VARIANT的使用

VARIANT是一个联合结构体，完整的定义为：

typedef struct tagVARIANT {union {struct __tagVARIANT {VARTYPE vt;WORD    wReserved1;WORD    wReserved2;WORD    wReserved3;union {LONGLONG            llVal;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            *punkVal;IDispatch           *pdispVal;SAFEARRAY           *parray;BYTE                *pbVal;SHORT               *piVal;LONG                *plVal;LONGLONG            *pllVal;FLOAT               *pfltVal;DOUBLE              *pdblVal;VARIANT_BOOL        *pboolVal;_VARIANT_BOOL       *pbool;SCODE               *pscode;CY                  *pcyVal;DATE                *pdate;BSTR                *pbstrVal;IUnknown            **ppunkVal;IDispatch           **ppdispVal;SAFEARRAY           **pparray;VARIANT             *pvarVal;PVOID               byref;CHAR                cVal;USHORT              uiVal;ULONG               ulVal;ULONGLONG           ullVal;INT                 intVal;UINT                uintVal;DECIMAL             *pdecVal;CHAR                *pcVal;USHORT              *puiVal;ULONG               *pulVal;ULONGLONG           *pullVal;INT                 *pintVal;UINT                *puintVal;struct __tagBRECORD {PVOID       pvRecord;IRecordInfo *pRecInfo;} __VARIANT_NAME_4;} __VARIANT_NAME_3;} __VARIANT_NAME_2;DECIMAL             decVal;} __VARIANT_NAME_1;
} VARIANT, *LPVARIANT, VARIANTARG, *LPVARIANTARG;

