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Pointer: THIS

The THIS pointer is a special variable which is used for object-oriented programming.

THIS is the pointer of a function block to its own function block instance. A THIS pointer is automatically available for each function block. You can use THIS only in methods and in function blocks. THIS is available for the implementation in the Input Assistant in the category Keywords.

Dereferencing of the pointer: THIS^

. Use of the THIS pointer

  • If a local variable overrides a function block variable in a method, you can set the function block variable with the THIS pointer. See example below (1)

  • If the pointer to the function block's own function block instance is referenced for use in a function. (See example below (2))

Example 101. Examples

ST

THIS^.METH_DoIt();

FBD/CFC/LD

_cds_img_pointer_this.png


Example 102. Example

The iVarB local variable overrides the iVarB function block variable.

FUNCTION_BLOCK  fbA
VAR_INPUT
    iVarA: INT;
END_VAR
iVarA := 1;

FUNCTION_BLOCK fbB EXTENDS fbA
VAR_INPUT
    iVarB: INT := 0;
END_VAR
iVarA := 11;
iVarB := 2;

    METHOD DoIt : BOOL
    VAR_INPUT
    END_VAR
    VAR
        iVarB: INT;
    END_VAR
    iVarB := 22;    // The local variable iVarB is set.
    THIS^.iVarB := 222;    // The function block variable iVarB is set even though iVarB is obscured.

PROGRAM PLC_PRG
VAR
   MyfbB: fbB;
END_VAR

MyfbB(iVarA:=0, iVarB:= 0);
MyfbB.DoIt();


Example 103. Example

A function call requires the reference to its own instance.

FUNCTION funA
VAR_INPUT
    pFB: fbA;
END_VAR
...;

FUNCTION_BLOCK  fbA
VAR_INPUT
    iVarA: INT;
END_VAR
...;

FUNCTION_BLOCK fbB EXTENDS fbA
VAR_INPUT
    iVarB: INT := 0;
END_VAR
iVarA := 11;
iVarB := 2;

    METHOD DoIt : BOOL
    VAR_INPUT
    END_VAR
    VAR
        iVarB: INT;
    END_VAR
    iVarB := 22;    //The local variable iVarB is set.
    funA(pFB := THIS^);    //funA is called via THIS^.

PROGRAM PLC_PRG
VAR
    MyfbB: fbB;
END_VAR
MyfbB(iVarA:=0 , iVarB:= 0);
MyfbB.DoIt();


Tip

THIS is not yet implemented for the instruction list (IL).

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