从Java到C++——从union到VARIANT与CComVariant的深层剖析
我第一次用VARIANT和CComVariant的时候全然不明确它是怎么回事，为它什么数据类型都能够存放。什么数据都能够被能够初始化？里面究竟是怎么实现的？感觉又奇妙又迷惑！我们在上一节中讲了union的使用方法之后你或许想到了大概是怎么回事了。没错。union能够帮我们实现这一个奇妙的功能。并且VARIANT定义中确实使用了union。假设你还不明确，那我们就自己来模拟实现一个相似于CComVariant功能的类把，废话不多说，看代码：
CVariate.h:
#ifndef CVARIATE_H
#define CVARIATE_H
#include <string.h>
//======================================================
const unsigned int DEFAULT_STRLEN = 256;
//=======================================================
//自定义一个类用于存放随意类型的数据，以模拟类型VARIANT的功能。
class CVariate
{
public:
CVariate() : type(INT), nVal(0){} //默认构造函数。初始化nVal，值为0
virtual ~CVariate(){}; //虚构函数，不做不论什么处理
CVariate(const CVariate& val) : type(val.type) //拷贝构造函数
{
CopyUnion(val);
}
CVariate &operator=(char c)
{
type = CHAR;
cVal = c;
return *this;
}
CVariate &operator=(short int sn)
{
type = SHORT;
snVal = sn;
return *this;
}
CVariate &operator=(int i)
{
type = INT;
nVal = i;
return *this;
}
CVariate &operator=(long l)
{
type = LONG;
lVal = l;
return *this;
}
CVariate &operator=(float f)
{
type = FLOAT;
fVal = f;
return *this;
}
CVariate &operator=(double d)
{
type = DOUBLE;
dVal = d;
return *this;
}
CVariate &operator=(const char* str);
void CopyUnion(const CVariate& val); //数据的拷贝
void DisplayValue();
private:
enum {CHAR, SHORT, INT, LONG, FLOAT, DOUBLE, STR}type; //为不同的数据类型定义一个枚举值
union //union，能够存入各种类型的数据
{
char cVal;
short snVal;
int nVal;
long lVal;
float fVal;
double dVal;
char strVal[DEFAULT_STRLEN];
};
};
#endif // CVARIATE_H
CVariate.cpp:#include "../include/CVariate.h"
#include <iostream>
CVariate& CVariate::operator=(const char* str)
{
if(strlen(str) >= DEFAULT_STRLEN)
{
std::cerr << "The length of string is out of memory." << std::endl;
} else
{
strcpy(strVal, str);
type = STR;
}
return *this;
}
void CVariate::CopyUnion(const CVariate& val)
{
switch(val.type)
{
case CVariate::CHAR:
cVal = val.cVal;
break;
case CVariate::SHORT:
snVal = val.snVal;
break;
case CVariate::INT:
nVal = val.nVal;
break;
case CVariate::LONG:
lVal = val.lVal;
break;
case CVariate::FLOAT:
fVal = val.fVal;
break;
case CVariate::DOUBLE:
dVal = val.dVal;
break;
case CVariate::STR:
if(strlen(val.strVal) >= DEFAULT_STRLEN)
{
std::cerr << "The length of string is out of memory." << std::endl;
break;
} else
{
strcpy(strVal, val.strVal);
}
break;
default:
return;
}
}
void CVariate::DisplayValue()
{
switch(type)
{
case CVariate::CHAR:
std::cout << cVal;
break;
case CVariate::SHORT:
std::cout << snVal;
break;
case CVariate::INT:
std::cout << nVal;
break;
case CVariate::LONG:
std::cout << lVal;
break;
case CVariate::FLOAT:
std::cout << fVal;
break;
case CVariate::DOUBLE:
std::cout << dVal;
break;
case CVariate::STR:
