Variable Scopes

They gotta exist somewhere!

In the CFML language, there are many persistence and visibility scopes that exist for variables to be placed in. These are differentiated by context: in a CFC, in a function, tag, thread or in a template. All CFML scopes are implemented as structures or hash maps of key-value name pairs. The default scope for variable storage is called variables. Thus you can refer variables like this in either CFC or Template context:

a = "hello";
writeOutput( a );
or 
writeOutput( variables.a );

Persistence Scopes

Can be used in any context, used for persisting variables for a period of time.

• session - stored in server RAM or external storages tracked by unique web visitor

• client - stored in cookies, databases, or external storages (simple values only)

• application - stored in server RAM or external storage tracked by the running ColdFusion application

• cookie - stored in a visitor's browser

• server - stored in server RAM for ANY application for that CFML instance

• request - stored in RAM for a specific user request ONLY

• cgi - read only scope provided by the servlet container and CFML

• form - Variables submitted via HTTP posts

• URL - Variables incoming via HTTP GET operations or the incoming URL

Template Scopes (CFM)

• variables - The default or implicit scope to which all variables are assigned.

Component Scopes (CFC)

• variables - Private scope, visible internally to the CFC only

• this - Public scope, visible from the outside world

• static - No need for a CFC instance; available as a CFC representation (Lucee only)

Function Scopes

• variables - Has access to private variables within a Component or Page

• this - Has access to public variables within a Component or Page

• local - Function-scoped variables only exist within the function execution. Referred to as var scoping

• arguments - Incoming variables to a function

Tag Scopes

• attributes - Incoming tag attributes

• variables - The default scope for variable assignments

• caller - Used within a custom tag to set or read variables within the template that called it.

Thread Scopes

• attributes - Passed variables via a thread

• thread - A thread-specific scope that can be used for storage and retrieval

• local - Variables local to the thread context

Evaluating Unscoped Variables

If you use a variable name without a scope prefix, ColdFusion checks the scopes in the following order to find the variable:

1. Local (function-local, UDFs, and CFCs only)

2. Arguments

3. Thread local (inside threads only)

4. Query (not a true scope; variables in query loops)

5. Thread

6. Variables

7. CGI

8. CFFILE

9. URL

10. Form

11. Cookie

12. Client

IMPORTANT: Because ColdFusion must search for variables when you do not specify the scope, you can improve performance by specifying the scope for all variables. It can also help you avoid nasty lookups or unexpected results.