default properties
Default properties allow "hoisting" or otherwise exposing of object members into the caller name space to facilitate wrapper types and (possibly) delegation patterns.
This feature is under developement and everything is subject to change.
Download development fork:
https://github.com/genericptr/freepascal/tree/defaultprops
Supports:
- records, objects, classes.
- arithmetic, compare, binary, unary, in overloads. (https://www.freepascal.org/docs-html/ref/refch15.html)
- visibility sections.
- array indexing.
- Statements:
- if, while, repeat, case, for..do
Precedence rules:
Precedence order for names is as follows: base, default (last to first).
type
THelper = class
num: integer;
procedure DoThis; overload;
procedure DoThis (param: integer); overload;
procedure DoThis (param: string); overload;
end;
procedure THelper.DoThis;
begin
writeln('THelper.DoThis:',num);
end;
procedure THelper.DoThis (param: integer);
begin
writeln('THelper.DoThis:',param,':',num);
end;
procedure THelper.DoThis (param: string);
begin
writeln('THelper.DoThis:',param,':',num);
end;
type
TParent = class
m_helper: THelper;
property helper: THelper read m_helper; default;
procedure DoThis (param:single);overload;
end;
procedure TParent.DoThis (param:single);
begin
writeln('TParent.DoThis:',param:1:1);
end;
type
TMyObject = class (TParent)
procedure Call;
procedure DoThis;overload;
end;
procedure TMyObject.DoThis;
begin
writeln('TMyObject.DoThis');
end;
procedure TMyObject.Call;
begin
DoThis(100); // THelper.DoThis
DoThis(10.5); // TParent.DoThis
DoThis('hello'); // THelper.DoThis
DoThis; // TMyObject.DoThis
end;
var
obj: TMyObject;
begin
obj := TMyObject.Create;
obj.m_helper := THelper.Create;
obj.Call;
end.
Operator overloads are possible also. Precedence can get complicated considering it's possible to have overloads in the current unit for the default type as well as overloads in the structure itself.
operator + (l : integer; r : ansistring): integer;
begin
result := l + StrToInt(r);
end;
type
TWrapper = record
m_value: integer;
property value: integer read m_value write m_value; default;
end;
var
wrapper: TWrapper;
begin
wrapper += 1; // default property takes precedence
wrapper += '128'; // unit + operator takes precedence because default property type (integer) doesn't support string overloads
writeln(wrapper.value); // 129
end.
Type helpers behave as normal on the default type.
type
TIntegerHelper = type helper for integer
function Str: string;
end;
function TIntegerHelper.Str: string;
begin
result := 'string->'+IntToStr(self);
end;
type
TWrapper = record
m_value: integer;
property value: integer read m_value write m_value; default;
end;
var
wrapper: TWrapper;
begin
wrapper := 100;
writeln(wrapper.str);
end.
Assignments:
Default properties introduce an ambiguity with assignments.
type
TWrapper = class
m_value: integer;
m_string: string;
property value: integer read m_value write m_value; default;
property str: string read m_string write m_string; default;
end;
var
wrapper: TWrapper;
begin
wrapper := TWrapper.Create; // TWrapper is same type so assignment is as normal
wrapper := 100; // integer matches "value"
wrapper := 'hello world'; // string matches "str"
writeln(wrapper.value);
writeln(wrapper.str);
end.
Multidimensional:
Currently during development while the concept is being explored default properties are "multidimensional", meaning you can declare more than one per type.
They have been developed this way from the outset because the concept of exposing namespaces is inherently a multidimensional concept. Limiting a structure to only one would be an articial limit impossed by the compiler so at least for development I wanted to keep the option available while the concept is being explored.
This is of course a highly contested idea given the capacity to introduce difficult to understand precednece rules and method calling.
Examples of usages:
Nullable types:
type
generic TNullable<T>= record
private
FValue : T;
public
isAssigned : boolean;
function IsNull: boolean;
procedure SetValue (newValue: T);
Property Value : T read FValue write SetValue; default;
class operator Initialize(var a: TNullable);
end;
procedure TNullable.SetValue (newValue: T);
begin
FValue := newValue;
isAssigned := true;
end;
function TNullable.IsNull: boolean;
begin
result := not isAssigned;
end;
class operator TNullable.Initialize(var a: TNullable);
begin
a.isAssigned := Default(T);
end;
type
TBoolean = specialize TNullable<boolean>;
Var
bool: TBoolean;
begin
bool := true;
if bool then
writeln('bool is true');
end.
