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4D v14.3

Undefined


Undefined ( variable ) -> Function result

Parameter	Type	Description
variable	Variable	Variable to test
Function result	Boolean	True = Variable is currently undefined False = Variable is currently defined

Description

Undefined returns True if variable has not been defined, and False if variable has been defined. A variable is defined if it has been created via a compilation directive or if a value is assigned to it. It is undefined in all other cases.

If the database has been compiled, the Undefined function returns False for all variables.

Example

The following code manages the creation of processes when a menu item for a particular module of your application is chosen. If the process already exists, you bring it to the front; if it does not exist, you start it. To do so, for each module of the application, you maintain an interprocess variable ◊PID_... that you initialize in the On Startup database method.

When developing the database, you add new modules. Instead modifying the On Startup database method (to add the initialization of the corresponding ◊PID_...) and then reopening the database to reinitialize everything each time you add a module, you use the Undefined command to manage the addition of the new module, on the fly:

  //M_ADD_CUSTOMERS global procedure

 

 If(Undefined(◊PID_ADD_CUSTOMERS)) // Takes care of intermediate development stages

    C_LONGINT(◊PID_ADD_CUSTOMERS)

    ◊PID_ADD_CUSTOMERS:=0

 End if

 

 If(◊PID_ADD_CUSTOMERS=0)

    ◊PID_ADD_CUSTOMERS:=New process("P_ADD_CUSTOMERS";64*1024;"P_ADD_CUSTOMERS")

 Else

    SHOW PROCESS(◊PID_ADD_CUSTOMERS)

    BRING TO FRONT(◊PID_ADD_CUSTOMERS)

 End if

  //Note: P_ADD_CUSTOMERS, the process master method,

  // sets ◊PID_ADD_CUSTOMERS to zero when it ends.