_variant_t
我们先看看COM所支持的一些类型的基本类：
Microsoft Specific
Standard classes are used to support some of the COM types. The classes are defined in comdef.h and the header files generated from the type library.
Class Purpose
_bstr_t
Wraps the BSTR type to provide useful operators and methods.
_com_error
Defines the error object thrown by _com_raise_error in most failures.
_com_ptr_t
Encapsulates COM interface pointers, and automates the required calls to AddRef, Release, and QueryInterface.
_variant_t
Wraps the VARIANT type to provide useful operators and methods.
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接下来我们在回过来看看_variant_t类的一些属性：
Microsoft Specific
A _variant_t object encapsulates the VARIANT data type. The class manages resource allocation and deallocation and makes function calls to VariantInit and VariantClear as appropriate.
Construction
_variant_t
Constructs a _variant_t object.
Operations
Attach
Attaches a VARIANT object into the _variant_t object.
Clear
Clears the encapsulated VARIANT object.
ChangeType
Changes the type of the _variant_t object to the indicated VARTYPE.
Detach
Detaches the encapsulated VARIANT object from this _variant_t object.
SetString
Assigns a string to this _variant_t object.
Operators
Operator =
Assigns a new value to an existing _variant_t object.
operator ==, !=
Compare two _variant_t objects for equality or inequality.
Extractors
Extract data from the encapsulated VARIANT object.
_variant_t类封闭了VARIANT数据类型,VARIANT是一个结构体类型,我们可以看一下它的定义
typedef struct tagVARIANT {
VARTYPE vt;
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.
};
};
vt用来存储存储变量的类型,联合体用来存储对用类型的值
对_variant_t变量初始化调用的是_variant_t的构造函数,我们一般习惯于用一个int型的变量对数字类的变量进行初始化,这对_variant_t变量是不允许的,原因很简单,_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.
_variant_t与CString之间的转化
CString--->_variant_t
CString str;
_variant_t str1=(LPCTSTR)str; //【但我更倾向于使用_variant_t的成员函数SetString来对_variant_t的对象赋值】
_variant_t--->CString
_variant_t vt;
CString tempstr=(LPCSTR)_bstr_t(vt); //_bstr_t的构造函数有对_variant_t类型的处理
===================================================================================
/*
将 _variant_t 类型的变量，转换成字符串（比如如果是数字，则转换成数字对应的字符串 ：如 10.2 -> "10.200000"）
*/
_variant_t var;
CString str; //转换以后的字符串
//以下代码演示如何转换为C标准字符串型
if (var.vt == VT_I4)
{
long lNum;
char szCh[21];
str=var.bstrVal;
WideCharToMultiByte
(CP_ACP, 0, var.bstrVal, -1,
szCh, sizeof(szCh), NULL, NULL);
}
//以下代码演示如何转换成逻辑型
if( var.vt == VT_BOOL)
{
BOOL bVar;
lNum=var.lVal;
bVar= var.boolVar==0? FALSE : TRUE;
}
//以下代码演示为其余类型（补充）
switch(var.vt)
{
case VT_BSTR: //var is BSTR type
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);
}
