MACROMEDIA ColdFusion MX, Develop Manual

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Calling functions and using variables 203
Referencing caller variables
A function can use and change any variable that is available in the calling page, including variables
in the caller’s Variables (local) scope, as if the function was part of the calling page. For example, if
you know that the calling page has a local variable called Customer_name (and there is no
function scope variable named Customer_name) the function can read and change the variable by
referring to it as Customer_name or (using better coding practice) Variables.Customer_name.
Similarly, you can create a local variable inside a function and then refer to it anywhere in the
calling page
after the function call. You cannot refer to the variable before you call the function.
However, you should generally avoid using the caller’s variables directly inside a function. Using
the caller’s variables creates a dependency on the caller. You must always ensure that the code
outside the function uses the same variable names as the function. This can become difficult if
you call the function from many pages.
You can avoid these problems by using only the function arguments and the return value to pass
data between the caller and the function. Do not reference calling page variables directly in the
function. As a result, you can use the function anywhere in an application (or even in multiple
applications), without concern for the calling code’s variables.
As with many programming practice, there are valid exceptions to this recommendation. For
example you might do any of the following:
•
Use a shared scope variable, such as an Application or Session scope counter variable.
•
Use the Request scope to store variables used in the function, as shown in “Using the Request
scope for static variables and constants” on page 208 .).
•
Create context-specific functions that work directly with caller data if you always synchronize
variable names.
Note: If your function must directly change a simple variable in the caller (one that is not passed to the
function by reference), you can place the variable inside a structure argument.
Using function-only variables
Make sure to use the
var
statement in CFScript UDFs to declare all function-specific variables,
such as loop indexes and temporary variables that are required only for the duration of the
function call. Doing this ensures that these variables are available inside the function only, and
makes sure that the variable names do not conflict with the names of variables in other scopes. If
the calling page has variables of the same name, the two variables are independent and do not
affect each other.
For example, if a ColdFusion page has a
cfloop
tag with an index variable i, and the tag body
calls a CFScript UDF that also has a loop with a function-only index variable i, the UDF does not
change the value of the calling page loop index, and the calling page does not change the UDF
index. So you can safely call the function inside the
cfloop
tag body.
In general, use the
var
statement to declare all UDF variables, other than the function arguments
or shared-scope variables, that you use only inside CFScript functions. Use another scope,
however, if the value of the variable must persist between function calls; for example, for a counter
that the function increments each time it is called.