Question Regarding Factory Method Pattern and Memory Management in Structured Text

[BhanuKiranChaluvadi]

I am trying to understand the memory allocation and deallocation process when using the Factory Method design pattern in Structured Text. Specifically, I am confused about how memory is allocated, returned, and subsequently freed in the following skeleton code.

Code Skeleton

INTERFACE I_Shape

{attribute 'enable_dynamic_creation'}
FUNCTION_BLOCK FB_Circle IMPLEMENTS I_Shape

FUNCTION_BLOCK FB_ShapeCreator
VAR_INPUT
    eShape : E_Shape;
END_VAR
VAR
    pCircle    : POINTER TO FB_Circle;
    pRectangle : POINTER TO FB_Rectangle;
    pSquare    : POINTER TO FB_Square;
    pStar      : POINTER TO FB_Star;
    pTriangle  : POINTER TO FB_Triangle;
    iShape     : I_Shape;
END_VAR

METHOD M_GetShapeObject : I_Shape
VAR_INPUT
END_VAR

CASE eShape OF
    E_Shape.Circle:
        // Dynamic instantiation  
        pCircle := __NEW(FB_Circle);
        iShape := pCircle^;
        
        // Return the object
        IF iShape <> 0 THEN
            M_GetShapeObject := iShape;
        END_IF

        // Release the memory
        IF (pCircle <> 0) THEN
            __DELETE(pCircle);
            pCircle := 0;
        END_IF

        eShape := E_Shape.init;
    ...
END_METHOD

Questions & Concerns

1. Memory Allocation & Return:

   • The statement pCircle := __NEW(FB_Circle); allocates memory on the heap, and pCircle holds the address of the allocated memory.
   • Later, we assign M_GetShapeObject := iShape;, but what exactly is being returned?
     • Is it a copy of the object on the stack being returned to the caller?
     • Or is the function returning a reference to the memory allocated on the heap?

2. Memory Deallocation Issue:

   • If iShape holds a reference to the heap-allocated memory, it seems problematic that we immediately deallocate the memory with:

     IF (pCircle <> 0) THEN
         __DELETE(pCircle);
         pCircle := 0;
     END_IF
     

   • Would this not result in a situation where M_GetShapeObject is returning a reference to an already freed object?

3. Clarification on M_GetShapeObject := iShape;:

   • What exactly happens on this line? Is it performing a deep copy of the object or just assigning a reference to the heap-allocated memory?

4. Further Reading:

   • If there are any official references or online materials explaining how memory management works with __NEW and __DELETE in Structured Text, I would appreciate the guidance.

Thank You!

I appreciate any insights or explanations that can help clarify this behavior.