Error on using structured variable in a Method of an Interface

[George]

I have another question about using structure type variable in a Method of an Interface, when I use object-oriented features I get

IDev.open.cfg – error A4109:  cfg => Invalid base object type

METHOD open: BOOL

VAR_INPUT
cfg : sDevCfg;
END_VAR

The strange behavior is that in some cases the structure type variable in Method prototype specification in an Interface. Adding in a Function Block a Method with VAR_INPUT structure type variable  is accepted without errors.

The problem is in interface definition.

[Sergio Bertana]

Hi George I’ve tried to recreate the same behaviour you report in the TestInterface program (Download).

In the screenshot you can see the program structure.

Can you verify if the program I made recreate exactly your situation?

[George]

Yes I tested your program and compiles correctly.

Now I gonna test this function in my program.

[George]

There an issue about memory allocation of a function block instance and assignment to an interface.

In CODESY and TWINCAT there is the NEW operator that reserves dynamic memory to instantiate function blocks.

I tried to something similar by using sysmalloc and pointer but I am not sure that is correct way.

IF SysFirstLoop THEN
pFbA:= SysMAlloc(SIZEOF(fbMyA));
pFbB:= SysMAlloc(SIZEOF(fbMyB));
END_IF;

CASE select    OF
0:
itf:= @pFbA;
select:=2;
1:
itf:= @pFbB;
select:=2;
ELSE
END_CASE;

itf4.mInit(myCfg);

[Autore]

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