怎样把 _variant_t 类型转换为 const char * 类型？（急，在线等待！）

2003-05-23 09:31:27 1.4k 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

C++数据类型深度解析：从基础类型到现代工具，提升代码质量与性能数据类型是编程语言中用于定义数据存储格式和操作方式的基础概念，它决定了数据的取值范围、内存占用和可执行的操作。在C++中，类型系统从内置基础类型（如整型、浮点型）延伸到用户自定义类型（如结构体、类），并通过类型修饰符（如const、volatile）赋予更多语义。理解类型原理对于编写高效、安全的代码至关重要，它能有效防止内存泄漏、数据溢出等常见问题，并直接影响程序性能。在实际工程中，类型选择需结合具体应用场景，例如金融计算需避免浮点误差，嵌入式系统需考虑内存占用，而高性能计算则需关注数据对齐与缓存友好性。本阅读详情

遇到一个问题，相关代码如下：
_variant_t RecordID；
CEditListCtrl m_Test;
(void)m_Test.AddItem( RecordID);
编译时出错，提示信息如下：
: error C2664: 'int __cdecl CEditListCtrl::AddItem(const char *,...)' : cannot convert parameter 1 from 'class _variant_t' to 'const char *'

CEditListCtrl类的成员函数AddItem声明如下
int CEditListCtrl::AddItem(LPCTSTR pszText, ...)

...全文

C++类型转换：从C风格到四种命名强制转换的安全迁移指南类型转换是编程语言中处理不同类型数据间操作的基础机制，其核心原理在于改变数据的解释方式或内存布局。在C++中，类型转换不仅是语法特性，更是保障程序安全性和可维护性的关键技术。C风格的类型转换虽然语法简单，但因其意图模糊且缺乏安全检查，容易引发未定义行为、内存访问违规等严重问题。C++为此引入了四种命名强制转换操作符——static_cast、dynamic_cast、const_cast和reinterpret_cast，每种工具都有明确的职责和适用场景，让编译器能在编译期或运行期进行更严格的检查。例如，s 阅读详情

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summer_love 2003-05-23

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先判断类型是不是VT_BSTR，如果是用OLE2CA(*)转换即可。前加USES_CONVERSION，包含头文件<Atl*.h>

rtdb 2003-05-23

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哟，学到新东西了。
ChangeType()这个函数好，就不用自己判断并处理类型了。
我先谢谢free_card(痛并快乐着)和jiangsheng(蒋晟.Net) 。

尤其是jiangsheng(蒋晟.Net)的那个函数，很成熟啊。

蒋晟 2003-05-23

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CString g_GetValueString(const _variant_t& val)
{
CString strVal;
_variant_t varDest(val);
if(!g_varIsValid(val)){
return strVal;
}
if(val.vt==VT_BOOL){
if(val.boolVal==VARIANT_FALSE){
return "是";
}
else
return "否";
}
else{

}
HRESULT hr=::VariantChangeType(&varDest,&varDest,VARIANT_NOUSEROVERRIDE|VARIANT_LOCALBOOL,VT_BSTR);
if(SUCCEEDED(hr)){
strVal=(LPCTSTR)_bstr_t(varDest);
}
return strVal;
}
BOOL g_varIsValid(const _variant_t& val)
{
if(val.vt==VT_NULL||val.vt==VT_EMPTY||val.vt==VT_ERROR)
return FALSE;
return TRUE;
}

mtsh 2003-05-23

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改成m_Test.AddItem( LPCTSTR(&RecordID));将存放对象的地址（而不是对象本身！）强制转换为(LPCTSTR)类型，试试看吧？

winthegame 2003-05-23

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不行偶把头砍下来给你当球踢！

SnowWolfSLF 2003-05-23

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你查_variant_t就可以了。

rtdb 2003-05-23

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在VARIANT的说明里。
有很多类型，常用的有：
VT_INT，VT_R8，VT_BSTR等。