为方便操作，定义了另外一个类_variant_t，完整定义为：

class _variant_t : public ::tagVARIANT {
public:// Constructors//_variant_t() throw();_variant_t(const VARIANT& varSrc) ;_variant_t(const VARIANT* pSrc) ;_variant_t(const _variant_t& varSrc) ;_variant_t(VARIANT& varSrc, bool fCopy) ;          // Attach VARIANT if !fCopy_variant_t(short sSrc, VARTYPE vtSrc = VT_I2) ;    // Creates a VT_I2, or a VT_BOOL_variant_t(long lSrc, VARTYPE vtSrc = VT_I4) ;     // 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) ; // Creates a VT_R8, or a VT_DATE_variant_t(const CY& cySrc) throw();                                // Creates a VT_CY_variant_t(const _bstr_t& bstrSrc) ;               // Creates a VT_BSTR_variant_t(const wchar_t *pSrc) ;                  // Creates a VT_BSTR_variant_t(const char* pSrc) ;                     // Creates a VT_BSTR_variant_t(IDispatch* pSrc, bool fAddRef = true) throw();           // Creates a VT_DISPATCH_variant_t(bool boolSrc) 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_variant_t(char cSrc) throw();                                      // Creates a VT_I1_variant_t(unsigned short usSrc) throw();                           // Creates a VT_UI2_variant_t(unsigned long ulSrc) throw();                            // Creates a VT_UI4_variant_t(int iSrc) throw();                                       // Creates a VT_INT_variant_t(unsigned int uiSrc) throw();                             // Creates a VT_UINT
#if (_WIN32_WINNT >= 0x0501)_variant_t(__int64 i8Src) throw();                                  // Creates a VT_I8_variant_t(unsigned __int64 ui8Src) throw();                        // Creates a VT_UI8
#endif// Destructor//~_variant_t() throw() ;// Extractors//operator short() const ;                           // Extracts a short from a VT_I2operator long() const ;                            // Extracts a long from a VT_I4operator float() const ;                           // Extracts a float from a VT_R4operator double() const ;                          // Extracts a double from a VT_R8operator CY() const ;                              // Extracts a CY from a VT_CYoperator _bstr_t() const ;                         // Extracts a _bstr_t from a VT_BSTRoperator IDispatch*() const ;                      // Extracts a IDispatch* from a VT_DISPATCHoperator bool() const ;                            // Extracts a bool from a VT_BOOLoperator IUnknown*() const ;                       // Extracts a IUnknown* from a VT_UNKNOWNoperator DECIMAL() const ;                         // Extracts a DECIMAL from a VT_DECIMALoperator BYTE() const ;                            // Extracts a BTYE (unsigned char) from a VT_UI1operator VARIANT() const throw();operator char() const ;                            // Extracts a char from a VT_I1operator unsigned short() const ;                  // Extracts a unsigned short from a VT_UI2operator unsigned long() const ;                   // Extracts a unsigned long from a VT_UI4operator int() const ;                             // Extracts a int from a VT_INToperator unsigned int() const ;                    // Extracts a unsigned int from a VT_UINT
#if (_WIN32_WINNT >= 0x0501)operator __int64() const ;                         // Extracts a __int64 from a VT_I8operator unsigned __int64() const ;                // Extracts a unsigned __int64 from a VT_UI8
#endif// Assignment operations//_variant_t& operator=(const VARIANT& varSrc) ;_variant_t& operator=(const VARIANT* pSrc) ;_variant_t& operator=(const _variant_t& varSrc) ;_variant_t& operator=(short sSrc) ;                // Assign a VT_I2, or a VT_BOOL_variant_t& operator=(long lSrc) ;                 // Assign a VT_I4, a VT_ERROR or a VT_BOOL_variant_t& operator=(float fltSrc) ;              // Assign a VT_R4_variant_t& operator=(double dblSrc) ;             // Assign a VT_R8, or a VT_DATE_variant_t& operator=(const CY& cySrc) ;           // Assign a VT_CY_variant_t& operator=(const _bstr_t& bstrSrc) ;    // Assign a VT_BSTR_variant_t& operator=(const wchar_t* pSrc) ;       // Assign a VT_BSTR_variant_t& operator=(const char* pSrc) ;          // Assign a VT_BSTR_variant_t& operator=(IDispatch* pSrc) ;           // Assign a VT_DISPATCH_variant_t& operator=(bool boolSrc) ;              // Assign a VT_BOOL_variant_t& operator=(IUnknown* pSrc) ;            // Assign a VT_UNKNOWN_variant_t& operator=(const DECIMAL& decSrc) ;     // Assign a VT_DECIMAL_variant_t& operator=(BYTE bSrc) ;                 // Assign a VT_UI1_variant_t& operator=(char cSrc) ;                 // Assign a VT_I1_variant_t& operator=(unsigned short usSrc) ;      // Assign a VT_UI2_variant_t& operator=(unsigned long ulSrc) ;       // Assign a VT_UI4_variant_t& operator=(int iSrc) ;                  // Assign a VT_INT_variant_t& operator=(unsigned int uiSrc) ;        // Assign a VT_UINT
#if (_WIN32_WINNT >= 0x0501)_variant_t& operator=(__int64 i8Src) ;             // Assign a VT_I8_variant_t& operator=(unsigned __int64 ui8Src) ;   // Assign a VT_UI8
#endif// Comparison operations//bool operator==(const VARIANT& varSrc) const throw();bool operator==(const VARIANT* pSrc) const throw();bool operator!=(const VARIANT& varSrc) const throw();bool operator!=(const VARIANT* pSrc) const throw();// Low-level operations//void Clear() ;void Attach(VARIANT& varSrc) ;VARIANT Detach() throw();VARIANT& GetVARIANT() throw();VARIANT* GetAddress() ;void ChangeType(VARTYPE vartype, const _variant_t* pSrc = NULL) ;void SetString(const char* pSrc) ; // used to set ANSI string
};

通常的操作方法为，使用VARIANT定义变量，使用 _variant_t 初始化变量值。

但给类成员变量赋值字符串值时会有问题。

例如：

class A{
public;   VARIANT m_var;
}A a;
a.m_var = _variant_t(“1234”);

这种情况下，m_var中存放的是个野指针，实际的指针在_variant_t的析构函数中被释放了。

如果是其他类型的数据，这种赋值方式是没问题的。

正确的做法应该是这样的：

m_var.vt=VT_STR;
m_var.bstrVal=SysAllocString("1234");

考虑代码的严谨性，整个类应该这样设计：

class A{
public;   VARIANT m_var;A::A(){}A::A(LPCTSTR str){VariantInit(&m_var);m_var.vt = VT_BSTR;m_var.bstrVal=SysAllocString(str);}A::~A(){if (m_var.vt = VT_BSTR){SysFreeString(m_var.bstrVal);}VariantClear(&m_var);}
}

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