8.6 Minutiae

Actually, Loki's Factory implementation does not use std::map. It uses a drop-in replacement for map, AssocVector, which is optimized for rare inserts and frequent lookups, the typical usage pattern of Factory. AssocVector is described in detail in Chapter 11.

In an initial draft of Factory, the map type was customizable by virtue of its being a template parameter. However, often AssocVector fits the bill exactly; in addition, using standard containers as template template parameters is not, well, standard. This is because implementers of standard containers are free to add more template arguments, as long as they provide defaults for them.

Let's focus now on the ProductCreator template parameter. Its main requirement is that it have functional behavior (accept operator() and take no arguments) and return a pointer convertible to AbstractProduct*. In the concrete implementation shown earlier, ProductCreator was a simple pointer to a function. This suffices if all we need is to create objects by invoking new, which is the most common case. Therefore, we choose

AbstractProduct* (*)()

as the default type for ProductCreator. The type looks a bit like a confusing emoticon because its name is missing. If you put a name after the asterisk within the parentheses,

AbstractProduct* (*PointerToFunction)()

the type reveals itself as a pointer to a function taking no parameters and returning a pointer to AbstractProduct. If this still looks unfamiliar to you, you may want to refer to Chapter 5, which includes a discussion on pointers to functions.

By the way, speaking of that chapter, there is a very interesting template parameter you can pass to Factory as ProductCreator, namely Functor<AbstractProduct*>. If you choose this, you gain great flexibility: You can create objects by invoking a simple function, a member function, or a functor, and bind appropriate parameters to any of them. The glue code is provided by Functor.

Our Factory class template declaration now looks like this:

template


<


   class AbstractProduct,


   class IdentifierType,


   class ProductCreator = AbstractProduct* (*)(),


   template<typename, class>


      class FactoryErrorPolicy = DefaultFactoryError


>


class Factory;

Our Factory class template is now ready to be of use.