Auto managed classes using management operators:
type
generic TAuto<T> = record
private
m_object: T;
class operator Initialize(var a: TAuto);
class operator Finalize(var a: TAuto);
public
property obj: T read m_object; default;
end;
type
TStringList = specialize TFPGList<String>;
TStringListAuto = specialize TAuto<TStringList>;
class operator TAuto.Initialize(var a: TAuto);
begin
a.m_object := T.Create;
end;
class operator TAuto.Finalize(var a: TAuto);
begin
a.m_object.Free;
end;
var
list: TStringListAuto;
str: string;
begin
list.Add('foo');
list.Add('bar');
for str in list do
writeln(str);
end.
Array indexing
Because it is technically possible arrays are exposed using default properties. This however doesn't conflict with the existing [] indexer properties.
type
TIntArray = array of integer;
TWrapper = record
m_value: TIntArray;
property value: TIntArray read m_value write m_value; default;
end;
var
wrapper: TWrapper;
i: integer;
begin
wrapper := TIntArray.Create(100,200,300); // NOTE: we can't use array constructors [] due to bug in compiler
wrapper[0] += 50;
wrapper[1] += 1;
for i in wrapper do
writeln(i);
end.
Modules/Delegation
If delegate properties are allowed be multidimensional in the final version they can be used for delegation patterns that would otherwise only be possible with multiple inheritance (in languages such as C++, Java).
// Stats:
type
TStats = record
hp: integer;
exp: integer;
procedure InitStats;
end;
// Base Module:
type
TEntity = class;
TBaseHandler = class
entity: TEntity;
constructor Create (inEntity: TEntity);
// NOTE: this is a dangerous idea because it will be convered
procedure Clear;
end;
// Physics Module:
TPhysicsHandler = class (TBaseHandler)
pos: TVec3;
acc: TVec3;
vel: TVec3;
procedure Integrate; virtual;
end;
// Renderer Module:
TRendererHandler = class (TBaseHandler)
procedure Render; virtual;
end;
// Entity:
TEntity = class
private
m_physics: TPhysicsHandler;
m_renderer: TRendererHandler;
m_stats: TStats;
public
property physics: TPhysicsHandler read m_physics; default;
property renderer: TRendererHandler read m_renderer; default;
property stats: TStats read m_stats; default;
public
procedure Update;
end;
// Implemention:
type
TMonster = class (TEntity)
public
procedure AfterConstruction; override;
end;
type
TMonsterRenderer = class (TRendererHandler)
procedure Render; override;
end;
procedure TStats.InitStats;
begin
hp := 100;
exp := 0;
end;
procedure TBaseHandler.Clear;
begin
writeln(classname,' clear');
end;
constructor TBaseHandler.Create (inEntity: TEntity);
begin
entity := inEntity;
end;
procedure TPhysicsHandler.Integrate;
begin
pos.x += vel.x;
pos.y += vel.y;
pos.z += vel.z;
end;
procedure TRendererHandler.Render;
begin
end;
procedure TEntity.Update;
begin
Integrate;
Render;
end;
procedure TMonster.AfterConstruction;
begin
m_physics := TPhysicsHandler.Create(self);
m_renderer := TMonsterRenderer.Create(self);
InitStats;
vel.x := 80;
vel.y := 20;
vel.z := 0;
end;
procedure TMonsterRenderer.Render;
begin
writeln('render monster at ', entity.pos.str, ' hp:', entity.hp);
end;
var
entity: TMonster;
i: integer;
begin
entity := TMonster.Create;
entity.Clear;
for i := 0 to 2 do
entity.Update;
end.
Default Implements Properties
"implements" properties are more complete by exposing their namespace using "default". If multiple defaults are allowed one could imagine this being a possible method to implement multiple inheritence in Pascal.
type
IWrapper = interface ['IWrapper']
procedure DoThis;
end;
type
TWrapper_Handler = class (IWrapper)
procedure DoThis;
end;
procedure TWrapper_Handler.DoThis;
begin
writeln('TWrapper_Handler.DoThis');
end;
type
TWrapper = class (IWrapper)
m_wrapper: TWrapper_Handler;
property handler: TWrapper_Handler read m_wrapper implements IWrapper; default;
procedure AfterConstruction; override;
end;
procedure TWrapper.AfterConstruction;
begin
m_wrapper := TWrapper_Handler.Create;
end;
procedure HandleWrapper (wrapper: IWrapper);
begin
wrapper.DoThis;
end;
var
wrapper: TWrapper;
begin
wrapper := TWrapper.Create;
HandleWrapper(wrapper);
end.