Here's the explanatory reply.
At 08:17 AM 7/18/2007, strk wrote:
I don't get the template<class,class> syntax on line 122.
It's complicated. The prior sentence may be an understatement.
A basic problem with getting an aspect class defined is the following tension:
- The owner class may not know the definition of the aspect at the point of
definition, because the aspect must be able to change.
- An owner class must have a full aspect class definition at its point of
instantiation, because an instance of the aspect class is a member of the
owner class.
- The aspect class must have a full owner class definition at its point of
instantiation, because it must be able to refer to the innards of the owner
class.
This all boils down to a definition cycle of length two. (Self-reference
is a cycle of length one.) There's an issue of infinite regress in
definition that must be addressed. The solution, in this case, is to make
the aspect parameter a template, where the final argument of said template
must be instantiated with the owner class itself. Observe the
specialization definition (around line 167 in my current copy):
template< class T >
class Null_Aspect_1< T, ACT::Handle_Registry_Leader< T,
Null_Aspect_1 > >
This class defines the null aspect for class Handle_Registry_Leader (which,
without an aspect, would be a template class with parameter T). The second
parameter is a class, which class is the owner, instantiated with its
template arguments, which include the null aspect.
Here's the magic. The use of Null_Aspect_1 in the specialization
definition refers to the specialized class itself, not the non-specialized
definition (which is in Aspect.hpp). Such a specialization creates a
reference cycle of length two, just as required.
Eric
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