[fpc-pascal] Indexing into records using [] property

[Ryan Joseph]

I’d like to know if there is a solution to the problem of indexing into records using [] properties.

In the example below I have an array of records (a dynamic array for the example) and I’d like to use the [] property to index into the values. The problem is because the array holds records I need to return a pointer to the record so I can access the fields. In the end the only problem is I need to dereference the pointer using ^.

Are there any solutions we could propose to this? Some ideas:

1) I looks like we could have a “var” modifier for function results but that concept has only ever existed in function params (like & in C++).

function GetValue(i: integer): TVec2; var;

2) Maybe a modifier added to GetValue which tells the compiler to call the function (and result) like a pointer when the indexer is used?

====================================

type
TVec2 = record
x, y: integer;
end;
TVec2Ptr = ^TVec2;
TVec2Array = array of TVec2;

type
TVecArray = class
values: TVec2Array;
function GetValue(i: integer): TVec2Ptr;
procedure Resize(newLength: integer);
property Indexer[i: integer]: TVec2Ptr read GetValue; default;
end;


function TVecArray.GetValue(i: integer): TVec2Ptr;
begin
result := @values[i];
end;

procedure TVecArray.Resize(newLength: integer);
begin
SetLength(values, newLength);
end;

var
arr: TVecArray;
begin
arr := TVecArray.Create;
arr.Resize(1);
arr[0]^.x := 100;
arr[0]^.y := 100;
end.

Regards,
Ryan Joseph

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http://l

[Ben Grasset]

If the absence of dereferencing is the most important thing to you, you
could do something like this:

program Example;

{$mode ObjFPC}{$H+}
{$modeswitch AdvancedRecords}

type
TVec2 = record
X, Y: Integer;
end;

TVec2Array = array of TVec2;

TVecArray = record
strict private
Values: TVec2Array;
public
class function Create(const ACapacity: Integer = 16): TVecArray;
inline; static;
procedure Resize(const NewLength: Integer); inline;
property I: TVec2Array read Values write Values;
end;

class function TVecArray.Create(const ACapacity: Integer = 16): TVecArray;
begin
Result := Default(TVecArray);
SetLength(Result.Values, ACapacity);
end;

procedure TVecArray.Resize(const NewLength: Integer);
begin
SetLength(Values, NewLength);
end;

begin
with TVecArray.Create(0) do
begin
Resize(1);
I[0].X := 100;
I[0].Y := 100;
end;
end.

The only downside there is that you obviously don't quite get the real
"default" property syntax.

I’d like to know if there is a solution to the problem of indexing into
records using [] properties.
In the example below I have an array of records (a dynamic array for the
example) and I’d like to use the [] property to index into the values. The
problem is because the array holds records I need to return a pointer to
the record so I can access the fields. In the end the only problem is I
need to dereference the pointer using ^.
1) I looks like we could have a “var” modifier for function results but
that concept has only ever existed in function params (like & in C++).
function GetValue(i: integer): TVec2; var;
2) Maybe a modifier added to GetValue which tells the compiler to call the
function (and result) like a pointer when the indexer is used?
====================================
type
TVec2 = record
x, y: integer;
end;
TVec2Ptr = ^TVec2;
TVec2Array = array of TVec2;
type
TVecArray = class
values: TVec2Array;
function GetValue(i: integer): TVec2Ptr;
procedure Resize(newLength: integer);
property Indexer[i: integer]: TVec2Ptr read GetValue; default;
end;
function TVecArray.GetValue(i: integer): TVec2Ptr;
begin
end;
procedure TVecArray.Resize(newLength: integer);
begin
SetLength(values, newLength);
end;
var
arr: TVecArray;
begin
arr := TVecArray.Create;
arr.Resize(1);
arr[0]^.x := 100;
arr[0]^.y := 100;
end.
Regards,
Ryan Joseph
_______________________________________________
http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal