Objects

DQ objects are reference types with fields, methods, constructors, destructors, properties, inheritance, and virtual dispatch.

Declaration

object OCounter:
private
    value : int = 0

public
    function Inc():
        value += 1
    endfunc

    function Value() -> int:
        return value
    endfunc
endobj

Members can be grouped with visibility labels such as public and private. The current codebase commonly uses these labels inside object declarations.

Allocation

Object variables are references. A reference may be null.

var obj : OCounter = null

Use new for heap allocation.

obj = new OCounter()

Use delete to destroy a heap object and release its storage.

delete obj

Embedded allocation uses <-.

var local_counter <- OCounter()
var global_counter <- OCounter()

Embedded objects live in the containing storage location, such as a local stack frame, global data, or another object.

Constructors and Destructors

*Create is the constructor. *Destroy is the destructor.

object OData:
    value : int

    function *Create(initial : int):
        value = initial
    endfunc

    function *Destroy():
        // cleanup
    endfunc
endobj

Constructors can be overloaded with [[overload]].

object OText:
    function *Create() [[overload]]:
    endfunc

    function *Create(text : strview) [[overload]]:
    endfunc
endobj

Methods

Methods are declared inside an object. They can also be implemented outside the object by qualifying the function name.

object OThing:
    function Run()
endobj

function OThing.Run():
    // ...
endfunc

Inside a method, object fields and methods are available without a self. prefix.

function OCounter.Inc():
    value += 1
endfunc

From the object methods variables and functions outside of the object cannot be accessed without full namespace qualification. Method body use statements can blend-in namespaces so they can be used without qualification.

function OCounter.Print()
  @print.PrintLn('counter={}', value)  // @print. can be used to access PrintLn()
  @.PrintLn('counter={}', value)       // "@." for access everything that available in the current module
  use print                            // now the symbols in @print are available without qualification
  PrintLn('counter={}', value)
endfunc

Inheritance

Objects support single inheritance.

object OBase:
    function Count() [[virtual]]:
    endfunc
endobj

object OChild(OBase):
    function Count() [[override]]:
        inherited
    endfunc
endobj

inherited calls the inherited implementation of the current method.

Virtual Methods

Virtual methods are marked with [[virtual]]. Overrides are marked with [[override]].

object OBase:
    function Write(text : strview) [[virtual]]:
    endfunc
endobj

object OChild(OBase):
    function Write(text : strview) [[override]]:
    endfunc
endobj

The compiler also recognizes [[abstract]] and [[final]] in object method contexts.

Dynamic Casts

Object references can be dynamically cast with TryCast or with tryfrom.

var base : OBase = child
var child2 : OChild = TryCast(OChild, base)

var child3 : OChild tryfrom base

If the value is not compatible with the requested object type, the result is null.

Properties

Properties expose field-like syntax backed by fields or methods.

object OBox:
private
    stored : int = 0

    function GetStored() -> int:
        return stored
    endfunc

    function SetStored(value : int):
        stored = value
    endfunc

public
    property value : int read GetStored write SetStored
endobj

A property must have at least a read or a write accessor.

Field-backed properties are also supported.

property direct : int read stored write stored

Properties are used like fields.

box.value = 10
box.value += 1
PrintInt(box.value)

Read-only and write-only properties are possible by omitting the opposite accessor.

property readonly : int read GetStored
property writeonly : int write SetStored

Indexed properties place an index signature before the property result type.

property item : [index : int]int read GetItem write SetItem
property cell : [row : int, col : int]int read GetCell write SetCell

Indexed accessors must be methods. A getter receives the index arguments and returns the property type. A setter receives the index arguments followed by the new value.

An indexed property can be marked default. The default indexed property is used when the object itself is indexed.

object OJson:
    ...
    property child : [int]OJson default read Child
endobj

function Test():
    var firstchild : OJson
    var rootobj = new JSon()
    rootobj.LoadFromFile('test.json')
    firstchild = rootobj[0]
endfunc

Properties themselves are not addressable, but values returned by a property keep their normal semantics. For example, a property returning an object reference can be used to access that object's members.

Object Function References

Method reference types use of object.

type FObjText = function(msg : cstring) of object

An object method reference contains both the method and the object instance. Virtual functions resolved at assignment.