_bstr_t是BSTR_T这个数据类型对应的类
_variant_t是VARIANT这个数据类型的封装
Microsoft Specific
Constructs a _bstr_t object.
_bstr_t( ) throw( ); _bstr_t( const _bstr_t& s1 ) throw( ); _bstr_t( const char* s2 ); _bstr_t( const wchar_t* s3 ); _bstr_t( const _variant_t& var ); _bstr_t( BSTR bstr, bool fCopy );
Parameters
- s1
- A _bstr_t object to be copied.
- s2
- A multibyte string.
- s3
- A Unicode string
- var
- A _variant_t object.
- bstr
- An existing BSTR object.
- fCopy
- If false, the bstr argument is attached to the new object without making a copy by calling SysAllocString.
Remarks
- _bstr_t( ) Constructs a default _bstr_t object that encapsulates a NULL BSTR object.
- _bstr_t( _bstr_t& s1 ) Constructs a _bstr_t object as a copy of another. This is a "shallow" copy, which increments the reference count of the encapsulated BSTR object instead
of creating a new one. - _bstr_t( char* s2 ) Constructs a _bstr_t object by calling SysAllocString to create a new BSTR object and encapsulate it. This constructor first performs a multibyte
to Unicode conversion.If s2 is too large, you may generate a stack overflow error. In such a situation, convert your char* to a wchar_t with MultiByteToWideChar and
then call the wchar_t * constructor. - _bstr_t( wchar_t* s3 ) Constructs a _bstr_t object by calling SysAllocString to create a new BSTR object and encapsulates it.
- _bstr_t( _variant_t& var ) Constructs a _bstr_t object from a _variant_t object
by first retrieving a BSTR object from the encapsulated VARIANT object. - _bstr_t( BSTR bstr | bool fCopy ) Constructs a _bstr_t object from an existing BSTR (as opposed to a wchar_t* string). If fCopy is false,
the supplied BSTR is attached to the new object without making a new copy with SysAllocString. This is the method used by the wrapper functions in the type library headers to encapsulate and take ownership of a BSTR,
returned by an interface method, in a _bstr_t object.
END Microsoft Specific
See Also
_variant_t
编辑本段简介
_variant_t和_bstr_t这两个类分别封装并管理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。
编辑本段定义
_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
};
编辑本段案例
我定义了一组函数可以方便的实现_variant_t类型转换
//_variant_t转WORD类型
BOOL AfxVariantConvert(WORD& val, const _variant_t& vt)
{
if (vt.vt == VT_UI2)
{
val = (WORD)vt.uiVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转DWORD类型
BOOL AfxVariantConvert(DWORD& val, const _variant_t& vt)
{
if (vt.vt == VT_UI4 )
{
val = (DWORD)vt.ulVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转DWORD64类型
BOOL AfxVariantConvert(DWORD64& val, const _variant_t& vt)
{
if (vt.vt != VT_UI8 )
{
val = (DWORD64)vt.ullVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转CString类型
BOOL AfxVariantConvert(CString& val, const _variant_t& vt)
{
if (vt.vt == VT_BSTR)
{
val = (LPCTSTR)vt.bstrVal;
return TRUE;
}
else
{
val = _T("");
return FALSE;
}
}
_variant_t转BOOL类型
BOOL AfxVariantConvert(BOOL& val, const _variant_t& vt)
{
if (vt.vt == VT_BOOL)
{
val = vt.boolVal;
return TRUE;
}
else
{
val = FALSE;
return FALSE;
}
}
_variant_t转CTime类型（精度变短，可能丢失数据）
BOOL AfxVariantConvert(CTime& val, const _variant_t& vt)
{
if (vt.vt == VT_DATE)
{
val = vt.date;
return TRUE;
}
else
{
val = NULL;
return FALSE;
}
}
_variant_t转double类型
BOOL AfxVariantConvert(DOUBLE& val, const _variant_t& vt)
{
if (vt.vt == VT_R8)
{
val = vt.dblVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转byte类型
BOOL AfxVariantConvert(BYTE& val, const _variant_t& vt)
{
if (vt.vt == VT_UI1 || vt.vt == VT_I1)
{
val = vt.bVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转float类型
BOOL AfxVariantConvert( FLOAT& val, const _variant_t& vt )
{
if (vt.vt == VT_R4)
{
val = vt.fltVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转long类型
BOOL AfxVariantConvert( long& val,const _variant_t& vt )
{
if (vt.vt == VT_I4)
{
val = vt.lVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
_variant_t转INT64类型
BOOL AfxVariantConvert( INT64& val,const _variant_t& vt )
{
if (vt.vt == VT_I8)
{
val = vt.llVal;
return TRUE;
}
else
{
val = 0;
return FALSE;
}
}
编辑本段功能：
实现_variant_t向int,String ,double,long,byte等类型转换
_variant_t到_bstr_t的转换
_bstr_t(vt)
CString str((char_t*)(_bstr_t)(vt)