Ancestor and descendant variables

All objects in PowerBuilder are descendants of PowerBuilder system objects the objects you see listed on the System page in the Browser.

Therefore, whenever you declare an object instance, you are declaring a descendant. You decide how specific you want your declarations to be.

As specific as possible

If you define a user object class named uo_empdata, you can declare a variable whose type is uo_empdata to hold the user object reference:

uo_empdata uo_emp1
uo_emp1 = CREATE uo_empdata

You can refer to the variables and functions that are part of the definition of uo_empdata because the type of uo_emp1 is uo_empdata.

When the application requires flexibility

Suppose the user object you want to create depends on the user's choices. You can declare a user object variable whose type is UserObject or an ancestor class for the user object. Then you can specify the object class you want to instantiate in a string variable and use it with CREATE:

uo_empdata uo_emp1
string ls_objname
ls_objname = ... // Establish the user object to open
uo_emp1 = CREATE USING ls_objname

This more general approach limits your access to the object's variables and functions. The compiler knows only the properties and functions of the ancestor class uo_empdata (or the system class UserObject if that is what you declared). It does not know which object you will actually create and cannot allow references to properties defined on that unknown object.

Abstract ancestor object

In order to address properties and functions of the descendants you plan to instantiate, you can define the ancestor object class to include the properties and functions that you will implement in the descendants. In the ancestor, the functions do not need code other than a return value they exist so that the compiler can recognize the function names. When you declare a variable of the ancestor class, you can reference the functions. During execution, you can instantiate the variable with a descendant, where that descendant implements the functions as appropriate:

uuo_empdata uo_emp1
string ls_objname
// Establish which descendant of uo_empdata to open
ls_objname = ...
uo_emp1 = CREATE USING ls_objname

// Function is declared in the ancestor class
result = uo_emp1.uf_special()

This technique is described in more detail in Dynamic versus static lookup.

Dynamic function calls

Another way to handle functions that are not defined for the declared class is to use dynamic function calls.

When you use the DYNAMIC keyword in a function call, the compiler does not check whether the function call is valid. The checking happens during execution when the variable has been instantiated with the appropriate object:

// Function not declared in the ancestor class
result = uo_emp1.DYNAMIC uf_special()

Performance and errors

You should avoid using the dynamic capabilities of PowerBuilder when your application design does not require them. Runtime evaluation means that work the compiler usually does must be done at runtime, making the application slower when dynamic calls are used often or used within a large loop. Skipping compiler checking also means that errors that might be caught by the compiler are not found until the user is executing the program.

Dynamic object selection for windows and visual user objects

A window or visual user object is opened with a function call instead of the CREATE statement. With the Open and OpenUserObject functions, you can specify the class of the window or object to be opened, making it possible to open a descendant different from the declaration's object type.

This example displays a user object of the type specified in the string s_u_name and stores the reference to the user object in the variable u_to_open. Variable u_to_open is of type DragObject, which is the ancestor of all user objects. It can hold a reference to any user object:

DragObject u_to_open
string s_u_name
s_u_name = sle_user.Text
w_info.OpenUserObject(u_to_open, s_u_name, 100, 200)

For a window, comparable code looks like this. The actual window opened could be the class w_data_entry or any of its descendants:

w_data_entry w_data
string s_window_name
s_window_name = sle_win.Text
Open(w_data, s_window_name)