_variant_t 数据类型介绍

_variant_t 和 _bstr_t 这两个类分别用于封装并管理 VARIANT 和 BSTR 这两种数据类型，VARIANT
和 BSTR 这两种类型是 COM 中使用的数据类型。为了将C++中的变量应用到ADO编程中，只能进行数据类型的转换。通过
_variant_t 和 _bstr_t 这两个类，就可以方便的把C++类型变量转换成COM中的变量了。

_variant_t的定义：COMUTIL.H

class _variant_t : public ::tagVARIANT {
public:
// Constructors
//
_variant_t() throw();
_variant_t(const VARIANT& varSrc) throw(_com_error);
_variant_t(const VARIANT* pSrc) throw(_com_error);
_variant_t(const _variant_t& varSrc) throw(_com_error);
_variant_t(VARIANT& varSrc, bool fCopy) throw(_com_error); // Attach VARIANT if !fCopy
_variant_t(short sSrc, VARTYPE vtSrc = VT_I2) throw(_com_error); // Creates a VT_I2, or a VT_BOOL
_variant_t(long lSrc, VARTYPE vtSrc = VT_I4) throw(_com_error); // Creates a VT_I4, a VT_ERROR, or a VT_BOOL
_variant_t(float fltSrc) throw(); // Creates a VT_R4
_variant_t(double dblSrc, VARTYPE vtSrc = VT_R8) throw(_com_error); // Creates a VT_R8, or a VT_DATE
_variant_t(const CY& cySrc) throw(); // Creates a VT_CY
_variant_t(const _bstr_t& bstrSrc) throw(_com_error); // Creates a VT_BSTR
_variant_t(const wchar_t*pSrc) throw(_com_error); // Creates a VT_BSTR
_variant_t(const char* pSrc) throw(_com_error); // Creates a VT_BSTR
_variant_t(IDispatch* pSrc, bool fAddRef = true) throw(); // Creates a VT_DISPATCH
_variant_t(bool bSrc) throw(); // Creates a VT_BOOL
_variant_t(IUnknown* pSrc, bool fAddRef = true) throw(); // Creates a VT_UNKNOWN
_variant_t(const DECIMAL& decSrc) throw(); // Creates a VT_DECIMAL
_variant_t(BYTE bSrc) throw(); // Creates a VT_UI1
// Destructor
//
~_variant_t() throw(_com_error);
// Extractors
//
operator short() const throw(_com_error); // Extracts a short from a VT_I2
operator long() const throw(_com_error); // Extracts a long from a VT_I4
operator float() const throw(_com_error); // Extracts a float from a VT_R4
operator double() const throw(_com_error); // Extracts a double from a VT_R8
operator CY() const throw(_com_error); // Extracts a CY from a VT_CY
operator _bstr_t() const throw(_com_error); // Extracts a _bstr_t from a VT_BSTR
operator IDispatch*() const throw(_com_error); // Extracts a IDispatch* from a VT_DISPATCH
operator bool() const throw(_com_error); // Extracts a bool from a VT_BOOL
operator IUnknown*() const throw(_com_error); // Extracts a IUnknown* from a VT_UNKNOWN
operator DECIMAL() const throw(_com_error); // Extracts a DECIMAL from a VT_DECIMAL
operator BYTE() const throw(_com_error); // Extracts a BTYE (unsigned char) from a VT_UI1
// Assignment operations
//
_variant_t& operator=(const VARIANT& varSrc) throw(_com_error);
_variant_t& operator=(const VARIANT* pSrc) throw(_com_error);
_variant_t& operator=(const _variant_t& varSrc) throw(_com_error);
_variant_t& operator=(short sSrc) throw(_com_error); // Assign a VT_I2, or a VT_BOOL
_variant_t& operator=(long lSrc) throw(_com_error); // Assign a VT_I4, a VT_ERROR or a VT_BOOL
_variant_t& operator=(float fltSrc) throw(_com_error); // Assign a VT_R4
_variant_t& operator=(double dblSrc) throw(_com_error); // Assign a VT_R8, or a VT_DATE
_variant_t& operator=(const CY& cySrc) throw(_com_error); // Assign a VT_CY
_variant_t& operator=(const _bstr_t& bstrSrc) throw(_com_error); // Assign a VT_BSTR
_variant_t& operator=(const wchar_t* pSrc) throw(_com_error); // Assign a VT_BSTR
_variant_t& operator=(const char* pSrc) throw(_com_error); // Assign a VT_BSTR
_variant_t& operator=(IDispatch* pSrc) throw(_com_error); // Assign a VT_DISPATCH
_variant_t& operator=(bool bSrc) throw(_com_error); // Assign a VT_BOOL
_variant_t& operator=(IUnknown* pSrc) throw(_com_error); // Assign a VT_UNKNOWN
_variant_t& operator=(const DECIMAL& decSrc) throw(_com_error); // Assign a VT_DECIMAL
_variant_t& operator=(BYTE bSrc) throw(_com_error); // Assign a VT_UI1
// Comparison operations
//
bool operator==(const VARIANT& varSrc) const throw(_com_error);
bool operator==(const VARIANT* pSrc) const throw(_com_error);
bool operator!=(const VARIANT& varSrc) const throw(_com_error);
bool operator!=(const VARIANT* pSrc) const throw(_com_error);
// Low-level operations
//
void Clear() throw(_com_error);
void Attach(VARIANT& varSrc) throw(_com_error);
VARIANT Detach() throw(_com_error);
void ChangeType(VARTYPE vartype, const _variant_t* pSrc = NULL) throw(_com_error);
void SetString(const char* pSrc) throw(_com_error); // used to set ANSI string
};

