P4089¶

Code: P4089
Message: Method differs from the method it overrides

This error occurs when a function block declares a method that overrides a method it inherits through EXTENDS, and the two methods do not have the same signature.

A method’s signature is its return type and its VAR_INPUT, VAR_OUTPUT and VAR_IN_OUT variables: their names, their sections, their types and their order, together with its R_EDGE and F_EDGE inputs. Local variables are not part of the signature, so an override may declare its own. A call on an instance of the derived block runs the override where it would have run the base method, so the call has to be valid for both.

An override is compared with the method it overrides: the nearest one up the EXTENDS chain. A method that differs from a member of an interface the block implements is reported as P4068 instead.

Example¶

The following code will generate error P4089:

FUNCTION_BLOCK FB_Motor
METHOD Describe : INT
    Describe := 1;
END_METHOD
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_AdvancedMotor EXTENDS FB_Motor
METHOD Describe : BOOL  (* Error: FB_Motor.Describe returns INT *)
    Describe := TRUE;
END_METHOD
END_FUNCTION_BLOCK

To fix this error, give the override the signature of the method it overrides, or give it a name of its own if it is a different method:

FUNCTION_BLOCK FB_Motor
METHOD Describe : INT
    Describe := 1;
END_METHOD
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_AdvancedMotor EXTENDS FB_Motor
METHOD Describe : INT
    Describe := 2;
END_METHOD
END_FUNCTION_BLOCK

P4089¶

Example¶

See Also¶