Your own types, 3 of 8

Properties

Give a type members that read like fields but run code, to check values and work them out.

What you’ll build

A Thermostat that holds a temperature in Celsius, shows it in Fahrenheit too, and keeps every setting within a safe range. Code that uses it reads and sets thermostat.Celsius as if it were a field, while the Thermostat checks each value it’s given.

A Property with a Get

A Property is a member that’s read like a field, but whose value comes from code: its Get. Get => value gives one expression; a Get block of several lines ends with End Get. A Property with only a Get can be read, and not assigned.

thermostat.fb

Import std.core

Class Thermostat
    Field celsius: Float = 20.0

    Property Fahrenheit: Float
        Get => celsius * 9.0 / 5.0 + 32.0
    End Property

    Property Comfortable: Bool
        Get
            Return celsius >= 18.0 && celsius <= 24.0
        End Get
    End Property
End Class

Local thermostat = Thermostat()
Print(thermostat.Fahrenheit, thermostat.Comfortable)

Output

68 True

Assigning thermostat.Comfortable = False is an error, since Comfortable has no Set.

A Set that checks

A Set runs when the Property is assigned, and its parameter is the new value. It can check the value, change it, or store it somewhere else. Here Celsius keeps the setting between 5 and 30 degrees, and Fahrenheit converts and passes its value on to Celsius.

thermostat.fb

Import std.core
Import std.math

Class Thermostat
    Field celsius: Float = 20.0

    Property Celsius: Float
        Get => celsius
        Set(value)
            celsius = Clamp(value, 5.0, 30.0)
        End Set
    End Property

    Property Fahrenheit: Float
        Get => celsius * 9.0 / 5.0 + 32.0
        Set(value)
            Celsius = (value - 32.0) * 5.0 / 9.0
        End Set
    End Property
End Class

Local thermostat = Thermostat()
thermostat.Celsius = 40.0
Print(thermostat.Celsius)
thermostat.Fahrenheit = 68.0
Print(thermostat.Celsius)
thermostat.Celsius += 1.5
Print(thermostat.Celsius, thermostat.Fahrenheit)

Output

30
20
21.5 70.7

thermostat.Celsius += 1.5 calls the Get, adds, and calls the Set with the sum. Clamp, from std.math, keeps a value between two others.

Static Properties

A Static Property belongs to the type, as a Static Field does, and is read through the type’s name. Its Get has no object to use.

thermostat.fb

Import std.core

Class Thermostat
    Static Field installed: Int

    New()
        installed++
    End New

    Static Property Installed: Int
        Get => installed
    End Property
End Class

Local rooms = [Thermostat(), Thermostat(), Thermostat()]
Print("{Thermostat.Installed} thermostats for {rooms.Count} rooms")

Output

3 thermostats for 3 rooms

Properties in a Struct

A Struct can have Properties too. Its Get reads the fields, and its Set can change them, like a ByRef Method, so a Struct’s Property can only be assigned on a variable.

size.fb

Import std.core

Struct Size
    Field Width: Int
    Field Height: Int

    Property Area: Int
        Get => Width * Height
    End Property

    Property Square: Int
        Get => Width
        Set(side)
            Width = side
            Height = side
        End Set
    End Property
End Struct

Local box = Size(3, 4)
Print(box.Area)
box.Square = 5
Print(box.Width, box.Height, box.Area)

Output

12
5 5 25

What you learned

A Property reads like a field, and its value comes from its Get.
A Set runs on assignment and can check or convert the value; without one, the Property can’t be assigned.
+= and ++ on a Property call its Get, then its Set.
A Static Property belongs to the type, and a Struct’s Set changes the Struct.

The rules are in the language reference.

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