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NOTICE

NOTICE
NOTICE
NOTICE

NOTICE

NOTICE

With compiler version >= V3.3.0.0, references are initialized (at 0).

NOTICE

NOTICE
NOTICE
NOTICE

NOTICE

NOTICE

If a reference refers to a device input, then the access is applied as write access. When the code is generated, this leads to the compiler warning: "‎...invalid assignment target‎".

Example: ‎rInput REF= Input;‎

If you require a construct of this kind, you have to first copy the input value (example: ‎rInput‎) to a variable with write access.

A reference with the ‎REFERENCE‎ data type implicitly refers to another object. The assignment is done with the ⮫ REF= operator. When accessed, the reference is implicitly dereferenced, and therefore does not need a special content operator ‎^‎ such as a pointer.

Syntax 

<identifier> : REFERENCE TO <data type> ;
<data type>: base type of the reference

Valid declaration 

PROGRAM PLC_PRG
VAR
    rspeA : REFERENCE TO DUT_SPECIAL;
    pspeA : POINTER TO DUT_SPECIAL;
    speB : DUT_SPECIAL;
END_VAR
rspeA REF= speB; // Reference rspeA is alias for speB. The code corresponds to pspeA := ADR(speB);
rspeA := speD; // The code corresponds to pspeA^ := speD;

Invalid declarations 

ariTest : ARRAY[0..9] OF REFERENCE TO INT;
priTest : POINTER TO REFERENCE TO INT;
rriTest : REFERENCE TO REFERENCE TO INT;
rbitTest : REFERENCE TO BIT;

A reference type must not be used as the base type of an array, pointer, or reference. Furthermore, a reference must not refer to a bit variable. These kinds of constructs generate compiler errors.

The readability of a program is made difficult when the same memory cell is accessed simultaneously by means of an identifier and its alias.

Example: ‎speB‎ and ‎rspeA‎

References and pointers to ‎BIT‎ variables are invalid declarations, as well as array elements with base type ‎BIT‎.

Examples

When references are assigned with :=, a value is always copied, no matter whether the reference is on the left, on the right, or on both sides:

  • Ref := value writes the value value to the location where the reference points to. In pointer notation: Ref^ := value

  • value := Ref writes the value which the reference points to after value. In pointer notation: value := Ref^

  • Ref1 := Ref2 writes the value which Ref2 points to. at the location where Ref1 points to. In pointer notation: Ref1^ := Ref2^

When REF= is used, the address is always applied and there has to be a reference on the left side:

  • Ref REF= value: The reference points to value. In pointer notation: Ref :=ADR(value)

  • Ref1 REF= Ref2: Ref1 points to the same value as Ref2. In pointer notation: Ref1 := Ref2

  • Value REF = Ref leads to a compile error

In the declaration, REFERENCE TO INT REF= value behaves like REFERENCE TO INT := value.

  • Comparison of reference and pointer
  • Testing the validity of a reference