从Java到C++——从union到VARIANT与CComVariant的深层剖析-白红宇

从Java到C++——从union到VARIANT与CComVariant的深层剖析

阅读量：6711 次

发布时间：2019-06-25

本文共 11594 字，大约阅读时间需要 38 分钟。

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 
     
      //======================================================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 
     
      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_UI8LONGLONG llVal; ---VT_I8LONG 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 */ 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; };