Incompatible POINTER TO types in class/function block hierarchy

This error occurs when a POINTER TO assignment involves two class or function block types that are not in a valid inheritance relationship. Only upcasts (child to parent) and same-type assignments are allowed.

POINTER TO is the recommended mechanism for polymorphism per IEC 61131-3. When working with class or function block hierarchies, the compiler checks that pointer assignments respect the EXTENDS chain: the right-hand side must be the same type as, or a subtype of, the left-hand side’s pointee type.

Invalid examples

Unrelated types

FUNCTION_BLOCK FbA
END_FUNCTION_BLOCK

FUNCTION_BLOCK FbX
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        instance : FbX;
        ptr      : POINTER TO FbA;
    END_VAR
    ptr := ADR(instance);  // Error: FbX is not a subtype of FbA
END_FUNCTION

Downcast (parent assigned to child pointer)

FUNCTION_BLOCK FbA
END_FUNCTION_BLOCK

FUNCTION_BLOCK FbB EXTENDS FbA
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        instance : FbA;
        ptr      : POINTER TO FbB;
    END_VAR
    ptr := ADR(instance);  // Error: FbA is not a subtype of FbB
END_FUNCTION

Sibling types

FUNCTION_BLOCK FbA
END_FUNCTION_BLOCK

FUNCTION_BLOCK FbB EXTENDS FbA
END_FUNCTION_BLOCK

FUNCTION_BLOCK FbC EXTENDS FbA
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        instance : FbC;
        ptr      : POINTER TO FbB;
    END_VAR
    ptr := ADR(instance);  // Error: FbC is not a subtype of FbB
END_FUNCTION

Valid examples

Same type

FUNCTION_BLOCK FbA
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        instance : FbA;
        ptr      : POINTER TO FbA;
    END_VAR
    ptr := ADR(instance);  // OK
END_FUNCTION

Upcast (child assigned to parent pointer)

FUNCTION_BLOCK FbA
END_FUNCTION_BLOCK

FUNCTION_BLOCK FbB EXTENDS FbA
END_FUNCTION_BLOCK

FUNCTION main
    VAR
        instanceB : FbB;
        ptrA      : POINTER TO FbA;
        ptrB      : POINTER TO FbB;
    END_VAR
    ptrA := ADR(instanceB);  // OK: FbB extends FbA
    ptrA := ptrB;            // OK: same relationship
END_FUNCTION