IS BOUND, IS INITIAL 和 IS ASSIGNED的区别

1 Introduction

The sets of these three predicate expressions can easily confuse ABAP newbies, if the explanation for them in corresponding ABAP help document are not seriously taken.

2 Explaination

The following text are copied from ABAP help:
IS BOUND

It checks whether a reference variable contains a valid reference. A data reference variable that contains a stack reference, on the other hand, can become invalid even if the reference data object is removed from the stack.

IS INITIAL

checks whether the operand operand is initial. The expression is true, if the operand contains its type-friendly initial value.

IS ASSIGNED

checks whether a memory area is assigned to a field symbol. The expression is true if the field symbol points to a memory area.