It looks like the c++ source for a single object file can not contain
more than one anonymous class that derives from a base class.  When I
attempted to do this, the linker failed with the following error:

fatal error LNK1179: invalid or corrupt file: duplicate COMDAT
'??0__unnamed@@QAE@XZ'

The "??0__unnamed@@QAE@XZ" symbol undecorates to "public: __thiscall
__unnamed::__unnamed(void)" so I think the compiler is using the same
class name for each anonymous class.  When the compiler then synthesizes
a default constructor for each class, it winds up generating two
functions with the same name.

Here is some example code:

#include <exception>

class : public exception
{
} anonymous1;

class : public exception
{
} anonymous2;

This was easy to work around by giving each previously anonymous class a
name but shouldn't the compiler generate unique class names for them?