JBC AND VISUAL BASIC GUIDE

Man Page Index

This table is a cross reference between jBC Basic commands and the equivalent functionality in Visual Basic using OBjEX. For each command that is applicable there is a reference to either a Visual Basic Intrinsic or to an OBjEX method.

TRAPS AND PITFALLS

Some of the common problems encountered by Basic programmers migrating to VB and OBjEX

ERROR HANDLING

It is important to read the VB documentation and understand the way that errors are handled in Visual Basic. In particular when you disable the default error handler (which aborts the program) ensure that you can handle all possible errors that can occur until you re-enable the default handler. One error to be particularly aware of is that any attempt to access an uninitialized object reference will cause an error rather than a familiar message of the form "Uninitialized variable - Zero Used"

Object References

Object variables that are declared in a VB program are just variables that can contain references to objects. They are effectively uninitialized variables until they are set to reference an object either as the result of a method on another object or as the result of a new operation. So make sure you do not attempt to use them as source operands until they refer to an object that actually exists.

THE SET STATEMENT

All references to objects have to be stored by the Set statement. If you omit the Set and create just an assignment statement then VB will attempt to assign the current default value of the object to the target variable.

EXAMPLE

Set MyDynArray = MyFile.Read(MyItemID)

Will make MyDynArray a reference to the jDynArray object that is
created by the Read method. If MyDynArray was referencing an object
prior to the Set statement then that object would be released if there were
no other references to it.

MyDynArray = MyFile.Read(MyItemID)

Will assign the string value of the dynamic array to the MyDynArray variable. If the MyDynArray actually references a existing dynamic array then the existing data will be overwritten by the string data converted back into a dynamic array. In effect the following will occur:

This may appear to be achieving the desired result but it will fail if the reference is null and is doing far more work than it needs to.

NULL REFERENCES VS NULL DYNAMIC ARRAYS

A Basic program would typically initialize dynamic arrays to, and test for, the null value in many circumstances. Using OBjEX , dynamic arrays are accessed through "references" that introduce a level of indirection creating two possible ways of representing null dynamic arrays.

1. As a null reference.
Methods that return jDynArray Object references will return a null reference when they fail to obtain the desired jDynArray (and cause an error). Code that tests for the existence of a record, for example, should use the IsObject() function to check for null references rather than looking for null dynamic arrays. The Set jDynArrayreference = Nothing statement should be used to set a reference to the null state.

2. As a reference to a jDynArray object with the value ".
Code that builds dynamic arrays from scratch should create the array using the new operator before accessing it.

Set jDynArrayreference = New jDynArray

This statement will release the current object (if any), create a jDynArray object and initialize it to the null state.

COMPONENTS

OBJEX