Generics proposals

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Already implemented!

Note-icon.png

Note: Generic classes are already implemented in the 2.3.1 version of FPC. See the FPC reference.

This page contains proposals and ideas, which have lead to the current implementation and for further features.


Why

  • Typesafety !
  • Speed
  • Readability

See "Already existing stuff" for possible but lacking ways right now.

Interested Parties

- Almindor - dannym - fpk - giantm - oliebol - plugwash - Synopsis - tom - PublicJoke - neli - dr_evil

Usage example (uncategorized)

TODO: combine the below stuff into one example using the <> syntax

(tom proposal)

  type
  // explicit addition of "generic" keyword. Maybe something like "uses generic <...>" 
  // better use the <>-brackets because otherwise it may cause confusion with arrays 
  // in the actual instantiation
  abc = class(..) generic <a, b>
  end;
  var
        //  because they're still unused ;)
        xyz : abc<String, Integer>;
        xyz2 : abc<AObj1, AObj2>;
  begin
    // don't repeat generics in instance creation (if not necessary)
    xyz := abc.Create;
    xyz2 := abc<descendant_of_Aobj1, descendant_of_Aobj2>.Create;
  end.

(dannym proposal new)

  type
  GList = generic class of T
  public
    procedure Add(const stuff: T);
  end;
 
  procedure GList.Add(const stuff: T); // note that here no 'of T'
  begin
    ...
  end;
 
  var
    intlist: GList of Integer;
  begin
    intlist := GList.Create; // note that here the detail type is missing and 
            // needs to be deducted from the var declaration
    //or intlist := typeof(intlist).Create;
    //intlist := GList of Integer.Create; <-- this is unclear if that works
  end;

(dannym proposal - old)

  type
  GList<T> = class
  public
    procedure Add(const stuff: T);
  end;
 
  procedure GList<T>.Add(const stuff: T);
  begin
    ...
  end;
 
  var
    intlist: GList<Integer>;
  begin
    intlist := GList<Integer>.Create;
  end;

plugwash:

  tmycollection=template(tobject)[x]
  tintegercollection=tmycollection[integer]

oliebol:

  template blaat<t:integer>(a:t);
  var x : t;
  begin
    do something with a, and use t with it
  end;

note that we should have a generics section (like interface and implementation sections) containing the generics:

  unit xyz;
  generics
  type
    GList......
  procedure GList.Add(const value: T);
  ....
  end.

(eiffel)

http://paste.lisp.org/display/6236

(PublicJoke proposal)

  unit Unit1(TMathType);
 
  interface
 
  type
    TMathClass = class
    public
      class function Add(a1, a2: TMathType): TMathType;
    end;
 
  implementation
 
  class function TMathClass.Add(a1, a2: TMathType): TMathType;
  begin
    Result:=a1+a2;
  end;
 
  end.
  unit Unit2;
 
  type
    TMathType = Longword;
 
  interface
 
  implementation
 
  end.
  program prog;
 
  uses
    Unit2, 
    Unit1(Byte) as ByteMath in 'Unit1.pas',
    Unit1(TMathType) as PolyMath in 'Unit1.pas';
 
  type
    TByteMathClass = ByteMath.TMathClass;
    TPolyMathClass = PolyMath.TMathClass;
 
  var
    b: Byte;
    p: TMathType;
 
  begin
    b:=TByteMathClass.Add(1, 2);
    p:=TPolyMathClass.Add(3, 4);
  end.

(Gadnio proposal) I personally think the C++ way of doing this is clear enough AND will be the easiest to parse/understand:

  template <T>
    TGenericObject< T > = record
      Data : T;
      OtherStuff : Pointer;
    end;{TGenericObject}
  template <TKey, TData = TGenericObject< boolean > >
    TGenericList = class( TWhee< TKey > )
    public
      function Lookup( const Key : TKey ): TData;
      template <THihi>
        class function DataToHihi( Data : TData ): THihi;
    end;{TGenericList}

Also, note that template specialization is a very, very important thing (for me especially). In C++ they've got vector< bool > that behaves different than the standard vector< T >. An example:

  template <T, TT>
    TSame = class
      class function Same: Boolean;
    end;{TSame}
  template <T>
    TSame = class
      class function Same: Boolean;
    end;{TSame}
  template <T, TT>
  class function TSame.Same: Boolean;
  begin
    Result := false;
  end;{TSame<T, TT>.Same}
  template <T>
  class function TSame.Same: Boolean;
  begin
    Result := true;
  end;{TSame<T>.Same}

This allows to make a compile-time check for same types that are not TObject, e.g.

