Freya: fields

A field is a variable associated with a class, when the field is static, or with an instance of a class, otherwise.

Field declarations

Fields can be declared both in the class declaration and in its implementation. Each field, however, can only be declared once.

Stack = class[X]
private
    Items: Array[X];
    Count, Capacity: Integer;
    // ...
end;

A field declared in an implementation section is considered as a private field:

implementation for Stack[X] is

    var Items: Array[X];

    // ...

Field declarations inside may be optionally preceded by the var keyword. You can add the static modifier for declaring static private fields, both in a class declaration and in its implementation.

Field initializers

A field declaration may be followed by an initialization expression:

implementation for Stack[X] is

    var Items: Array[X] = new Array[X](128);

You can't use any instance member in a field initializer. Field initializers are evaluated before the constructor from the base class is called.

Static fields can also have an initializer. In that case, the initializer expression may refer to other static members, both from the declaring type or from any other type. Static field initializers are executed in their declared order:

MyClass = class
public
    static A: Integer = B + 1;
    static B: Integer = A + 1;
end;

In the above class, A is initialized with 1, since B has not yet been initialized and it holds the default value for integers. Then, B is initialized with 2, because A at that point, A has already been initialized.

Read only fields

You can also declare read only fields:

Stack = class[X]
private
    Capacity: Integer; readonly;
    // ...
end;

A read only field cannot be modified except inside a constructor from its class:

constructor Stack(Capacity: Integer);
begin
    Self.Capacity := Capacity;
    // ...
end;

You can also use a field initializer with a read only field.

Classes and records can declare static read only fields that will behave as constants. For instance:

Vector = record
public
    constructor(X, Y, Z: Double);

public
    static Zero: Vector = new Vector; readonly;
    static XAxis: Vector = new Vector(1, 0, 0); readonly;
    static YAxis: Vector = new Vector(0, 1, 0); readonly;
    static ZAxis: Vector = new Vector(0, 0, 1); readonly;
end;

We could have initialized those static read only fields in a static constructor, as an alternative.

Fields

Field declarations

Field initializers

Read only fields

See also