Policy Based Class Design
PolicyBasedClassDesign is the name of the first chapter in ModernCeePlusPlusDesign
This is a technique of implementing design choices in CeePlusPlus. The design choices are said to be orthogonal if they can be implemented independently. If each policy is given a name, that name can define a minimal interface, and there can be several implementations which meet the needs of the policy. One of these can be chosen at compile time to provide alternative implementations.
One example I have used goes as follows. I want to make use of pointers to objects within a C++ class. I decide that the interface for this will be
Pointer_Class::Pointer_Type
An implementation of this using BoostSharedPtr could be
template
class Boost_Pointer_Class {
public:
typedef boost::shared_ptr Pointer_Type;
};
In the following, class Foo uses Boost_Pointer_Class as a default option.
template class Pointer_Class = Boost_Pointer_Class>
class Foo
{
typedef typename Pointer_Class::Pointer_Type ptr_t;
};
Class Foo can be declared without reference to this policy as long as the default will do:
Foo foo_int;
One project which makes use of this is MetaAgent which is to be found at http://codeproject.com/useritems/metaagent.asp#__top
By way of contrast, TheBoostGraphLibrary describes SetsOfRequirements which are ways of testing what a template type must do if it is to be put into a given class.