I am new to Object-Oriented Programming (OOP) in Structured Text (ST), but both familiar with OOP in C/C++ and ST. I would like to understand what is the difference between Properties (with a corresponding VAR to get/set) and a VAR_IN_OUT? Also, when should I use one instead of the other (in Structured Text OOP)?

Let’s say that we have the abstract object “Device” that has a name:

FUNCTION_BLOCK ABSTRACT Device
VAR
    Name_ : STRING(8);
END_VAR

with the following property:

PROPERTY PUBLIC Name : STRING(8)

with the getter:

Name := Name_;

and the setter:

Name_ := Name;

What would be the difference with this:

FUNCTION_BLOCK ABSTRACT Device
VAR_IN_OUT
    Name : STRING(8);
END_VAR

In other words, when should I use the property and when should I use the VAR_IN_OUT? Similarly, when should I use a property with only a getter/setter or a VAR_OUTPUT/VAR_INPUT, respectively?