char s[255];
strcpy(s, strVal);
std::cout << s;
break;
default:
return;
}
std::cout << std::endl;
}
Test.cpp:int main()
{
CVariate cVal;
cVal.DisplayValue();
cVal = 125;
cVal.DisplayValue();
CVariate cVal2(cVal);
cVal2.DisplayValue();
cVal2 = 188.598;
cVal2.DisplayValue();
cVal2 = "Hello World.";
cVal2.DisplayValue();
return 0;
}结果例如以下：VARIANT的结构能够參考头文件VC98\Include\OAIDL.H中关于结构体tagVARIANT的定义例如以下：
struct tagVARIANT {
union {
struct __tagVARIANT {
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union {
ULONGLONG ullVal; ---VT_UI8
LONGLONG llVal; ---VT_I8
LONG lVal; ---VT_I4 */
BYTE bVal; ---VT_UI1 */
SHORT iVal; ---VT_I2 */
FLOAT fltVal; ---VT_R4 */
DOUBLE dblVal; ---VT_R8 */
VARIANT_BOOL boolVal; ---VT_BOOL */
_VARIANT_BOOL bool; ---(obsolete) */
SCODE scode; ---VT_ERROR */
CY cyVal; ---VT_CY */
DATE date; ---VT_DATE */
BSTR bstrVal; ---VT_BSTR */
IUnknown * punkVal; ---VT_UNKNOWN */
IDispatch * pdispVal; ---VT_DISPATCH */
SAFEARRAY * parray; ---VT_ARRAY */
BYTE * pbVal; ---VT_BYREF|VT_UI1 */
SHORT * piVal; ---VT_BYREF|VT_I2 */
LONG * plVal; ---VT_BYREF|VT_I4 */
LONGLONG * pllVal; ---VT_BYREF|VT_I8 */
FLOAT * pfltVal; ---VT_BYREF|VT_R4 */
DOUBLE * pdblVal; ---VT_BYREF|VT_R8 */
VARIANT_BOOL *pboolVal; ---VT_BYREF|VT_BOOL */
_VARIANT_BOOL *pbool; ---(obsolete) */
SCODE * pscode; ---VT_BYREF|VT_ERROR */
CY * pcyVal; ---VT_BYREF|VT_CY */
DATE * pdate; ---VT_BYREF|VT_DATE */
BSTR * pbstrVal; ---VT_BYREF|VT_BSTR */
IUnknown ** ppunkValVARIANT的结构能够參考头文件VC98\Include\OAIDL.H中关于结构体tagVARIANT的定义。
struct tagVARIANT {
union {
struct __tagVARIANT {
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union {
ULONGLONG ullVal; ---VT_UI8
LONGLONG llVal; ---VT_I8
LONG lVal; ---VT_I4 */
BYTE bVal; ---VT_UI1 */
SHORT iVal; ---VT_I2 */
FLOAT fltVal; ---VT_R4 */
DOUBLE dblVal; ---VT_R8 */
VARIANT_BOOL boolVal; ---VT_BOOL */
_VARIANT_BOOL bool; ---(obsolete) */
SCODE scode; ---VT_ERROR */
CY cyVal; ---VT_CY */
DATE date; ---VT_DATE */
BSTR bstrVal; ---VT_BSTR */
IUnknown * punkVal; ---VT_UNKNOWN */
IDispatch * pdispVal; ---VT_DISPATCH */
SAFEARRAY * parray; ---VT_ARRAY */
BYTE * pbVal; ---VT_BYREF|VT_UI1 */
SHORT * piVal; ---VT_BYREF|VT_I2 */
LONG * plVal; ---VT_BYREF|VT_I4 */
LONGLONG * pllVal; ---VT_BYREF|VT_I8 */
FLOAT * pfltVal; ---VT_BYREF|VT_R4 */
DOUBLE * pdblVal; ---VT_BYREF|VT_R8 */
VARIANT_BOOL *pboolVal; ---VT_BYREF|VT_BOOL */
_VARIANT_BOOL *pbool; ---(obsolete) */
SCODE * pscode; ---VT_BYREF|VT_ERROR */
CY * pcyVal; ---VT_BYREF|VT_CY */
DATE * pdate; ---VT_BYREF|VT_DATE */
BSTR * pbstrVal; ---VT_BYREF|VT_BSTR */
IUnknown ** ppunkVal; ---VT_BYREF|VT_UNKNOWN */
IDispatch ** ppdispVal; ---VT_BYREF|VT_DISPATCH */
