THIS is a special variable and 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 obscures a function block variable in a method, you can set the function block variable with the
THISpointer. 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))
Examples
ST:
THIS^.METH_DoIt();
FBD/CFC/LD:
THIS is not yet implemented for the instruction list (IL).
Examples
(1) The local variable iVarB obscures the function block variable iVarB.
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();
(2) 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();
See also