User Guide

208 Chapter 10: Building and Using ColdFusion Components
You can refer to instance data of a CFC only if you create an instance of the CFC. From inside the
CFC, you refer to instance data of the CFC using the
this prefix, for example
this.firstvariable. From the calling page, you refer to instance data using dot notation,
including the name of the instance of the component and the name of the instance data, as in
objectname.ivarname. Components whose methods you invoke without first instantiating the
component do not typically have instance data.
You initialize instance data at the top of the component definition, before the method definitions.
ColdFusion executes this code when it instantiates the component; for example, when a
cfobject tag creates the component instance. Because this code executes only when the instance
is created and it typically “constructs” properties of the component, instance data initialization
code is sometimes called constructor code.
You can use any CFML tag or function in constructor code, and the code can perform any
ColdFusion processing, such as querying a database or data validation and manipulation. If one
component extends another, the parent component’s constructor code executes before the child
component’s constructor code.
Note: ColdFusion does not require you to put the initialization code at the top of the component
definition; however, it is good programming practice to do so.
The following example shows constructor code for a shopping cart CFC:
<cfcomponent>
<!--- Initialize the array for the cart item IDs and quantities. --->
<cfset This.CartData = ArrayNew(2)>
<!--- The following variable has the ID of the "Special Deal" product for
this session. --->
<cfset This.Special_ID = RandRange(1, 999)>
For information on scopes, see “The This scope” on page 227 and “The Variables scope”
on page 228.
A useful technique is to define a method named
init(), which initializes an instance of a CFC,
acting as a constructor. The
init() method can initialize constants and return an instance of the
component to the calling page. The following code illustrates an example of an
init() method:
<cfcomponent displayname="shoppingCart">
<cffunction name="init" access="public" output="no"
returntype="shoppingCart">
<cfargument name="shoppingCartID" type="UUID" required="yes">
<cfset variables.shoppingCartID = arguments.shoppingCartID>
<cfreturn this>
</cffunction>
<!--- Additional methods go here. --->
</cfcomponent>
In this example, the init() method uses the variables scope to make the shopping cart ID
available anywhere in the CFC. For more information about scope, see “CFC variables and
scope” on page 227.