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ColdFusion Authors: Yakov Fain, Jeremy Geelan, Maureen O'Gara, Nancy Y. Nee, Tad Anderson

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ColdFusion: Article

Inheritance in ColdFusion: Code Reuse for CFCs

When it comes to code, less is more

Multiple Inheritance
So far we have only considered inheriting into a child CFC a single parent CFC via the extends attribute of the <cfcomponent> tag. Can we inherit multiple CFCs by using the extends attribute of the <cfcomponent> tag? The answer is very intentionally no; multiple inheritance has certain complications (a discussion of these is beyond the scope of this article) and so a deliberate decision was made to allow for only single inheritance in CFML. However, there is a new type of limited multiple inheritance that can be used in CFML deployed via New Atlanta's BlueDragon 7.0: interfaces. An interface is a special type of CFC that cannot be directly instantiated and that can only specify, but not implement, methods. An interface cannot contain any other pseudo-constructor code, including statements that create object properties. An unlimited number of CFC interfaces can be inherited in any CFC by using BlueDragon 7.0's new implements attribute of the <cfcomponent> tag. Such inheritance does not gain a child CFC any new functionality since the methods of the interface are not implemented. What is gained by the child CFC is the requirement that it implement all methods specified in the interfaces it implements. In a sense, an interface defines a contract that any CFC implementing the interface must follow. In order to understand why this is useful, we have to understand another important concept: type matching.

Implementing type matching in CFML code is another way to make it more reliable and maintainable. Type matching involves explicitly specifying how the code is intended to be used, which makes the code easier to read, easier to use properly, and easier to maintain. In addition, an explicit specification of how the code is intended to be used makes it more likely that improper use of the code will be identified sooner rather than later, either via the generation of errors in testing or by inspection of the code. For example, when we define a CFML function/method with the <cffunction> tag, we may use the returnType attribute to specify the type of data that is returned by the function/method. In the same way, when we define a CFML function/method argument with the <cfargument> tag, we may use the type attribute to specify the type of the data that must be supplied for that argument. For either of these attributes, if the specified type is not passed, an error is generated. In addition, for either of these attributes, we may specify the name of a CFC and by doing so we are indicating that the data being passed must be an instance of the specified CFC.

What does type matching with CFC types gain us? It gains us the ability to reliably reference the public properties and methods the specified type defines. In other words, if we specify 'type="simple"' in a <cfargument> tag, we can reference the myMethod method inside the function to which that <cfargument> tag applies because that method is defined in simple.cfc.
Here is where inheritance comes in: because a child CFC inherits all of the properties and methods of its parent CFC, the public properties and methods of the parent CFC can be reliably referenced on an instance of the child CFC. That is, if we specify 'type="exsimple" in a <cfargument> tag, we can reference the myMethod method inside the function to which that <cfargument> applies because that method is defined in simple.cfc and exsimple.cfc inherits simple.cfc. As a result of this behavior, an instance of a child CFC matches the type of its parent CFC and, for that matter, the type of any CFC from which that parent CFC inherits, and any CFC from which that CFC inherits, and so on all the way on up the inheritance chain. It is this very fact that type matching honors inheritance that explains the usefulness of an interface: since an interface is a type that can be inherited, when an object matches the type of an interface, that interface's properties and methods can be referenced reliably on that object.

We have standard CFCs that support standard single inheritance and interfaces that allow for a special type of multiple inheritance that is useful for type matching. But what about something that is between a standard CFC and an interface? BlueDragon 7.0 provides that, too, and it's called an abstract CFC. Like a standard CFC, an abstract CFC allows for the implementation of properties and methods, and like a standard CFC, an abstract CFC may only inherit from a single other CFC. However, like a CFC interface, an abstract CFC also allows for the specification of methods that must be implemented by any CFC that inherits from that abstract CFC, and like a CFC interface, an abstract CFC cannot be instantiated directly. An abstract CFC is a CFC that is not a completed CFC implementation; it is a partial CFC implementation designed specifically to be inherited by CFCs that complete that partial implementation.

Conclusion
We have now seen how inheritance allows us to minimize the amount of CFML we have to write when working with CFCs. Any CFC can inherit the full CFML contents of a single other CFC by extending that CFC. With BlueDragon 7.0, we can use abstract CFCs, a special type of CFC that can be extended but not directly instantiated. In addition, we have seen that with BlueDragon 7.0, a CFC can inherit unimplemented methods from an unlimited number of CFC interfaces, which is useful for CFC type matching.

In order not to detract from the focus of this article, certain object-oriented considerations such as overriding, abstraction, encapsulation (including the use of getter and setter methods), and polymorphism intentionally were not covered and certain statements impacted by such considerations intentionally were not fully qualified.

Resources
If you are interested in a fuller introductory treatment of object-oriented development with CFCs than I was able to give in this article, see the following presentation by Matt Woodward:

www.mattwoodward.com/presentations/cfc_101.pdf

If you are interested in more information on using inheritance as well as on aggregation (a concept not discussed in this article), see Hal Helms's newsletter from March 22, 2003:

www.halhelms.com?fuseaction=newsletters.show&issue=032203_Inheritance

Finally, if you are interested in more information on CFC interfaces and abstract CFCs as supported in BlueDragon 7.0 see the BlueDragon 7.0 cfml Enhancement Guide:

www.newatlanta.com/products/bluedragon/self_help/docs/7_0/ BlueDragon_70_CFML_Enhancements_Guide.pdf

More Stories By Josh Adams

Josh Adams is the developer evangelist for New Atlanta's BlueDragon family of CFML application server products. He presents on a regular basis at technical conferences and user groups throughout North America. Josh is also an active CFML developer both in his role at New Atlanta and in other endeavors.

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