_variant_t 是一个类，是 VARIANT 的完全包装类，它提供多种构造函数和数据转换函数。它继承自一个结构体，其结构如下：

struct tagVARIANT
{
union
{
struct __tagVARIANT
{
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union
{
LONG lVal;
BYTE bVal;
SHORT iVal;
FLOAT fltVal;
DOUBLE dblVal;
VARIANT_BOOL boolVal;
_VARIANT_BOOL bool;
SCODE scode;
CY cyVal;
DATE date;
BSTR bstrVal;
IUnknown __RPC_FAR *punkVal;
IDispatch __RPC_FAR *pdispVal;
SAFEARRAY __RPC_FAR *parray;
BYTE __RPC_FAR *pbVal;
SHORT __RPC_FAR *piVal;
LONG __RPC_FAR *plVal;
FLOAT __RPC_FAR *pfltVal;
DOUBLE __RPC_FAR *pdblVal;
VARIANT_BOOL __RPC_FAR *pboolVal;
_VARIANT_BOOL __RPC_FAR *pbool;
SCODE __RPC_FAR *pscode;
CY __RPC_FAR *pcyVal;
DATE __RPC_FAR *pdate;
BSTR __RPC_FAR *pbstrVal;
IUnknown __RPC_FAR *__RPC_FAR *ppunkVal;
IDispatch __RPC_FAR *__RPC_FAR *ppdispVal;
SAFEARRAY __RPC_FAR *__RPC_FAR *pparray;
VARIANT __RPC_FAR *pvarVal;
PVOID byref;
CHAR cVal;
USHORT uiVal;
ULONG ulVal;
INT intVal;
UINT uintVal;
DECIMAL __RPC_FAR *pdecVal;
CHAR __RPC_FAR *pcVal;
USHORT __RPC_FAR *puiVal;
ULONG __RPC_FAR *pulVal;
INT __RPC_FAR *pintVal;
UINT __RPC_FAR *puintVal;
struct __tagBRECORD
{
PVOID pvRecord;
IRecordInfo __RPC_FAR *pRecInfo;
} __VARIANT_NAME_4;
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
};

其中，vt 表示的是 _variant_t 的数据类型，它的取值是以下枚举结构的值：

enum VARENUM
{
VT_EMPTY = 0 [V] [P] nothing
VT_NULL =1 [V] [P] SQL style Null
VT_I2 =2 [V][T][P][S] 2 byte signed int
VT_I4 =3 [V][T][P][S] 4 byte signed int
VT_R4 = 4 [V][T][P][S] 4 byte real
VT_R8 = 5 [V][T][P][S] 8 byte real
VT_CY = 6 [V][T][P][S] currency
VT_DATE =7 [V][T][P][S] date
VT_BSTR =8 [V][T][P][S] OLE Automation string
VT_DISPATCH =9 [V][T][P][S] IDispatch *
VT_ERROR =10 [V][T][P][S] SCODE
VT_BOOL =11 [V][T][P][S] True=-1, False=0
VT_VARIANT =12 [V][T][P][S] VARIANT *
VT_UNKNOWN =13 [V][T] [S] IUnknown *
VT_DECIMAL =14 [V][T] [S] 16 byte fixed point
VT_RECORD =36 [V] [P][S] user defined type
VT_I1 = 16 [V][T][P][s] signed char
VT_UI1 =17 [V][T][P][S] unsigned char
VT_UI2 =18 [V][T][P][S] unsigned short
VT_UI4 =19 [V][T][P][S] unsigned short
VT_I8 =20 [T][P] signed 64-bit int
VT_UI8 =21 [T][P] unsigned 64-bit int
VT_INT =22 [V][T][P][S] signed machine int
VT_UINT =23 [V][T] [S] unsigned machine int
VT_VOID =24 [T] C style void
VT_HRESULT =25 [T] Standard return type
VT_PTR =26 [T] pointer type
VT_SAFEARRAY =27 [T] (use VT_ARRAY in VARIANT)
VT_CARRAY = 28 [T] C style array
VT_USERDEFINED =29 [T] user defined type
VT_LPSTR = 30 [T][P] null terminated string
VT_LPWSTR =31 [T][P] wide null terminated string
VT_FILETIME = 64 [P] FILETIME
VT_BLOB = 65 [P] Length prefixed bytes
VT_STREAM =66 [P] Name of the stream follows
VT_STORAGE =67 [P] Name of the storage follows
VT_STREAMED_OBJECT =68 [P] Stream contains an object
VT_STORED_OBJECT =69 [P] Storage contains an object
VT_BLOB_OBJECT =70 [P] Blob contains an object
VT_CF = 71 [P] Clipboard format
VT_CLSID =72 [P] A Class ID
VT_VECTOR = 0x1000 [P] simple counted array
VT_ARRAY = 0x2000 [V] SAFEARRAY*
VT_BYREF = 0x4000 [V] void* for local use
VT_BSTR_BLOB =0xfff Reserved for system use
VT_RESERVED = 0x8000,
VT_ILLEGAL = 0xffff,
VT_ILLEGALMASKED = 0xfff,
VT_TYPEMASK = 0xfff
};

_variant_t 变量初始化时调用的是 _variant_t 的构造函数。如果用一个int型的变量对数字类的变量进行初始化，会出错，因为 _variant_t 的构造函数中没用用整型对其初始化的构造函数，可先将整型转化成long对其进行初始化，赋值操作与构造函数类似，也可如下操作：

int a = 2000;
_variant_t var;
var.vt = VT_I2;
var.lVal = a;

对于没有构造函数的数据类型都可以这样赋值。

对于有构造函数的就可以直接利用构造函数赋值，例如：

long l=222;
_variant_t lVal(l);