  TFoo = class;
  TBar = class( TFoo );
  var
    S : Boolean;
  begin
    S := TSame< String, Integer >.Same; //s = false
    S := TSame< Int64, Integer >.Same; //s = false
    S := TSame< String, String >.Same; //s = true
    S := TSame< TBar, TFoo >.Same;//s = false (not the same as "var x: TFoo; begin x := TBar.Create; end;" )
  end;

Another thing to note: It would be very good if something like this (c++ syntax) would be implemented:

  template <bool b>
  class h
  {
    public:
    static const int i = 0;
  };//h
  template <>
  class h< true >
  {
    public:
    static const int i = 1;
  };
  template <>
  class h< false >
  {
    public:
    static const int i = 2;
  };

Here

  h<false>.i = 2
  h<true>.i = 1

ALWAYS.

Usage example

If you add more syntax examples, please add equivalent to both. Two main syntaxes seem to be popular, "<>" which looks more or less like C++, c#, etc.. and adding a "generic" keyword which is more along the Pascal style of code. Go to Generics Vote to vote for one or the other syntax.

Using brackets: <>

  type
    TGenericCollection<T: TCollectionItem> = class(TComponent)
    ...implement TCollection and use T
    end;
 
    TCollection = TGenericCollection<TCollectionItem>;
    TFieldDefs = TGenericCollection<TFieldDef>; 
 
    TGenericList<T: PtrInt> = class(TObject)
    ...implement TList and use PtrInt size for code generation
    end;
 
    TList = TGenericList<Pointer>;

Implementation of TGenericCollection can be compiled as if (T: TCollectionItem) were used.

Would this solve the circular dependency ? It seems so to me, because one knows at least as much as in current implementation of these classes, but I'm no compiler engineer.

For procedures:

  function MyFunc<T: TTreeItem>(A: T): T;
  begin
   // do something with the tree item
  end;  
 
  function Max<T>(A, B: T): T;
  begin
    if A < B then
      Result := B
    else
      Result := A;
  end;

My restrictions won't allow implementing generic Max and Min, I guess. That really needs macro-alike stuff (for the compiler implementation).

Another example for linked lists: (ripped from mail to fpc-devel of dannym, but modified a bit)

  type
    TListItem<T> = record
      Data: T;
      Next: TListItem;
    end;
    PListItem = ^TListItem;
 
    TList<T> = class
    private
      fHead: PListItem<T>;
      fTail: PListItem<T>;
    published
      procedure Add(Item: T);
    end;
 
  procedure TList<T>.Add(Item: T);
  var
    node: PListItem<T>;
  begin
    New(node);
    node^.Data := Item;
    node^.Next := nil;
 
    if Assigned(fTail) then begin
      fTail^.Next := node;
    end else begin
      fHead := node;
    end;
 
    fTail := node;
  end;
 
  type
    TApple = class
    end;
    TOrange = class
    end;
 
    TAppleList = TList<TApple>;
 
  var
    apples: TAppleList;
    apple: TApple;
  begin
    apples.Add(TApple.Create); // works
    apples.Add(TOrange.Create); // compile error
 
    apple := apples[0]; // works
    apple := apples[1]; // not applicable
    apple := apples[0] as TApple; // works, but unneccessary
    apple := apples[1] as TApple; // not applicable
  end;

Using the "generic" keyword

  type
    TGenericCollection = generic(T: TCollectionItem) class(TComponent)
    ...implement TCollection and use T
    end;
 
    TCollection = specialize TGenericCollection(TCollectionItem);
    TFieldDefs = specialize TGenericCollection(TFieldDef); 
 
    TGenericList = generic(T: PtrInt) class(TObject)
    ...implement TList and use PtrInt size for code generation
    end;
 
    TList = specialize TGenericList(Pointer);

For procedures:

  function generic(T: TTreeItem) MyFunc(A: T): T;
  begin
   // do something with the tree item
  end;  
 
  function generic(T) Max(A, B: T): T;
  begin
    if A < B then
      Result := B
    else
      Result := A;
  end;

Another example for linked lists: (ripped from mail to fpc-devel of dannym, but modified a bit)

  type
    TListItem = generic(T) record
      Data: T;
      Next: TListItem;
    end;
    PListItem = ^TListItem;
 
