default properties

From Lazarus wiki
Jump to navigationJump to search

Default properties allow "hoisting" or exposing of object members into the caller name space to facilitate wrapper types and (possibly) delegation patterns. They could be seen as "with" statements which encompass an entire structures namespace.

This feature is under development and everything is subject to change.

Download Development Branch

https://github.com/genericptr/freepascal/tree/defaultprops

Supports

Hoisting Members

By making "m_obj" a default property we can omit m_obj. to subscript into the members. This is like wrapping every instance of "m_obj" in a "with" statement. This makes it possible to bring members from other structures into the current structure without prefixing "m_obj." before accessing fields.

 type
   THelper = record
     num: integer;
   end;
 
 type
   TWrapper = record
     m_obj: THelper;
     property helper: THelper read m_obj; default;
     procedure Inc;
   end;
 
 procedure TWrapper.Inc;
 begin
   num += 1;
 end;
 
 var
   wrapper: TWrapper;
 begin
   wrapper.num := 100;
   wrapper.Inc;
   writeln(wrapper.num);
 end.

Precedence Rules

For default properties which are structures (objects, records, classes) it's possible to have method overloads in both the base structure and the default property (or unit scope even). For this reason overload resolution must follow a predicable pattern.

Precedence order for overloads 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
   if wrapper = 129 then       // = operator works also
     writeln('got 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.

Passing

Passing a structure with default properties follows normal move semantics. For example, if there is a record with a default property of the type "integer" passing the record will not copy the integer value but rather the entire record will be passed.

 type
   TIntRec = record
     m_int: integer;
     property value: integer read m_obj; default;
   end;
 
 procedure Print (int: TIntRec); overload;
 begin
   writeln(int.value);
 end;
 
 procedure Print (int: integer); overload;
 begin
   writeln(int);
 end;

 var
   int: TIntRec;
 begin
   int := 100;
   Print(int);    // Print(TIntRec) is called
   writeln(int)   // ERROR: can't write this type (TIntRec)
 end.

Assignment Rules

Default properties introduce an ambiguity with assignments which is resolved by type.

 type
   TWrapper = class
     m_value: integer;
     m_string: string;
     property value: integer read m_value write m_value; default;
   end;
 
 var
   wrapper: TWrapper;
 begin
   wrapper := TWrapper.Create;   // TWrapper is same type so assignment is as normal
   wrapper := 100;               // integer matches "value"
   writeln(wrapper.value);
 end.

Multiple Defaults

Currently for development default properties are "multidimensional", meaning you can declare more than one per type and overloads will be resolved according to precedence.

They have been developed this way from the outset because the nature of the search is inherently multidimensional, i.e base object, default property 1, default property 2 etc... so 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 namespace collisions.

Hiding Fields

If multiple defaults are allowed then it would be possible to hide fields from other default properties and cause some potential bugs.

 type
   THelper = record
     data: integer;
   end;
 
 type
   TWrapper = record
     m_objA: THelper;
     m_objB: THelper;
     property helperA: THelper read m_objA; default;
     property helperB: THelper read m_objB; default;
   end;
 
 var
   wrapper: TWrapper;
 begin
   wrapper.data := 1;
   writeln('helperA:',wrapper.helperA.data); // 0
   writeln('helperB:',wrapper.helperB.data); // 1
 end.

Examples 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 does conflict with the existing [] indexer properties so this may be a problem.

As it stands however default properties on array types are an interesting method to make array wrappers.

 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.

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).

  
 type
   TStats = record
     hp: integer;
     exp: integer;
     procedure InitStats;
   end;
 
 // Base Module:
 
 type
   TEntity = class;
   TBaseHandler = class
     entity: TEntity;
     constructor Create (inEntity: TEntity);
     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;
 RSS
 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 inheritance 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.

COMPILER: Implemention Details

Default properties are implemented primarily at the parser layer and overloads (proc and operator) are handled through tcallcandiates (must be explictly requested in create params). They could be handled in the node layer during typechecking instead if that was deemed more appropriate. I used the parser layer because regular properties are handled this way and I wasn't sure about the ramifications of making nodes convert themselves to properties in all instances across the entire platform. Furthermore I personally see default properties as a sort of "meta-layer" which doesn't really exist but is more of a re-routing scheme which is applied in advance of normal syntax.

For reference here are some of the important changes made to support default properties.

  • symtable.pas

searchsym_xxx() functions now contain parameters which explicitly request defaults to be included in the result (see ssf_search_defaults) and return a tpropertysym. searchsym_with_defaults() is added for unit level searching.

  • pexpr.pas
 do_member_read_internal() - handles the default property sym returned by searchsym_xxx using handle_read_property().
 do_proc_call() - after proc params are parsed default read property is handled via handle_default_proc_call().
 

Other relevant functions:

 handle_default_assignment()
 handle_default_proc_call()
 handle_default_unary_operator()
 handle_default_binary_operator()
  • htypcheck.pas

Procdefs returned from tcallcandidates (if requested in constructor) can return procdefs from default properties. To help facilitate this tprocoverloadentry class was added (see collect_overloads_in_struct).

The new overload for choose_best will return a tpropertysym (if the the best procdef belongs to a default property) so it can be handled properly.

  • pstatmnt.pas

Expressions of type torddef are converted to properties using handle_expr_default_property_access().

  • nutils.pas

Various helper functions for default properties:

 handle_read_property()
 handle_write_property()
 try_default_read_property()
  • nflw.pas

Uses try_default_read_property() to resolve default read properties for for..in enumerators. This is only possible for default properties that support the for..in loop, such as arrays, sets etc...