Enforcing different C++ template arguments

Question

I'm try to create a BidirectionalMap class using (only) STL (no, boost is not an option.) I have 99% percent of it working the way that I want, but what I really can't figure out is how to force the template to require two different types so that operator[] can be properly overridden. Something like...

template < class KeyType, class ValueType >
class BidirectionalMap
{
  public:

  ...

  const ValueType& operator[](const KeyType& _k ) { return( m_keyMap[ _k ] ); }
  const KeyType& operator[](const ValueType& _v ) { return( m_valMap[ _v ] ); }

  private:

  std::map< KeyType > m_keyMap;
  std::map< ValueType > m_valueMap;
};

main()
{
  BidirectionalMap< Foo, Foo > fooMap; // won't work, ambiguous.
  BidirectionalMap< Foo, Bar > fooBarMap; // does work.
} 

Thoughts? -R

Answer 1

Just add the following partial specialisation:

template <typename T>
class BidirectionalMap<T, T>;

This will case the compiler to instantiate a template that isn’t defined (since the above is only declared) and bail if the user tries to pass the same type as both template arguments.