    TList = generic(T) class
    private
      fHead: specialize PListItem(T);
      fTail: specialize PListItem(T);
    published
      procedure Add(Item: T);
    end;
 
  procedure generic(T) TList.Add(Item: T);
  var
    node: specialize PListItem(T);
  begin
    New(node);
    node^.Data := Item;
    node^.Next := nil;
 
    if Assigned(fTail) then begin
      fTail^.Next := node;
    end else begin
      fHead := node;
    end;
 
    fTail := node;
  end;
 
  type
    TApple = class
    end;
    TOrange = class
    end;
 
    TAppleList = specialize TList(TApple);
 
  var
    apples: TAppleList;
    apple: TApple;
  begin
    apples.Add(TApple.Create); // works
    apples.Add(TOrange.Create); // compile error
 
    apple := apples[0]; // works
    apple := apples[1]; // not applicable
    apple := apples[0] as TApple; // works, but unneccessary
    apple := apples[1] as TApple; // not applicable
  end;

Ugly things in need to be resolved:

  { a generic type "TPointer" which is a pointer to type instantiated with }
  type TPointer = generic(Type) ^Type;
  { a generic type that instantiates all the enums for Type }
  type TAllEnums = generic(Type)(low(Type),high(Type));

So, before a keyword can be real "nice" the following things need addressing:

  1. Double brackets ()(), looks C-ish again
  2. In math, one usually defines functions with the parameters on the left of the equal sign, not on the right

Suggestion 2

Inspired by idea from oliebol in combination with ugly examples, in list of const, var, type sections, add "template" section:

  const
    ThisIsAConst = 1;
 
  type
    TMyEnum = (ab, ac);
 
  template(T: TCollectionItem)
    TGenericCollection = class(TComponent)
    ...implement TCollection and use T
    end;
 
  type
    TCollection = specialize TGenericCollection(TCollectionItem);
    TFieldDefs = specialize TGenericCollection(TFieldDef); 
 
  template(T: PtrInt)
    TGenericList = class(TObject)
    ...implement TList and use PtrInt size for code generation
    end;
 
  type
    TList = specialize TGenericList(Pointer);

For procedures:

  function generic(T: TTreeItem) MyFunc(A: T): T;
  begin
   // do something with the tree item
  end;  
 
  function generic(T) Max(A, B: T): T;
  begin
    if A < B then
      Result := B
    else
      Result := A;
  end;

Another example for linked lists: (ripped from mail to fpc-devel of dannym, but modified a bit)

  template(T)
    TListItem = record
      Data: T;
      Next: TListItem;
    end;
    PListItem = ^specialize TListItem(T);
 
    TList = class
    type
      PItem = specialize PListItem(T);
    private
      fHead: PItem;
      fTail: PItem;
    published
      procedure Add(Item: T);
    end;
 
  ...implementation...
 
  procedure generic(T) TList.Add(Item: T);
  var
    node: PItem;
  begin
    New(node);
    node^.Data := Item;
    node^.Next := nil;
 
    if Assigned(fTail) then begin
      fTail^.Next := node;
    end else begin
      fHead := node;
    end;
 
    fTail := node;
  end;
 
  type
    TApple = class
    end;
    TOrange = class
    end;
 
    TAppleList = specialize TList(TApple);
 
  var
    apples: TAppleList;
    apple: TApple;
  begin
    apples.Add(TApple.Create); // works
    apples.Add(TOrange.Create); // compile error
 
    apple := apples[0]; // works
    apple := apples[1]; // not applicable
    apple := apples[0] as TApple; // works, but unneccessary
    apple := apples[1] as TApple; // not applicable
  end;

Suggestion 3

  { Chu Jetcheng, 2008-07-24 }
 
  program Generics;
 
  type
    { The compiler should maintain a global table storing each specialized type for each generic.  The 
      `of' operator is used to create a specialization for a specified generic type.  Specialized types 
      of the same `type layout' (no other words) point to the same entry in the table of specializations
      of the specialized generic type, and thus are compatible.  Generic objects and records should also 
      be allowed. }
 
    TGeneric = generic class(TAncestorClass) of T1, T2
    private
      FData1: T1;
      FData2: T2;
    public
      property Data1: T1 read FData1 write FData1;
      property Data2: T2 read FData2 write FData2;
      constructor Create(const A: T1; const B: T2);
      procedure WriteData; virtual;
    end;
 
    TGenericDescendent = generic class(TGeneric of Integer, T3) of T3
    private
      FData3: T3;
      FObjectOfSameType: TGenericDescendent of T3;
    public
      property Data3: T3 read FData3 write FData3;
      property ObjectOfSameType: TGenericDescendent of T3 read FObjectOfSameType write FObjectOfSameType;
      constructor Create(const C: T3);
      procedure WriteData; override;
    end;
 
