Operation-Driven Evaluation

<cfset mynow = now()>
<cfoutput>#mynow#</cfoutput>


<cfset mynow = mynow + 0>
<cfoutput>#mynow#</cfoutput>

<cfif myVariable>
<cfoutput>myVariable equals #myVariable# and is TRUE 

</cfoutput>
</cfif>
<cfif myVariable IS TRUE>
<cfoutput>myVariable equals #myVariable# and is TRUE 
</cfoutput>
</cfif>

<cfif myVariable IS NOT FALSE>
<cfoutput>myVariable equals #myVariable# and IS NOT FALSE
</cfoutput>
</cfif>

To prevent such ambiguities when you compare strings, use the ColdFusion string comparison functions Compare and CompareNoCase instead of the comparison operators.

You can also use the IsDate() function to determine whether a string can be interpreted as a date-time value, or to add characters to a string before comparison to avoid incorrect interpretation.

Date-time functions and queries when ODBC is not supported

ColdFusion creates Open Database Connectivity (ODBC) timestamp values when it converts date-time objects, such as the results returned by the ColdFusion Now, CreateDate, CreateTime, and CreateDateTime functions, to strings. As a result, you can get unexpected results when using dates with a database driver that does not support ODBC escape sequences or when you are using SQL in a query of queries.

If you use SQL to insert data into a database or in a WHERE clause to select data from a database, and the database driver does not support ODBC-formatted dates, use the DateFormat function to convert the date-time value to a valid format for the driver. This rule also applies to queries of queries.

For example, the following SQL statement uses the DateFormat function in a query of queries to select rows that have MyDate values in the future:

<cfquery name="MyQofQQ" dbtype="query">
SELECT * 
FROM DateQuery
WHERE MyDate >= '#DateFormat(Now())#'
</cfquery>

The following query of queries fails with the error message "Error: {ts is not a valid date," because ColdFusion Now function returns an ODBC timestamp:

<cfquery name="MyQofQQ" dbtype="query">
SELECT * 
FROM DateQuery
WHERE MyDate >= '#now()#'
</cfquery>

Using JavaCast with overloaded Java methods

You can overload Java methods so a class can have several identically named methods that differ only in parameter data types. At runtime, the VM attempts to resolve the specific method to use, based on the types of the parameters passed in the call. Because ColdFusion does not use explicit types, you cannot predict which version of the method the JVM will use.

The ColdFusion JavaCast function helps you ensure that the right method executes by specifying the Java type of a variable, as in the following code:

<cfset  emp.SetJobGrade(JavaCast("int", JobGrade)>

The JavaCast function takes two parameters: a string representing the Java data type and the variable whose type you are setting. You can specify the following Java data types: bool, int, long, float, double, and String.

For more information on the JavaCast function, see CFML Reference.

The effect of quotes

To ensure that ColdFusion properly interprets string data, surround strings in single or double quotes. For example, ColdFusion evaluates "10/2/2001" as a string that can be converted into a date-time object. However, it evaluates 10/2/2001 as a mathematical expression 5/2001, which evaluates to 0.00249875062469.

Examples of Type Conversion in Expression Evaluation

The following examples demonstrate ColdFusion expression evaluation.

Example 1
2 * TRUE + "YES" - ('y' & "es")
Result value as string: "2"
Explanation: (2*TRUE) is equal to 2; ("YES"-"yes") is equal to 0; 2 + 0 equals 2.

Example 2
TRUE AND 2 * 3
Result value as string: "YES"
Explanation: 6 is converted to Boolean TRUE; TRUE AND TRUE is TRUE.

Example 3
"Five is " & 5
Result value as string: "Five is 5"
Explanation: 5 is converted to the string "5".

Example 4
DateFormat("October 30, 2001" + 1)
Result value as string: "31-Oct-01"
Explanation: The addition operator forces the string "October 30, 2001" to be converted to a date-time object and then to a number. The number is incremented by one. The DateFormat function requires its argument to be a date-time object; thus the result of the addition is converted to a date-and-time object. 1 is added to the date-time object, moving it ahead by one day to October 31, 2001.

Variables differ in the source of the data, the places in your code where they are meaningful, and how long their values persist. These considerations are generally referred to as a variable's scope.

Scope Types

The following table lists the types of ColdFusion scopes and describes their uses. For more information on the function of each scope type, see Developing ColdFusion Applications.

Creating and Using Scope Variables

The following table shows how you create and refer to variables in different scopes in your code. For more information on the mechanisms for creating variables in most scopes see Creating Variables.

Using Scopes

The following sections provide details on how you can create and use variables in different scopes.

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. Variables (local scope)
2. CGI
3. URL
4. Form
5. Cookie
6. Client

Because ColdFusion must search for variables when you do not specify the scope, you can improve performance by specifying the scope for all variables. ColdFusion releases through 5.0 have some limitations on using the Variables scope identifier, as described in the following section.

To access variables in the Attribute, Request, Session, Application, and Server scopes, you must use the scope identifier prefix.

Limitations on using scope names

The use of the scope identifiers is limited, as follows.

Using the Variables identifier with queries and structures

If you do not specify the Variables prefix when you create a query or structure, you cannot use the prefix when you reference the query object or structure.

If you do specify the Variables prefix when you create the query or structure, you must always use the prefix when you reference the query or structure.

For example, if you use the following cfquery code, you must always refer to the query variable as myQuery, not as Variables.myQuery.

<cfquery name="myquery" datasource="CompanyInfo">

Similarly, if you use the following cfquery code, you must always refer to the query variable as Variables.myQuery, not as myQuery.

<cfquery name="Variables.myquery" datasource="CompanyInfo">

The following two scripts correctly create and use structures:

<cfscript> st1 = structnew(); st1.key0 = "foo";</cfscript>

<cfscript> Variables.st1 = structnew(); Variables.st1.key0 = "foo";</ cfscript>

The following script does not work as expected:

<cfscript> st1 = structnew(); Variables.st1.key0 = "foo";</cfscript>

The script appears to create a new structure, st1 and populate its element, st1.key0. In fact, it creates an empty structure st1 and a separate simple string variable Variables.st1.key0. The following code illustrates this behavior:

<cfscript> 
	st1 = structnew(); 
	Variables.st1.key0 = "foo";
	st1.key0 = "bar"
</cfscript>
<cfoutput>
	Variables.st1.key0: #Variables.st1.key0#

	st1.key0: #st1.key0#

</cfoutput>

Scopes and CFX tags

ColdFusion scopes do not apply to CFX tags. The ColdFusion page that calls a CFX tag must use tag attributes to pass data to the CFX tag. The CFX tag must use the Java Request and Response interfaces or the C++ Request class to get and return data.

The Java setVariable Response interface method and C++ CCFX::SetVariable method to return data to the Variables scope of the calling page. Therefore, they are equivalent to setting a Caller scope variable in a custom ColdFusion tag.

Scopes available as structures

ColdFusion makes the following scopes available as structures:

• Form
• URL
• Attributes
• Request
• CGI
• Cookie
• Session
• Application

You can reference the variables in these scopes as elements of a structure. To do so, specify the scope name as the structure name and the variable name as the key.