I just made a first attempt to use generics. Trying different things I figured out that the appended code compiles without any error message in FPC 2.6.0RC1
1. Is this behavior intended (generic objects)?
I didn't find anything like that in the docs
2. Replacing "object" at (1) with "class" gives an error when compiling
3. In my case the specialization should only work if _ResType is TType or derived, as certain fields and functions/procedures are read / executed
Is there currently a way to do this typesafe (my version is a hack, I believe) ?
If not, maybe this would be an interesting feature for FPC.
Maybe I'm trying to do the manager in a way, that isn't the right one - other suggestions are welcome - but as this is on a developer maillist (I know), my question mainly concerns the general generics part.
Thanks
A.v.P.
============================================================
program Test;
{$mode objfpc}{$H+}
uses Classes;
{$R *.res}
type
TType = object
RefCount : Word;
constructor Init;
destructor Done;
function GetCount : Word;
end;
// manages a list of _ResType-Objects
generic TManager<_ResType> = object //(1)
// _ResType must be TType or detrieved, howto define this??
type
PResType = ^_ResType;
public
constructor Create;
destructor Destroy;
function GetRefCount : Word;
// stuff to manage Resource-array
private
Resource : array of PResType; // array of pointer to specialized type
of generic
end;
TResManager = specialize TManager<TType>;
constructor TType.Init;
begin
RefCount := 3;
WriteLn('TType.Init done...');
end;
destructor TType.Done;
begin
WriteLn('TType.Done...');
end;
function TType.GetCount : Word;
begin
Result := Self.RefCount;
end;
constructor TManager.Create;
begin
SetLength(Resource, 2);
Resource[0]^.Init; // No problem here
WriteLn('Manager.Create done...');
end;
destructor TManager.Destroy;
begin
Resource[0]^.Done; // No problem here
WriteLn('Manager.Destroy done...');
end;
function TManager.GetRefCount : Word;
begin
Result := Resource[0]^.RefCount; // direct access
// or:
//Result := Resource[0]^.GetCount; // helper function
end;
Var
TT : TResManager;
begin
TT.Create;
WriteLn(TT.GetRefCount);
// if commented then 'no problem' else 'Access violation'
TT.Destroy;
end.============================================================
An unhandled exception occurred at $0000000000411D90 : EAccessViolation : Access violation
$0000000000411D90 line 66 of Test.lpr
$0000000000411DF6 line 76 of Test.lpr