P4066¶

Code: P4066
Message: Value does not implement the interface it is used as

This error occurs when a value is used where an interface is required, but the value’s type does not implement that interface.

A variable of an interface type refers to an instance of a function block type that implements the interface. The compiler accepts a value for it when the value is:

an instance of a function block type that lists the interface in IMPLEMENTS, or inherits it through EXTENDS from a base type that does
an instance whose type implements an interface that extends the required one
a variable of the same interface, or of an interface that extends it
the literal 0, which makes the variable refer to nothing

The check applies to an assignment to a variable of an interface type, and to an argument passed to an input of an interface type when calling a function block or method.

Example¶

The following code will generate error P4066:

INTERFACE I_Comm
END_INTERFACE

FUNCTION_BLOCK FB_Serial
END_FUNCTION_BLOCK

PROGRAM main
VAR
    comm   : I_Comm;
    serial : FB_Serial;
END_VAR
    comm := serial;  (* Error: FB_Serial does not implement I_Comm *)
END_PROGRAM

FB_Serial does not list I_Comm in IMPLEMENTS, so an instance of it cannot be used as an I_Comm.

To fix this error, declare that the function block type implements the interface:

FUNCTION_BLOCK FB_Serial IMPLEMENTS I_Comm
END_FUNCTION_BLOCK

An interface does not convert to an interface it is extended by. The following code will also generate error P4066, because not every I_Base is an I_Comm:

INTERFACE I_Base
END_INTERFACE

INTERFACE I_Comm EXTENDS I_Base
END_INTERFACE

PROGRAM main
VAR
    base : I_Base;
    comm : I_Comm;
END_VAR
    comm := base;  (* Error: I_Base does not implement I_Comm *)
END_PROGRAM

The assignment in the other direction, base := comm, is allowed.

P4066¶

Example¶

See Also¶