VT_EMPTY No value was specified. If an optional argument to an Automation method is left blank, do not pass a VARIANT of type VT_EMPTY. Instead, pass a VARIANT of type VT_ERROR with a value of DISP_E_PARAMNOTFOUND.
VT_EMPTY | VT_BYREF Not valid.
VT_UI1 An unsigned 1-byte character is stored in bVal.
VT_UI1 | VT_BYREF A reference to an unsigned 1-byte character was passed. A pointer to the value is in pbVal.
VT_UI2 An unsigned 2-byte integer value is stored in uiVal.
VT_UI2 | VT_BYREF A reference to an unsigned 2-byte integer was passed. A pointer to the value is in puiVal.
VT_UI4 An unsigned 4-byte integer value is stored in ulVal.
VT_UI4 | VT_BYREF A reference to an unsigend 4-byte integer was passed. A pointer to the value is in pulVal.
VT_UINT An unsigned integer value is stored in uintVal.
VT_UINT | VT_BYREF A reference to an unsigned integer value was passed. A pointer to the value is in puintVal.
VT_INT An integer value is stored in intVal.
VT_INT | VT_BYREF A reference to an integer value was passed. A pointer to the value is in pintVal.
VT_I1 A 1-byte character value is stored in cVal.
VT_I1 | VT_BYREF A reference to a 1-byte character was passed. A pointer the value is in pcVal.
VT_I2 A 2-byte integer value is stored in iVal.
VT_I2 | VT_BYREF A reference to a 2-byte integer was passed. A pointer to the value is in piVal.
VT_I4 A 4-byte integer value is stored in lVal.
VT_I4 | VT_BYREF A reference to a 4-byte integer was passed. A pointer to the value is in plVal.
VT_R4 An IEEE 4-byte real value is stored in fltVal.
VT_R4 | VT_BYREF A reference to an IEEE 4-byte real value was passed. A pointer to the value is in pfltVal.
VT_R8 An 8-byte IEEE real value is stored in dblVal.
VT_R8 | VT_BYREF A reference to an 8-byte IEEE real value was passed. A pointer to its value is in pdblVal.
VT_CY A currency value was specified. A currency number is stored as 64-bit (8-byte), two's complement integer, scaled by 10,000 to give a fixed-point number with 15 digits to the left of the decimal point and 4 digits to the right. The value is in cyVal.
VT_CY | VT_BYREF A reference to a currency value was passed. A pointer to the value is in pcyVal.
VT_BSTR A string was passed; it is stored in bstrVal. This pointer must be obtained and freed by the BSTR functions, which are described in Conversion and Manipulation Functions.
VT_BSTR | VT_BYREF A reference to a string was passed. A BSTR* that points to a BSTR is in pbstrVal. The referenced pointer must be obtained or freed by the BSTR functions.
VT_DECIMAL Decimal variables are stored as 96-bit (12-byte) unsigned integers scaled by a variable power of 10. VT_DECIMAL uses the entire 16 bytes of the Variant.
VT_DECIMAL | VT_BYREF A reference to a decimal value was passed. A pointer to the value is in pdecVal.
VT_NULL A propagating null value was specified. (This should not be confused with the null pointer.) The null value is used for tri-state logic, as with SQL.
VT_NULL | VT_BYREF Not valid.
VT_ERROR An SCODE was specified. The type of the error is specified in scodee. Generally, operations on error values should raise an exception or propagate the error to the return value, as appropriate.
VT_ERROR | VT_BYREF A reference to an SCODE was passed. A pointer to the value is in pscode.
VT_BOOL A 16 bit Boolean (True/False) value was specified. A value of 0xFFFF (all bits 1) indicates True; a value of 0 (all bits 0) indicates False. No other values are valid.
VT_BOOL | VT_BYREF A reference to a Boolean value. A pointer to the Boolean value is in pbool.
VT_DATE A value denoting a date and time was specified. Dates are represented as double-precision numbers, where midnight, January 1, 1900 is 2.0, January 2, 1900 is 3.0, and so on. The value is passed in date.
This is the same numbering system used by most spreadsheet programs, although some specify incorrectly that February 29, 1900 existed, and thus set January 1, 1900 to 1.0. The date can be converted to and from an MS-DOS representation using VariantTimeToDosDateTime, which is discussed in Conversion and Manipulation Functions.

