5. Data items and types of data items

In this chapter, we describe data-items that may represent references to objects, primitive values, string values, and composite values.

As mentioned in previous chapters, objects may have attributes in the form of data-items that represent properties characterizing the corresponding phenomena. For the Account example, we have seen examples of data-items such as owner, balance, and interestRate:

class Account(owner: ref Customer):
   balance: var float
   interestRate: var float
   -"-

There is a fundamental difference between the data-items owner and balance. While owner is a reference to the object representing the owner of the account (represented by an object of class Customer), balance represents a property that is given by a float value. We return to this in section .

Some values are denoted primitive values, and their types are often predefined. balance and interestRate are examples of data-items holding primitive values, in this case of type float. We have also seen examples of primitive values of type integer and boolean. We summarise these primitive values below.

A phenomenon may also be characterised by values that are composite values constructed from primitive values or other composite values. Examples of this is a date, which consist of a year, month and day all of type integer. Other examples are points, complex numbers, GPS-positions, etc. The type of a composite value is called a composite value type or just composite type. We describe composite values and types in section .

A String is another example of a value type. The name attribute of Customer was in an early version the class of type String. We describe String in section .

We use the term datum to cover values as well as references. Data-items in general may hold datums and a datum may thus be a value or reference.