THIS and SUPER

THIS and SUPER name the function block instance a method is running on. THIS^ is that instance itself, which is how a method reaches a variable of the function block that one of its own parameters or locals hides. SUPER^ is the same instance seen as its base type, which is how a derived type calls the method it overrode. Both are pointers, so both are written with the dereference operator ^.

Neither is a way to reach a second variable of an inherited name: a derived type cannot redeclare a variable it inherits (P4044).

Note

THIS and SUPER is a keyword only when the --allow-fb-inheritance flag is enabled. When the flag is not set, THIS and SUPER is an ordinary identifier and may be used as a variable or type name, exactly as in standard IEC 61131-3. Pass --allow-fb-inheritance to enable the object-oriented syntax, or select a dialect that includes it. See Enabling Dialects and Features for the dialects and flags reference.

IEC 61131-3

Edition 3 (object-oriented programming)

Support

Parsed only — not yet analyzed or executed (P9999). Enable with --allow-fb-inheritance; see Enabling Dialects and Features.

Syntax

The dereference operator is required — THIS and SUPER are pointers, so THIS.count is not valid where THIS^.count is. Whitespace between the keyword and the ^ is accepted:

THIS ^ . member
SUPER ^ . member

A dereferenced reference is used wherever a variable is: read from, assigned to, subscripted, or called.

Example

FUNCTION_BLOCK FB_Motor
    VAR
        speed : INT;
    END_VAR

    METHOD Stop
        THIS^.speed := 0;
    END_METHOD
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_LoggingMotor EXTENDS FB_Motor
    METHOD Stop
        SUPER^.Stop();
    END_METHOD
END_FUNCTION_BLOCK

FB_LoggingMotor declares its own Stop, which overrides the one it inherits. SUPER^.Stop() calls the overridden base implementation; writing Stop() there would call itself.

THIS^ is most useful when a local name hides a member of the instance — for example a method parameter named after a variable of the function block. THIS^.speed then names the function block’s variable, and speed alone names the parameter.

See Also