How to best convert CString to BSTR to pass it as an “in” parameter into a COM method?

Question

I need to convert a CString instance into a properly allocated BSTR and pass that BSTR into a COM method. To have code that compiles a

Answer 1

One of _bstr_t constructors allows you to simply attach to existing BSTR so that you can have the exception that you want from CString::AllocSysString when BSTR allocation fails.

// _bstr_t simply attaches to BSTR, doesn't reallocate it
interface->CallMethod( _bstr_t(sourceString.AllocSysString(), false) );

The _bstr_t constructor documentation says:

_bstr_t(
   BSTR bstr,
   bool fCopy 
);

fCopy
If false, the bstr argument is attached to the new object without making a copy by calling SysAllocString.

On the other hand, CComBSTR constructor doesn't seem to have the corresponding signature; although it can be used as well if BSTR allocation failure exception is not really needed, as mentioned by Phil Booth in his answer.