SAFEARRAY ** pparray; ---VT_BYREF|VT_ARRAY */
VARIANT * pvarVal; ---VT_BYREF|VT_VARIANT */
PVOID byref; ---Generic ByRef */
CHAR cVal; ---VT_I1 */
USHORT uiVal; ---VT_UI2 */
ULONG ulVal; ---VT_UI4 */
INT intVal; ---VT_INT */
VARIANT的结构能够參考头文件VC98\Include\OAIDL.H中关于结构体tagVARIANT的定义。
struct tagVARIANT {
union {
struct __tagVARIANT {
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union {
ULONGLONG ullVal; ---VT_UI8
LONGLONG llVal; ---VT_I8
LONG lVal; ---VT_I4 */
BYTE bVal; ---VT_UI1 */
SHORT iVal; ---VT_I2 */
FLOAT fltVal; ---VT_R4 */
DOUBLE dblVal; ---VT_R8 */
VARIANT_BOOL boolVal; ---VT_BOOL */
_VARIANT_BOOL bool; ---(obsolete) */
SCODE scode; ---VT_ERROR */
CY cyVal; ---VT_CY */
DATE date; ---VT_DATE */
BSTR bstrVal; ---VT_BSTR */
IUnknown * punkVal; ---VT_UNKNOWN */
IDispatch * pdispVal; ---VT_DISPATCH */
SAFEARRAY * parray; ---VT_ARRAY */
BYTE * pbVal; ---VT_BYREF|VT_UI1 */
SHORT * piVal; ---VT_BYREF|VT_I2 */
LONG * plVal; ---VT_BYREF|VT_I4 */
LONGLONG * pllVal; ---VT_BYREF|VT_I8 */
FLOAT * pfltVal; ---VT_BYREF|VT_R4 */
DOUBLE * pdblVal; ---VT_BYREF|VT_R8 */
VARIANT_BOOL *pboolVal; ---VT_BYREF|VT_BOOL */
_VARIANT_BOOL *pbool; ---(obsolete) */
SCODE * pscode; ---VT_BYREF|VT_ERROR */
CY * pcyVal; ---VT_BYREF|VT_CY */
DATE * pdate; ---VT_BYREF|VT_DATE */
BSTR * pbstrVal; ---VT_VARIANT的结构能够參考头文件VC98\Include\OAIDL.H中关于结构体tagVARIANT的定义。
struct tagVARIANT {
union {
struct __tagVARIANT {
VARTYPE vt;
WORD wReserved1;
WORD wReserved2;
WORD wReserved3;
union {
ULONGLONG ullVal; ---VT_UI8
LONGLONG llVal; ---VT_I8
LONG lVal; ---VT_I4 */
BYTE bVal; ---VT_UI1 */
SHORT iVal; ---VT_I2 */
FLOAT fltVal; ---VT_R4 */
DOUBLE dblVal; ---VT_R8 */
VARIANT_BOOL boolVal; ---VT_BOOL */
_VARIANT_BOOL bool; ---(obsolete) */
SCODE scode; ---VT_ERROR */
CY cyVal; ---VT_CY */
DATE date; ---VT_DATE */
BSTR bstrVal; ---VT_BSTR */
IUnknown * punkVal; ---VT_UNKNOWN */
IDispatch * pdispVal; ---VT_DISPATCH */
SAFEARRAY * parray; ---VT_ARRAY */
BYTE * pbVal; ---VT_BYREF|VT_UI1 */
SHORT * piVal; ---VT_BYREF|VT_I2 */
LONG * plVal; ---VT_BYREF|VT_I4 */
LONGLONG * pllVal; ---VT_BYREF|VT_I8 */
FLOAT * pfltVal; ---VT_BYREF|VT_R4 */
DOUBLE * pdblVal; ---VT_BYREF|VT_R8 */
VARIANT_BOOL *pboolVal; ---VT_BYREF|VT_BOOL */
_VARIANT_BOOL *pbool; ---(obsolete) */
SCODE * pscode; ---VT_BYREF|VT_ERROR */
CY * pcyVal; ---VT_BYREF|VT_CY */
DATE * pdate; ---VT_BYREF|VT_DATE */
BSTR * pbstrVal; ---VT_BYREF|VT_BSTR */
IUnknown ** ppunkVal; ---VT_BYREF|VT_UNKNOWN */
IDispatch ** ppdispVal; ---VT_BYREF|VT_DISPATCH */
SAFEARRAY ** pparray; ---VT_BYREF|VT_ARRAY */
VARIANT * pvarVal; ---VT_BYREF|VT_VARIANT */
PVOID byref; ---Generic ByRef */
CHAR cVal; ---VT_I1 */
USHORT uiVal; ---VT_UI2 */