VT_DATE | VT_BYREF A reference to a date was passed. A pointer to the value is in pdate.
VT_DISPATCH A pointer to an object was specified. The pointer is in pdispVal. This object is known only to implement IDispatch. The object can be queried as to whether it supports any other desired interface by calling QueryInterface on the object. Objects that do not implement IDispatch should be passed using VT_UNKNOWN.
VT_DISPATCH | VT_BYREF A pointer to a pointer to an object was specified. The pointer to the object is stored in the location referred to by ppdispVal.
VT_VARIANT Invalid. VARIANTARGs must be passed by reference.
VT_VARIANT | VT_BYREF A pointer to another VARIANTARG is passed in pvarVal. This referenced VARIANTARG, pvarVal, cannot be another VT_VARIANT|VT_BYREF. This value can be used to support languages that allow functions to change the types of variables passed by reference.
VT_UNKNOWN A pointer to an object that implements the IUnknown interface is passed in punkVal.
VT_UNKNOWN | VT_BYREF A pointer to the IUnknown interface is passed in ppunkVal. The pointer to the interface is stored in the location referred to by ppunkVal.
VT_ARRAY | <anything> An array of data type <anything> was passed. (VT_EMPTY and VT_NULL are invalid types to combine with VT_ARRAY.) The pointer in pparray points to an array descriptor, which describes the dimensions, size, and in-memory location of the array. The array descriptor is never accessed directly, but instead is read and modified using the functions described in Conversion and Manipulation Functions.

winthegame 2003-05-23

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LPCTSTR(_bstr_t(RecordID));

csdnnewman 2003-05-23

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to 东临碣石：
sorry,该怎样使用RecordID.vt呢？msdn里查不到vt这个成员函数呀！多谢！

rtdb 2003-05-23

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重要的是_variant_t RecordID中存放的是什么类
型的数据。

若是字符串，应可以用：
m_Test.AddItem( (LPCTSTR)(_bstr_t)RecordID));

若是整形，需要：
CString data ;
data.Format("%d", (long)RecordID);
m_Test.AddItem(data);

若是double, 则要
data.Format("%f", (double)RecordID);

可以根据RecordID.vt来判断数据类型。

free_card 2003-05-23

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_variant_t vfirstname;//存储的是数据库中的数据
CString str
vfirstname=pRs->GetCollect (_variant_t("Phone_Num"));
vfirstname.ChangeType (VT_BSTR);
str=vfirstname.bstrVal;

csdnnewman 2003-05-23

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这个CEditListCtrl类是我借用别人封装好了的类

csdnnewman 2003-05-23

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谢谢,好的

rtdb 2003-05-23

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对不起，看错题了，我测试一下，就给你答案。

csdnnewman 2003-05-23

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我改成如下形式：
m_Test.AddItem( (_bstr_t)RecordID));
编译通过可以运行，但显示出来的都是乱码

csdnnewman 2003-05-23

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to 东临碣石：
我改成m_Test.AddItem( LPCTSTR(RecordID));

但还是出错，提示信息如下：
: error C2440: 'type cast' : cannot convert from 'class _variant_t' to 'const char *'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

rtdb 2003-05-23

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强制转换就可以：
m_Test.AddItem( LPCTSTR(RecordID));

csdnnewman 2003-05-23

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谢谢各位前辈指点！
学到了很多东西！
问题已经解决，几种方法都可以。

怎样把 _variant_t 类型转换为 const char * 类型？（急，在线等待！）

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