_variant_t類封閉了VARIANT數據類型,VARIANT是一個結構體類型,我們可以看一下它的定義
- {
- VARTYPE vt; //存儲數據類型
- unsigned short wReserved1;
- unsigned short wReserved2;
- unsigned short wReserved3;
- union
- {
- Byte bVal; // VT_UI1.
- Short iVal; // VT_I2.
- long lVal; // VT_I4.
- float fltVal; // VT_R4.
- double dblVal; // VT_R8.
- VARIANT_BOOL boolVal; // VT_BOOL.
- SCODE scode; // VT_ERROR.
- CY cyVal; // VT_CY.
- DATE date; // VT_DATE.
- BSTR bstrVal; // VT_BSTR.
- DECIMAL FAR* pdecVal // VT_BYREF|VT_DECIMAL.
- IUnknown FAR* punkVal; // VT_UNKNOWN.
- IDispatch FAR* pdispVal; // VT_DISPATCH.
- SAFEARRAY FAR* parray; // VT_ARRAY|*.
- Byte FAR* pbVal; // VT_BYREF|VT_UI1.
- short FAR* piVal; // VT_BYREF|VT_I2.
- long FAR* plVal; // VT_BYREF|VT_I4.
- float FAR* pfltVal; // VT_BYREF|VT_R4.
- double FAR* pdblVal; // VT_BYREF|VT_R8.
- VARIANT_BOOL FAR* pboolVal; // VT_BYREF|VT_BOOL.
- SCODE FAR* pscode; // VT_BYREF|VT_ERROR.
- CY FAR* pcyVal; // VT_BYREF|VT_CY.
- DATE FAR* pdate; // VT_BYREF|VT_DATE.
- BSTR FAR* pbstrVal; // VT_BYREF|VT_BSTR.
- IUnknown FAR* FAR* ppunkVal; // VT_BYREF|VT_UNKNOWN.
- IDispatch FAR* FAR* ppdispVal; // VT_BYREF|VT_DISPATCH.
- SAFEARRAY FAR* FAR* pparray; // VT_ARRAY|*.
- VARIANT FAR* pvarVal; // VT_BYREF|VT_VARIANT.
- void FAR* byref; // Generic ByRef.
- char cVal; // VT_I1.
- unsigned short uiVal; // VT_UI2.
- unsigned long ulVal; // VT_UI4.
- int intVal; // VT_INT.
- unsigned int uintVal; // VT_UINT.
- char FAR * pcVal; // VT_BYREF|VT_I1.
- unsigned short FAR * puiVal; // VT_BYREF|VT_UI2.
- unsigned long FAR * pulVal; // VT_BYREF|VT_UI4.
- int FAR * pintVal; // VT_BYREF|VT_INT.
- unsigned int FAR * puintVal; //VT_BYREF|VT_UINT.
- };
- };
typedef struct tagVARIANT
{
VARTYPE vt; //存儲數據類型
unsigned short wReserved1;
unsigned short wReserved2;
unsigned short wReserved3;
union
{
Byte bVal; // VT_UI1.
Short iVal; // VT_I2.
long lVal; // VT_I4.
float fltVal; // VT_R4.
double dblVal; // VT_R8.
VARIANT_BOOL boolVal; // VT_BOOL.
SCODE scode; // VT_ERROR.
CY cyVal; // VT_CY.
DATE date; // VT_DATE.
BSTR bstrVal; // VT_BSTR.
DECIMAL FAR* pdecVal // VT_BYREF|VT_DECIMAL.
IUnknown FAR* punkVal; // VT_UNKNOWN.
IDispatch FAR* pdispVal; // VT_DISPATCH.
SAFEARRAY FAR* parray; // VT_ARRAY|*.
Byte FAR* pbVal; // VT_BYREF|VT_UI1.
short FAR* piVal; // VT_BYREF|VT_I2.
long FAR* plVal; // VT_BYREF|VT_I4.
float FAR* pfltVal; // VT_BYREF|VT_R4.
double FAR* pdblVal; // VT_BYREF|VT_R8.
VARIANT_BOOL FAR* pboolVal; // VT_BYREF|VT_BOOL.
SCODE FAR* pscode; // VT_BYREF|VT_ERROR.
CY FAR* pcyVal; // VT_BYREF|VT_CY.
DATE FAR* pdate; // VT_BYREF|VT_DATE.
BSTR FAR* pbstrVal; // VT_BYREF|VT_BSTR.
