_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_I2operator long() const throw(_com_error); // Extracts a long from a VT_I4operator float() const throw(_com_error); // Extracts a float from a VT_R4operator double() const throw(_com_error); // Extracts a double from a VT_R8operator CY() const throw(_com_error); // Extracts a CY from a VT_CYoperator _bstr_t() const throw(_com_error); // Extracts a _bstr_t from a VT_BSTRoperator IDispatch*() const throw(_com_error); // Extracts a IDispatch* from a VT_DISPATCHoperator bool() const throw(_com_error); // Extracts a bool from a VT_BOOLoperator IUnknown*() const throw(_com_error); // Extracts a IUnknown* from a VT_UNKNOWNoperator DECIMAL() const throw(_com_error); // Extracts a DECIMAL from a VT_DECIMALoperator 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] nothingVT_NULL =1 [V] [P] SQL style NullVT_I2 =2 [V][T][P][S] 2 byte signed intVT_I4 =3 [V][T][P][S] 4 byte signed intVT_R4 = 4 [V][T][P][S] 4 byte realVT_R8 = 5 [V][T][P][S] 8 byte realVT_CY = 6 [V][T][P][S] currencyVT_DATE =7 [V][T][P][S] dateVT_BSTR =8 [V][T][P][S] OLE Automation stringVT_DISPATCH =9 [V][T][P][S] IDispatch *VT_ERROR =10 [V][T][P][S] SCODEVT_BOOL =11 [V][T][P][S] True=-1, False=0VT_VARIANT =12 [V][T][P][S] VARIANT *VT_UNKNOWN =13 [V][T] [S] IUnknown *VT_DECIMAL =14 [V][T] [S] 16 byte fixed pointVT_RECORD =36 [V] [P][S] user defined typeVT_I1 = 16 [V][T][P][s] signed charVT_UI1 =17 [V][T][P][S] unsigned charVT_UI2 =18 [V][T][P][S] unsigned shortVT_UI4 =19 [V][T][P][S] unsigned shortVT_I8 =20 [T][P] signed 64-bit intVT_UI8 =21 [T][P] unsigned 64-bit intVT_INT =22 [V][T][P][S] signed machine intVT_UINT =23 [V][T] [S] unsigned machine intVT_VOID =24 [T] C style voidVT_HRESULT =25 [T] Standard return typeVT_PTR =26 [T] pointer typeVT_SAFEARRAY =27 [T] (use VT_ARRAY in VARIANT)VT_CARRAY = 28 [T] C style arrayVT_USERDEFINED =29 [T] user defined typeVT_LPSTR = 30 [T][P] null terminated stringVT_LPWSTR =31 [T][P] wide null terminated stringVT_FILETIME = 64 [P] FILETIMEVT_BLOB = 65 [P] Length prefixed bytesVT_STREAM =66 [P] Name of the stream followsVT_STORAGE =67 [P] Name of the storage followsVT_STREAMED_OBJECT =68 [P] Stream contains an objectVT_STORED_OBJECT =69 [P] Storage contains an objectVT_BLOB_OBJECT =70 [P] Blob contains an objectVT_CF = 71 [P] Clipboard formatVT_CLSID =72 [P] A Class IDVT_VECTOR = 0x1000 [P] simple counted arrayVT_ARRAY = 0x2000 [V] SAFEARRAY*VT_BYREF = 0x4000 [V] void* for local useVT_BSTR_BLOB =0xfff Reserved for system useVT_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);