MACROMEDIA ColdFusion MX, Develop Manual, Page: 44 / 860

44 Chapter 2: Elements of CFML
Constants
The value of a constant does not change during program execution. Constants are simple scalar
values that you can use within expressions and functions, such as “Robert Trent Jones” and
123.45. Constants can be integers, real numbers, time and date values, Boolean values, or text
strings. ColdFusion does not allow you to give names to constants.
Variables
Variables are the most frequently used operands in ColdFusion expressions. Variable values can be
set and reset, and can be passed as attributes to CFML tags. Variables can be passed as parameters
to functions, and can replace most constants.
ColdFusion has a number of built-in variables that provide information about the server and are
returned by ColdFusion tags. For a list of the ColdFusion built-in variables, see
Chapter 1,
“Reserved Words and Variables,” in CFML Reference .
The following two characteristics classify a variable:
•
The scope of the variable, which indicates where the information is available and how long the
variable persists
•
The data type of the variable’s value, which indicates the kind of information a variable
represents, such as number, string, or date
The following section lists and briefly describes the variable scopes. “Data types” on page 45 lists
data types (which also apply to constant values). For detailed information on ColdFusion
variables, including data types, scopes, and their use, see
Chapter 3, “Using ColdFusion
Variables,” on page 53 .
Variable scopes
The following table describes ColdFusion variable scopes:
Scope Description
Variables
(local) The default scope for variables of any type that are created with the
cfset
and
cfparam
tags. A local variable is available only on the page on which it is created and
any included pages.
Form The variables passed from a form page to its action page as the result of submitting
the form.
URL The parameters passed to the current page in the URL that is used to call it.
Attributes The values passed by a calling page to a custom tag in the custom tag’s attributes.
Used only in custom tag pages.
Caller A reference, available in a custom tag, to the Variables scope of the page that calls
the tag. Used only in custom tag pages.
ThisTag Variables that are specific to a custom tag, including built-in variables that provide
information about the tag. Used only in custom tag pages. A nested custom tag can
use the
cfassociate
tag to return values to the calling tag’s ThisTag scope.
Request Variables that are available to all pages, including custom tags and nested custom
tags, that are processed in response to an HTTP request. Used to hold data that
must be available for the duration of one HTTP request.