ULONG ulVal; ---VT_UI4 */
INT intVal; ---VT_INT */
UINT uintVal; ---VT_UINT */
DECIMAL * pdecVal; ---VT_BYREF|VT_DECIMAL */
CHAR * pcVal; ---VT_BYREF|VT_I1 */
USHORT * puiVal; ---VT_BYREF|VT_UI2 */
ULONG * pulVal; ---VT_BYREF|VT_UI4 */
ULONGLONG * pullVal; --- VT_BYREF|VT_UI8 */
INT * pintVal; ---VT_BYREF|VT_INT */
UINT * puintVal; ---VT_BYREF|VT_UINT
struct __tagBRECORD {
PVOID pvRecord;
IRecordInfo * pRecInfo;
} __VARIANT_NAME_4; --- VT_RECORD
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
};BYREF|VT_BSTR */
IUnknown ** ppunkVal; ---VT_BYREF|VT_UNKNOWN */
IDispatch ** ppdispVal; ---VT_BYREF|VT_DISPATCH */
SAFEARRAY ** pparray; ---VT_BYREF|VT_ARRAY */
VARIANT * pvarVal; ---VT_BYREF|VT_VARIANT */
PVOID byref; ---Generic ByRef */
CHAR cVal; ---VT_I1 */
USHORT uiVal; ---VT_UI2 */
ULONG ulVal; ---VT_UI4 */
INT intVal; ---VT_INT */
UINT uintVal; ---VT_UINT */
DECIMAL * pdecVal; ---VT_BYREF|VT_DECIMAL */
CHAR * pcVal; ---VT_BYREF|VT_I1 */
USHORT * puiVal; ---VT_BYREF|VT_UI2 */
ULONG * pulVal; ---VT_BYREF|VT_UI4 */
ULONGLONG * pullVal; --- VT_BYREF|VT_UI8 */
INT * pintVal; ---VT_BYREF|VT_INT */
UINT * puintVal; ---VT_BYREF|VT_UINT
struct __tagBRECORD {
PVOID pvRecord;
IRecordInfo * pRecInfo;
} __VARIANT_NAME_4; --- VT_RECORD
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
};
UINT uintVal; ---VT_UINT */
DECIMAL * pdecVal; ---VT_BYREF|VT_DECIMAL */
CHAR * pcVal; ---VT_BYREF|VT_I1 */
USHORT * puiVal; ---VT_BYREF|VT_UI2 */
ULONG * pulVal; ---VT_BYREF|VT_UI4 */
ULONGLONG * pullVal; --- VT_BYREF|VT_UI8 */
INT * pintVal; ---VT_BYREF|VT_INT */
UINT * puintVal; ---VT_BYREF|VT_UINT
struct __tagBRECORD {
PVOID pvRecord;
IRecordInfo * pRecInfo;
} __VARIANT_NAME_4; --- VT_RECORD
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
};; ---VT_BYREF|VT_UNKNOWN */
IDispatch ** ppdispVal; ---VT_BYREF|VT_DISPATCH */
SAFEARRAY ** pparray; ---VT_BYREF|VT_ARRAY */
VARIANT * pvarVal; ---VT_BYREF|VT_VARIANT */
PVOID byref; ---Generic ByRef */
CHAR cVal; ---VT_I1 */
USHORT uiVal; ---VT_UI2 */
ULONG ulVal; ---VT_UI4 */
INT intVal; ---VT_INT */
UINT uintVal; ---VT_UINT */
DECIMAL * pdecVal; ---VT_BYREF|VT_DECIMAL */
CHAR * pcVal; ---VT_BYREF|VT_I1 */
USHORT * puiVal; ---VT_BYREF|VT_UI2 */
ULONG * pulVal; ---VT_BYREF|VT_UI4 */
ULONGLONG * pullVal; --- VT_BYREF|VT_UI8 */
INT * pintVal; ---VT_BYREF|VT_INT */
UINT * puintVal; ---VT_BYREF|VT_UINT
struct __tagBRECORD {
PVOID pvRecord;
IRecordInfo * pRecInfo;
} __VARIANT_NAME_4; --- VT_RECORD
} __VARIANT_NAME_3;
} __VARIANT_NAME_2;
DECIMAL decVal;
} __VARIANT_NAME_1;
};
http://blog.csdn.net/nppg2008/article/details/7885139
http://blog.csdn.net/china_chentao/article/details/4059698
