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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

But how are a CFC's properties and methods created? When the instance of the CFC is created, the "pseudo-constructor" of the CFC is called. Effectively, this means that all the code in the CFC that defines methods is used to create the object's methods and that all other code in the CFC, including any code that creates the object's properties, is executed.

Once an object has been created from a CFC, it and its public properties and methods can be referenced. By assigning the object to a variable, it and its public properties and methods can be referenced an unlimited number of times during the life of that variable. In line 1 of callsimple.cfm, we see that the object created from simple.cfc is assigned to a variable named myObj. Because no scope is declared for myObj, it is placed in the Variables scope. On the fourth line of callsimple.cfm, we see the public myVar property of the myObj instance of simple.cfc being referenced, and on the fifth line of callsimple.cfm, we see the public myMethod method of that same myObj instance of simple.cfc being called. Referencing an object's public properties and methods is as simple as using the variable name to which the object was assigned followed by a period (a.k.a. "dot") and then the name of the property or method.

When calling a method of an object, we also must include a pair of parentheses after the method name. Inside those parentheses we must include, if any for the given method, a comma-delimited list of the arguments to be passed to the method. There are other ways to pass arguments to methods, but for simplicity's sake we won't cover those in this article.

Now that we know what is going on in callsimple.cfm, let's look at its output:

text output
statement
string

The first line of the output results from simply creating an instance of simple.cfc on the first line of callsimple.cfm and the resultant pseudo-constructor execution of all code in simple.cfc. The second line of the output results from referencing the public myVar property of myObj on the fourth line of callsimple.cfm. Finally, the third line of the output results from referencing the public myMethod method of myObj on the fifth line of callsimple.cfm.

With a basic understanding of CFCs in place, let's move on to the purpose of this article: reusing code with CFCs. As mentioned earlier, we can reuse code in CFCs not only via all the standard CFML code reuse methods - including files, calling custom tags, and calling user-defined functions - but also via inheritance. But what exactly is inheritance?

CFCs and Inheritance
In the real world, inheritance involves one entity gaining things as the result of a relationship with another entity. Inheritance in CFCs is very similar: one CFC gains things (specifically, CFML code) as the result of a relationship with another CFC. We call a CFC that inherits from another a "child CFC" and we call the CFC from which that child inherits a "parent CFC." CFML inheritance is virtual, meaning both the child CFC and the parent CFC contain the complete CFML code of the parent CFC, without duplication of the CFML code of the parent CFC in the child CFC file.

Why is inheritance useful? Because it allows us to write code in one CFC and then use that code in numerous other CFCs. Thus, we can write that CFML code one time and have it virtually contained in an unlimited number of CFCs through inheritance. By writing that CFML only one time, it's less likely we will write bugs into it and, should a bug be discovered in it or should a requirement for its functionality change, we need only modify a single instance of the code.

Now that we have an understanding of CFC inheritance, let's take a look at how it's used in a file called exsimple.cfc:

<cfcomponent extends="simple">
    <cffunction name="newMethod">
       <cfreturn "newstring">
    </cffunction>
</cfcomponent>

Note the difference between the opening <cfcomponent> tag of simple.cfc and the opening <cfcomponent> tag of exsimple.cfc - exsimple.cfc has 'extends="simple"' in that opening <cfcomponent> tag. The extends attribute of a CFC's <cfcomponent> tag is used to specify that CFC's parent. As such, by using 'extends="simple"' in the opening <cfcomponent> tag, we cause exsimple.cfc to virtually contain all the CFML in simple.cfc. Thus, when an instance of exsimple.cfc is created with either the CreateObject() function or the <cfobject> tag, the pseudo-constructor of simple.cfc is called, causing the CFML code contained in simple.cfc to be executed, just as if that code were located in exsimple.cfc. In addition, after an instance of exsimple.cfc has been created, we can reference on that instance any of the public properties and methods defined in simple.cfc just as if they were defined in exsimple.cfc. Let's see what this looks like in a file named callexsimple.cfm:

<cfset myObj = CreateObject("component", "exsimple")>
<cfoutput>
    <br>
    #myObj.myVar#<br>
    #myObj.myMethod()#<br>
    #myObj.newMethod()#<br>
</cfoutput>

We create an instance of exsimple.cfc. Because exsimple.cfc extends simple.cfc, we can reference the public myVar property and the public myMethod method of simple.cfc as well as the public newMethod method of exsimple.cfc. The output of callexsimple.cfm is exactly what we expect:

text output
statement
string
newstring

Now if we update simple.cfc, exsimple.cfc will automatically get the benefits of that change without being changed at all.
Because we didn't have to duplicate code from simple.cfc in order to use it in exsimple.cfc, we were able to write less code. Less code is better code because it gives us fewer places to introduce bugs and because it's easier to maintain.


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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