Returning Strings to an ASP page from a C++ COM object

[erich]

I am trying to return a string to an ASP page from a COM method call. If I code the COM dll in VB everything works, I can pass in a buffer (ByRef) or have the function return a string. If I code the object in C++ and make it return a string ([out, retval]) and I try and put the return value into a variable, e.g.

StringValue = Obj.Function(Param1, Param2)

I get 'an error occurred in an external object...'.
If however I use a function like 'Left', e.g.

StringValue = Left(Obj.Function(Param1, Param2), 100)

it works fine.

Any ideas?