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。
责任编辑：小草