IUnknown FAR* FAR* ppunkVal; // VT_BYREF|VT_UNKNOWN.
IDispatch FAR* FAR* ppdispVal; // VT_BYREF|VT_DISPATCH.
SAFEARRAY FAR* FAR* pparray; // VT_ARRAY|*.
VARIANT FAR* pvarVal; // VT_BYREF|VT_VARIANT.
void FAR* byref; // Generic ByRef.
char cVal; // VT_I1.
unsigned short uiVal; // VT_UI2.
unsigned long ulVal; // VT_UI4.
int intVal; // VT_INT.
unsigned int uintVal; // VT_UINT.
char FAR * pcVal; // VT_BYREF|VT_I1.
unsigned short FAR * puiVal; // VT_BYREF|VT_UI2.
unsigned long FAR * pulVal; // VT_BYREF|VT_UI4.
int FAR * pintVal; // VT_BYREF|VT_INT.
unsigned int FAR * puintVal; //VT_BYREF|VT_UINT.
};
};
注意:vt用來存儲內部變量(聯合體中保存的變量)的類型,聯合體用來存儲對應類型的值二、數據類型轉換
_bstr_t類可以作爲_variant_t類與基本數據類型轉換的中介
(1) _variant_t與CString之間的轉化
1> CString轉換爲_variant_t:
- CString str;
- _variant_t str1=(LPCTSTR)str;
CString str;
_variant_t str1=(LPCTSTR)str;
使用_variant_t的成員函數SetString來對_variant_t的對象賦值會更好;2> _variant_t轉換爲CString:
- _variant_t vt;
- CString tempstr=(LPCSTR)_bstr_t(vt); //_bstr_t的構造函數有對_variant_t類型的處理
_variant_t vt;
CString tempstr=(LPCSTR)_bstr_t(vt); //_bstr_t的構造函數有對_variant_t類型的處理
(2) _variant_t與char *之間的轉換
1> char * 轉換爲_variant_t
方法與CString轉換爲_variant_t的方法類似:
- char * cValue;
- _variant_t vValue=(LPSTR)cValue;
char * cValue;
_variant_t vValue=(LPSTR)cValue;
2> _variant_t轉換爲char *:
錯誤方法:
- _variant_t vValue;
- char * value=(LPSTR)(LPCSTR)_bstr_t(vValue)
_variant_t vValue;
char * value=(LPSTR)(LPCSTR)_bstr_t(vValue)
value指向一堆亂碼...
原因:不能用char *直接指向(LPSTR)(LPCSTR)_bstr_t( _variant_t ),因爲這樣轉換之後實際上是一個string,而非一個char *
正確方法:
進行轉換時,只能用strcpy(),將LPSTR指向的字符複製到char * 所指向的內存中;如下例:
- _variant_t vValue;
- char cValue[16]={0};
- strcpy(cValue, (LPCSTR)_bstr_t(vValue));
_variant_t vValue;
char cValue[16]={0};
strcpy(cValue, (LPCSTR)_bstr_t(vValue));
(3) 判斷_variant_t的值類型
下面的轉換代碼根據_variant_t的vt進行類型判斷,然後將數據值轉換爲CString類型(可作爲轉換爲其他數據類型的中介)
- CString str;
- //以下代碼演示如何轉換爲C標準字符串型
- switch(var.vt)
- {
- case VT_BSTR:
- {
- str=var.bstrVal;
- break;
- }
- case VT_I2: //var is short int type
- {
- str.Format("%d",(int)var.iVal);
- break;
- }
- case VT_I4: //var is long int type
- {
- str.Format("%d", var.lVal);
- break;
- }
- case VT_R4: //var is float type
- {
- str.Format("%10.6f", (double)var.fltVal);
- break;
- }
- case VT_R8: //var is double type
- {
- str.Format("%10.6f", var.dblVal);
- break;
- }
- case VT_CY: //var is CY type
- {
- str=COleCurrency(var).Format();
- break;
- }
- case VT_DATE: //var is DATE type
- {
- str=COleDateTime(var).Format();
- break;
- }
- case VT_BOOL: //var is VARIANT_BOOL
- {
- str= (var.boolVal==0) ?"FALSE": "TRUE";
- break;
- }
- default:
- {
- str.Format("Unk type %d\n",var.vt);
- TRACE("Unknown type %d\n",var.vt);
- break;
- }
- }