P4068¶

Code: P4068
Message: Function block member differs from the interface member it implements

This error occurs when a function block type implements an interface and declares one of the interface’s members, but with a different signature.

A method must have the same return type as the interface’s method, and the same input, output and in-out variables: the same names, the same kinds and the same types, in the same order. A string parameter must have the same length, and an R_EDGE or F_EDGE input the same edge. Local variables are not compared. A property must have the same type, and every accessor (GET, SET) that the interface declares. An extra accessor is allowed.

A function block that extends a base which implements an interface implements it too, so a method or property it overrides must also match the interface’s.

Example¶

The following code will generate error P4068:

INTERFACE I_Comm
    METHOD Send : BOOL
        VAR_INPUT
            data : INT;
        END_VAR
    END_METHOD
END_INTERFACE

FUNCTION_BLOCK FB_Serial IMPLEMENTS I_Comm
    METHOD Send : BOOL
        VAR_INPUT
            data : DINT;  (* Error: the interface declares INT *)
        END_VAR
        Send := TRUE;
    END_METHOD
END_FUNCTION_BLOCK

To fix this error, declare the member exactly as the interface does:

FUNCTION_BLOCK FB_Serial IMPLEMENTS I_Comm
    METHOD Send : BOOL
        VAR_INPUT
            data : INT;
        END_VAR
        Send := TRUE;
    END_METHOD
END_FUNCTION_BLOCK

P4068¶

Example¶

See Also¶