MACROMEDIA COLDFUSION MX 61-DEVELOPING COLDFUSION MX, Руководство по разработке

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Dynamic expressions and dynamic variables 93
Selecting how to create variable names
The following two examples describes cases when you need dynamic variable names:
•
Form applications where the number and names of fields on the form vary dynamically. In this
case, the form posts only the names and values of its fields to the action page. The action page
does not know all the names of the fields, although it does know how the field names (that is,
the variable names) are constructed.
•
If the following are true:
■
ColdFusion calls a custom tag multiple times.
■
The custom tag result must be returned to different variables each time.
■
The calling code can specify the variable in which to return the custom tag result.
In this case, the custom tag does not know the return variable name in advance, and gets it as
an attribute value.
In both cases, it might appear that dynamic expressions using the
Evaluate
function are needed
to construct the variable names. However, you can achieve the same ends more efficiently by
using dynamic variable naming, as shown in
“Example: a dynamic shopping cart” on page 99 .
This does not mean that you must always avoid dynamic evaluation. However, given the
substantial performance costs of dynamic evaluation, you should first ensure that one of the
following techniques cannot serve your purpose:
•
An array (using index variables)
•
Associative array references containing expressions to access structure elements
•
Dynamically generated variable names
Dynamic variable naming without dynamic evaluation
While ColdFusion does not always allow you to construct a variable name in-line from variable
pieces, it does let you to do so in the most common uses, as described in the following sections.
Using pound signs to construct a variable name in assignments
You can combine text and variable names to construct a variable name on the left side of a
cfset
assignment. For example, the following code sets the value of the variable Product12 to the string
"Widget":
<cfset ProdNo = 12>
<cfset "Product#ProdNo#" = "Widget">
To construct a variable name this way, all the text on the left side of the equal sign must be in
quotation marks.
This usage is less efficient than using arrays. The following example has the same purpose as the
previous one, but requires less processing:
<cfset MyArray=ArrayNew(1)>
<cfset prodNo = 12>
<cfset myArray[prodNo] = "Widget">