
VT_R8	= 5,
VT_CY	= 6,
VT_DATE	= 7,
VT_BSTR	= 8,
.........................................
    } ;