Question

I’m trying to make a class that takes a map as a template parameter. In particular it should be able to take std::map and boost::ptr_map. Currently I’m trying this:

template <template<typename, typename> class MAP, typename KEYTYPE, typename DATATYPE>
class RestrictedMapBase
{
   /* bunch of methods ... */
}

This class is being inherited by two other classes, one for std::map and one for boost::ptr_map.

template <typename KEYTYPE, typename DATATYPE>
class RestrictedMap: public RestrictedMapBase<std::map, KEYTYPE, DATATYPE>
{
   /* Bunch of methods ... */
};

template <typename KEYTYPE, typename DATATYPE>
class RestrictedPointerMap: public RestrictedMapBase<boost::ptr_map, KEYTYPE, DATATYPE>
{
   /* Bunch of methods ... */
};

But on compilation I get these errors:

RestrictedMap.h(166) : error C3201: the template parameter list for
class template ‘std::map’ does not match the template parameter list
for template parameter ‘MAP’ RestrictedMap.h(183) : see reference
to class template instantiation
‘STLUtils::RestrictedMap’ being compiled

RestrictedMap.h(186) : error C3201: the template parameter list for
class template ‘boost::ptr_map’ does not match the template parameter
list for template parameter ‘MAP’ RestrictedMap.h(203) : see
reference to class template instantiation
‘STLUtils::RestrictedPointerMap’ being compiled

Can anybody point me in the right direction of what I’m doing wrong? Thanks.

Answer 1

Class template std::map have 4 arguments:

template <typename _Key, typename _Tp, typename _Compare = std::less<_Key>,
          typename _Alloc = std::allocator<std::pair<const _Key, _Tp> > >
class map

Thus it can not be pass via template<typename, typename> just like you can not assign a pointer to a 3-argument function to a pointer to 2-argument function.

The solution is to pass the exact type to use to RestrictedMapBase:

template <typename MAP>
class RestrictedMapBase
{
   typedef typename MAP::key_type KEYTYPE;
   typedef typename MAP::mapped_type DATATYPE;
};

Your initial design also limits users of your class because they can not specify a compare function for keys (or a hash function if they want to use hash tables) and an allocator.