    TGenericIR = TGeneric of Integer, Real;
    TGenericIRClass = class of TGenericIR;
 
    TGenericISClass = class of (TGeneric of Integer, string);
 
    PGenericObj = generic ^TGenericObj; 
    TGenericObj = generic object of T 
      MyField: T;
      procedure WriteData;
    end;
 
 
  { TGeneric }
 
  { Use the same syntax for method implementations as with immediate classes. }
 
  constructor TGeneric.Create(const A: T1; const B: T2);
  begin
    FData1 := A;
    FData2 := B;
  end;
 
  procedure TGeneric.WriteData;
  begin
    Writeln(FData1);
    Writeln(FData2);
  end;
 
  { TGenericDescendent }
 
  constructor TGenericDescendent.Create(const C: T3);
  begin
    inherited Create(16, C);
    FData3 := C;
  end;
 
  procedure TGenericDescendent.WriteData;
  begin
    inherited;
    Writeln(FData3);
    if Assigned(FObjectOfSameType) then FObjectOfSameType.WriteData;
  end;
 
  { TGenericObj }
 
  procedure TGenericObj.WriteData;
  begin
    Writeln(MyField);
  end;
 
 
  var 
    Obj1: TGeneric of Integer, Real;
    Obj2, Obj3: TGenericDescendent of string;
    OldStyleObj: TGenericObj of Integer;
    OldStyleObjPtr: PGenericObj of Integer;
 
  begin
    Obj1 := TGeneric.Create(32, 3.14);
    Obj1.WriteData;
    Writeln(#9, Obj1 is TGenericIR);
    Writeln(#9, Obj1 is TGenericIS);
 
    Obj2 := TGenericDescendent.Create('Generic test.');
    Obj2.WriteData;
    Writeln(#9, Obj2 is TGenericIR);
    Writeln(#9, Obj2 is TGenericIS);
 
    Obj3 := TGenericDescendent.Create('Another generic test.');
    Obj3.ObjectOfSameType := Obj2;
    Obj3.WriteData;
 
    Obj3.Free;
    Obj2.Free;
    Obj1.Free;
 
    OldStyleObj.MyField := 100;
    OldStyleObj.WriteData;
 
    OldStyleObjPtr := New(PGenericObj of Integer);
    with OldStyleObjPtr^ do begin
      MyField := 202;
      WriteData;
    end;
    Dispose(ObjStyleObjPtr);
  end.

Terms

type user:

  • the function in the compiler that defines a variable or return type and thus 'uses a (already defined) type'.

immediate type:

  • not generic and not specialized, i.e. 'normal type'

generic type:

  • template for a class with some unspecified types, never to be filled in into this class type. Only by specialisation.

specialization:

  • copy generic type class to new specialized type class

type parameters:

  • T and Q, the unknown types for the generic TSomeGeneric<T,Q>

specialized type:

  • a generic type with all type parameters know, written into a real class type (tree) and reused if possible

Changes

Changes in class/record definition representation

Each class definition representation has added fields for:

 - class instantiation mode
    0  immediate type
    1  generic type, no instantiation, just generate specialized type
    2  specialized type
 - when mode 2:
     generic type uplink (XXXX, what, unitname, typename?)
   when mode 1:
     list of specialized types known so far (and where),
     XXXX list items
   when mode 0: 
     nothing

Changes in 'type user' compilation (for class/record types only)

Each class type user will have to check mode of the class type.

For the used class type, If mode is immediate

 - proceed as always till now.

If mode is specialized

 - proceed as always till now 
 - keep in mind the generic type for some checks later (and debugging).

If mode is generic, this:

 - check 'list of specialized types' for the type parameter to use.
   If there is already a specialized type, use that.
   (given that it is compatible with 'compile parameters' XXXX)
 - If not available, clone the generic type in the tree, and fill in the
   type params with the actual types to use.
   Generate machine code as normal.
   Remember the new specialized type for later in the list by the 
   generic type and type params used.

(the generic type is best written to some ppu as parse tree so that it can be refetched for cloning somewhen. To keep it easy, initially it should be limited to having its .pas file around when wanting to use a generic)

type params

Generics store a list of the type params (names).

 ('T', 'Q')

And Specializations store a list of type mappings from the real types to the type params, in the way of

  type
    T = Integer;
    Q = Boolean;

of course these are local to this class/record specialization.

