_bstr_t 和CComBSTR

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_bstr_t在VC中是为了兼容BSTR类型而增加的，也就是为了实现LPCSTR与BSTR转换。

它需要头文件#include <comdef.h>

_bstr_t 是BSTR的包装类
转换方法

LPSTR strDemo="Test";
_bstr_t bstr(strDemo);
建议加上try，catch，用于catch(_com_error &e)

The following pseudocode shows the typical use of CComBSTR:

HRESULT CMyObject::MyMethod(IOtherObject* pSomething){    CComBSTR bstrText(L"Hello");    bstrText += " again";                     // LPCSTR conversion    bstrText.ToUpper();    pSomething->Display(bstrText);            // [in] parameter    MessageBoxW(0, bstrText, L"Test", MB_OK); // Assumes Windows NT}

As you can see, CComBSTR significantly simplifies the use of BSTRs. Four uses of CComBSTR, however, require special care:

Freeing the BSTR explicitly
Using CComBSTR as an [out] parameter
Using a CComBSTR automatic variable in right-side assignments
Using a CComBSTR member variable in right-side assignments

当在BSTR*所在的位置作为一个[out]参数传递CComBSTR时，你必须先调用Empty释放string的内容，就象下面这样：

HRESULT CMyObject::MyMethod2(ISomething* p, /*[out]*/ BSTR* pbstr){    CComBSTR bstrText;        bstrText = L"Some assignment";     // BSTR is allocated.        bstrText.Empty();                  // Must call empty before    pSomething->GetText(&bstrText);    //  using as an [out] parameter.    if(bstrText != L"Schaller")        bstrText += "Hello";           // Convert from LPCSTR.}因为在重写BSTR内容之前方法COM 为[out] 参数的规则是并不调用sysfreestring,这讲出现泄漏。