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One of the primary goals of using a factory is to organize your code so you don't need to select teh exact constructor type when creating an object.
Two kinds of Factory Patterns, actually, they are similar, with difference that the abstract factory can contain servral factory methods.
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Factory Method
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Abstract Factory.
The key roles in the Factory Patterns include:
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Product
: Actually it acts as the working horse in the real time processing, I mean to do the work belonging to the products.
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Product specific factory
:? The place where the special product is made. Please note the word "special", which means that even there is a Super Class of Factory, each kind of product needs a special defined factory as derived factory which is 1-to-1 tightly connected to this specific product.
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Super Class of Factory:
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Actually, it acts as a place where the product specific factories gather.? A table of Concret Product Specific Factory Objects is maintained, as form of static member, by this class.
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Also, the class provide the virtual method of the Create() for the derivded Product Specific Factory class to implement respectively.
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Factory Initializer:
The responsibility of this Class is to realize all the necessary Product?Specific Factories, and place them into the static Factory Table of the Super Class Factory. This repsonsibility should and MUST be finished before all the necessary Fatories begin to work on manufacturing, normally occurs when the system is initialized.
Some rules(if can call them as rules)?needs to be noted when implemented by C++:
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The constructor of the specific product normally is designed as Private member, to only allow friend class to access. The purpose of this rule is to prevent the product to be made by incorrect factory.
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The 1-to-1 product specific factory is designed as the friend class of the its specific product class.? The implementation of the Product specifc factory could be combined into the internal body of the product as the form of Product::Factory. We can also to place the factory out the product, as the form of ProductFactory. As you wish, :)
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The list of the concret factories of the super class of factory is designed as static. The index of the list is used to find the correct concrete factory by passing the Identifier parameter.
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The signleton is also used: Factory and the Factory Initializer should be singleton. In other words,?we can only?has only one set of factories and only one factory initializer.?Otherwise, there will exist two interactive factory framework in a single system. The result is that the system is crashed.
Any supplementary information or correction is to be attached later, with further understanding on this important Patterns.
Alex Zhang