Other things to note (for *later*)

What if a method implementation depends on the type used to know how the implementation should look like ?

  procedure GList<T is TObject>.Add(const value: T);
  procedure GList<T is Integer>.Add(const value: T);

?

or more cryptic like

  procedure GList<TObject>.Add(const value: T);
  procedure GList<Integer>.Add(const value: T);

?

or like

  procedure GList<T>.Add(const value: Integer);
  procedure GList<T>.Add(const value: TObject);

the latter would need extra compiler support but would be nicer, overall. Potentially clashing with normal overloads though.

How to do internal storage classes

  type
  PNode<T> = ^TNode<T>;
  TNode<T> = record
    prev,next: PNode<T>;
    data: T;
  end;
 
  type
  GList<T> = class
  private
    head, tail: PNode<T>;
    cnt: Integer;
  public
    procedure Add(const stuff: T);
  end;

?

This implies that generics are best implemented both for records and for classes alike! Also implies that pointer types need to support generic too.

Also one specialization should chain other specializations (probably also when deriving from a generic).

How to derive from the generic type

It would be good if generic types could be derived from.

  type
  TGenericEventyList<T> = class(TGenericList<T>)
  public
    procedure Add(value: T); override;
  end;
  ...

Interfaces should support generics too

  type
  IBa<T> = interface
    procedure Add(value: T);
  end;
  TBa<T> = class(...,IBla<T>,IInterface)
  end;

(what to do about the guids of the specializations? probably just generate whatever, they dont have any real meaning after all)

Of course this has limitations to check, like

  • the generic interface cannot be used, just its specializations
  • the specialized interface cannot be used in a generic class interface list
  • (the generic interface can be used in a generic class interface list)
  • the generic interface itself is invalid to use anywhere (just as generic classes are) except for specializing and deriving from
  • normal interfaces cannot derive from generic interfaces
  • (generic interfaces can derive from generic interfaces) - given the type params are the same
  • specialized interfaces cannot be derived from

Limitation of possible specialisations

  type
  TGenericList<T: TObject or Integer> = ...
  end;

Probably useful...

What to do with class functions

I dont know a whole lot of how class functions are stored internally. What happens to class functions in generics?

'class of'

<Almindor> don't forget class of, we need class of compatibility for dynamic class factories

<Almindor> There are possible problems with 'class of'

Implementation details

Sanity checks

  • Generic types cannot be instantiated from
  • (Generic types can be derived from)
  • Specialized types cannot be derived from
  • Normal types cannot use generic types in its definition
  • (Normal types can use specialized types in its definition)
  • There are no half specialized types (one type param specified, the other not)
  • prevent loops while 'unpacking' specializations (loop counter field in every specialization)

Things to note and not forgot

  • A class and record generic can use itself as type within its definition.

example:

  type
  TBla<T> = class
    parent: TBla<T>;
  end;
  • A class and record generic can use other generics as types within its definition.
  type
  TBla<T> = class
    parent: TBlo<T>;
  end;

Implementation steps

Extend parser to support '<T>' and '<T: bla or bla or bla>' and '<T,Q>' notation

Easy to do. Done.

Different ways to do it, one excluding the other

Way 1

Change class/record/object/pointertype/interface parser to mark if it is a generic, a specialization or normal

Mediate to do. Have done it for class/object and pointertype. Needs all kinds of clone functions for the defs and syms. fpk says he has patches. waiting.

insert dummy symbols (typesym + typedef) for the type params in the generic

Cannot be inserted into the object because parsing of types within objects has been disabled to make that possible:

  type
  Row = Cardinal;
  TBla = class 
    Row: Integer;
  end;

weird.. (function searchsym_type, is not possible to have type definitions in records, objects, parameters)

parse implementation section methods *for the generic*

Is harder than it sounds, because the class/object the method is member of has to be searched for the type symbol for the type params (and these *not* derefed but just used as is).

when parsing generics, use a dummy type in place of the type params

For that, defer the type checking of the compiler in that case to 'as late as possible' or 'never for the generic' <--- TODO... hard...

Change derivation handlers to do the sanity checks

Should pose no problems other than finding all the places.

Add error messages for it

Change pexpr (tnode creating)

<Synopsis+> see the do_resulttypepass() calls in pexpr.pas But the compiler needs to known for example if it allows a . after the type

  • add a new tnode type for postfix operators (instead of resolving the operators into the fields/methods they reference) and defer type checking until later (just before binary generation - which is not done for the generic type).

