Adobe 38043740 Developing Applications - Page 170

DEVELOPING COLDFUSION 9 APPLICATIONS 165 Building Blocks of ColdFusion Applications In CFScript, you create function-only variables with the var statement. Unlike other variables, you never prefix function-only variables with a scope name. 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 so 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. 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 no function scope variable named Customer_name exists) 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 use it anywhere in the calling page after the function call. You cannot use the variable before you call the function. However, generally avoid using the caller's variables directly inside a function. Using the caller's variables creates a dependency on the caller. Ensure that the code outside the function uses the same variable names as the function. Doing so 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 variables. As with other programming practices, valid exceptions to this recommendation exist. For example, you can 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. For more information, see "Using the Request scope for static variables and constants" on page 174. • 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 arguments Function arguments can have the same names, but different values, as variables in the caller. Avoid such uses for clarity, however. The following rules apply to argument persistence: • Because ColdFusion passes simple variable and array arguments by value, their names and values exist only while the function executes. Last updated 1/20/2012