Data Type: REFERENCE TO

NOTICE

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

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.

Comparison of reference and pointer
Testing the validity of a reference