Examples for postfix operators are:

  • '[]' array member access (normal array, class default array property?)
  • '.' record/class member method/variable access
  • '^' pointer dereference
  • (automatic pointer dereference)

<Synopsis> grep for m_auto and you'll find the places where it is used <Synopsis> only in pexpr.pas

Change ppu streaming routines to save the tree of generics to the ppu

Change type user to instantiate specialized types for generics when used

Somewhat hard to do. (main work here :))

For the instantiation

  • there is a dummy type sym needed (ok),
  • a new objectdef
  • clone of all the syms and defs (at least proc and property) of the original class/object
  • and replacing all the type params by the correct types.
  • Store that into the module as def (not on disk).
  • (<Synopsis+> an additional symtable shall be added to tmodule: tempsymtable; all temporary tdef's shall be stored in that symtable)
  • Then resume compiling for the class/object.

Way 2

just keep the source code of the generic around and do blind replacing of the type params and then compile a specialization from that as 'source'

Notes

<Synopsis+> but the code to generate the specialization at runtime can be generated at the time the specialization is created. It doesn't need to be stored in ppu. all information is available to generate the tree at that time.

<Synopsis+> In the 2.1.x development series a rewrite of the ppu loading is planned to make it more clean and more maintainable. Currently it is risky to fix bugs without introducing new bugs

<plugwash-> It needs to be stored in some form so that the specialisations can be generated from it. <Synopsis+> but you can't store them in the ppu. Or with the same limitations as inlining. No references to the implementation symtable.

fpk has patches to make cloning defs possible.

01:28:24 <Synopsis+> users will complain; things like:

  interface
    generictype declar
  implementation
    procedure helper;
    begin
    end
    constructor generictype.create
    begin
      helper
    end
  end.

Will not be allowed.

Another problem:

  unit a;
  interface
    generictypeA
  implementation
  uses B
  end.
 
  unit b;
  interface
  uses a
  implemenation
  begin
    generictypeA<integer>.craete
  end.

At the time that the implementation of B is parsed it needs already the code of generictypeA. but that is not yet parsed! The compiler doesn't know anything yet. It only knows that generictypeA exists.

<fpk-> dannym, Synopsis: we can simply forbid such code in generics which cause the problem

<fpk-> i.e. everything used by generics must be already included in the interface uses clause ... or forbid references to units used only in the implementation section :)

<Synopsis+> fpk had a good idea that can solve some of the issues: unit bla; interface ... generics ... implementation ... initialisation ... finalizatrion end.

<Synopsis+> The generics delcarations shall be put in a separate section before implementation is parsed

<dannym> at 'procedure TTest.Add(x: T);', T needs to be known. but it doesnt seem that the object symtable is searched for the class method parameters. correct ?

<Synopsis+> dannym: correct, object symtables can not contain types, because that created problems. there is special code for that: remove the objectsymtable before parsing type declarations.

  { for records, don't search the recordsymtable for the symbols of the types }
  oldsymtablestack:=symtablestack;
  symtablestack:=symtablestack.next;

Because you can have a field with the same name as a type

  type
  Wrd = Word;
  R = Record
    Wrd : Wrd;
  end

this is allowed. To support this the record or object symtable is remvoed from the symtablestack when the type is parsed.

Already existing stuff

metaclasses

  type
  TObjectClass = class of TObject;
  TTypedList = class
  private
    Fwanttype: TObjectClass;
    ...
  public
    constructor Create(itemtype: TObjectClass);
    procedure Add(item: TObject);
    ...
  end;
  constructor TTypedList.Create(itemtype: TObjectClass);
  begin
    Fwanttype := itemtype;
    ...
  end;
  procedure TTypedList.Add(item: TObject);
  begin
    assert(item.Class = Fwanttype);
    ...
  end;
 
  ...
  TSomeStuff = class
  end;
  ...
  Flist := TTypedList.Create(TSomeStuff);

Advantages

  • already works

Disadvantages

  • slow at runtime
  • cannot do simple types
  • need to cast all the time even for the supported types (i.e. when reading). eeek.

To be incorporated here

Chat logs

<Synopsis> http://www.eleforum.com/cgi-bin/eleweb_lift?action=3&script=wall&num=18&reversesort=true&type=today&starttime=00:00&endtime=23:59&startdate=01-02-05&enddate=01-02-05 <Synopsis> or <Synopsis> http://neli.hopto.org:3980/~micha/irc-fpc/2005/01/%23fpc.20050102.log

See also