MUTABLE

MUTABLE is an attribute that can be used on variables that are declared in a VAR_INPUT section of a function block or a thread block.
 

By using this attribute it is legal to assign a value to this variable from inside the function block / thread block.

The MUTABLE attribute can only be used on variables of simple data types.

This is a simple example of using mutable variables:

FUNCTION_BLOCK CB_TEST;

VAR_INPUT

  v1 : MUTABLE INT;

  v2 : MUTABLE STRING;

  v3 : INT;

END_VAR;

  v1 := v1 * 5;

  v2 := "Hello world";

  v3 := 51;   <--- NOT ALLOWED

END_FUNCTION_BLOCK;

VAR

  test : CB_TEST;

END_VAR;

Calling the function block:

test( v1 := 1, v2 := "Nice",  v3 := 1 );

After the call to "test", the contents of the input variables are as follows: "v1" is 5, "v2" is "Hello world", and "v3" is 1.

"v1" and "v2" are both mutable and their values are therefore changed, while "v3" is not mutable and its contents remain unchanged.