map and virtual operator< for base class

[Bernd Prager <bernd at prager dot ws>]

I am a little stuck with implementing a simple cache and would 
appreciate a little help.
I defined a simple map:

typedef boost::bimap<int, yoda::Base> Cache;
Cache cache;

where I am trying to store two different (!) from Base inherited classes.

AFAIK the map requires the less operator to be implemented.
I declared a virtual operator in my Base class:

class Base {
public:
virtual bool operator<(const Base& b) const;
...};

and implemented the operator in the derived classes:

class Tst: public Base {
public:
bool operator<(const Tst& t) const; // less then
private:
std::string rep;
...};

bool Tst::operator<(const Tst& t) const
{
return (this->rep < t.rep );
}

The compiler throws me:
/usr/local/include/boost/bimap/bimap.hpp:156: instantiated from 
‘boost::bimaps::bimap<int, yoda::Base, mpl_::na, mpl_::na, mpl_::na>’
maptest.cpp:20: instantiated from here
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_function.h:230: 
error: no match for ‘operator<’ in ‘__x < __y’
make: *** [maptest.o] Error 1

I am not sure where my mistake lies. Is it the virtual operator?
How else would I implement a cache to store different objects?

Thank you for any advice,
-- Bernd