default properties
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
- records, objects, classes.
- arithmetic, compare, binary, unary, in overloads. (https://www.freepascal.org/docs-html/ref/refch15.html)
- visibility sections.
- array indexing with []
- if, while, repeat, case, for..do statements
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...
