management operators
Management Operators
From Free Pascal version 3.1.1 onwards, there is a new language feature called management operators for advanced records.
The new operators are: Initialize, Finalize, AddRef and Copy.
These are a fairly unique feature, and are called "management operators" because:
- Each record, even non-managed or empty, with management operator becomes a managed type.
- They make it possible to implement new custom types with their own memory management, e.g: new string types, fast TValue implementations without hacks on the RTL, etc.
Management operators have no result type as opposed to normal operators, and operate via a very simple virtual method table.
Thanks to this, it is possible to combine management operators with all low-level RTL functions, such as InitializeArray / FinalizeArray / etc.
Management Operators can be used for many things:
- More granularly controlling the lifetimes of simple value types / primitives
- Implementing "nullable" value types
- Custom ARC implementations
- A very fast RTTI.TValue implementation
- As a replacement for manually-called Init/Done record methods like in mORMot for many types (for example SynCommons.TSynLocker).
- Auto init/finit for pointers/classes/simple types or anything else we have in Pascal.
- Much more
They work correctly in all possible ways with the RTL:
- New (Initialize).
- Dispose (Finalize).
- Initialize (Initialize).
- Finalize (Finalize).
- InitializeArray (Initialize).
- FinalizeArray (Finalize).
- SetLength (Initialize/Finalize).
- Copy (AddRef).
- RTTI.IsManaged.
Management operators are often called implicitly, for example:
- Global variables (Initialize/Finalize).
- Local variables (Initialize/Finalize).
- For fields inside records, objects or classes (Initialize/Finalize).
- Variable assignment (Copy).
- For parameters for routines - AddRef/Finalize/none - this depends on modifiers like var / constref / const.
Initialize
The initialize operator is called directly after stack memory allocation for a record happens.
It allows for custom automatic initialization code.
program TestInitialize;
{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}
type
PRec = ^TRec;
TRec = record
I: Integer;
class operator Initialize(var aRec: TRec);
end;
class operator TRec.Initialize(var aRec: TRec);
begin
aRec := Default(TRec); // initialize to default
end;
procedure PrintRec(r: PRec);
begin
WriteLn('Initialized TRec field i: ', r^.I = 0); // should always be zero, stack or heap
end;
var
a, b: PRec;
begin
New(a);
New(b); // standard "new" does not initialize, but now it does!
PrintTRec(a);
PrintTRec(b);
Dispose(a);
Dispose(b);
end.
Finalize
Finalize is called when a record goes out of scope.
It is useful for automatic custom finalization code.
program TestFinalize;
{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}
type
PRec = ^TRec;
TRec = record
I: Integer;
class operator finalize(var aRec: TRec);
end;
class operator TRec.finalize(var aRec: TRec);
begin
writeln('Just to let you know: I am finalizing..');
end;
var
a, b: PRec;
c: array of Trec;
begin
New(a);
New(b);
Dispose(a);
Dispose(b);
writeln('Just before program termination this will also be finalized');
Setlength(c, 4);
end.
AddRef
AddRef is called after the contents of a record have been duplicated by a bitwise copy (for example after, not during, an assigment.)
By itself it does not do any lifetime management, but you can use it to implement it. See also Copy.
program TestAddref;
{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}
uses
SysUtils;
type
PRec = ^TRec;
TRec = record
I: Integer;
class operator AddRef(var aRec: TRec): T;
end;
class operator TRec.Addref(var aRec: TRec): T;
begin
writeln('Just to let you know: maybe you can do lifetime management here..');
end;
var
a, b: array of Trec;
begin
setlength(a, 4);
b := copy(a);
end.
Copy
The Copy operator, if implemented, is called instead of the default copy behavior. This operator is responsible for copying everything that's needed from the source to the target.
todo: add a simple example!
There is a (complex) example in "/tests/test/tmoperator8.pas" within the FPC sources.
A simple example of using Initialize and Finalize together
unit UResourceHandlers;
{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}
interface
uses
Classes, SysUtils;
type
{ TObjectHandler }
TObjectHandler = record
obj: TObject;
class operator Initialize(var hdl: TObjectHandler);
class operator Finalize(var hdl: TObjectHandler);
end;
implementation
{ TObjectHandler }
class operator TObjectHandler.Initialize(var hdl: TObjectHandler);
begin
hdl.obj := nil;
end;
class operator TObjectHandler.finalize(var hdl: TObjectHandler);
begin
FreeAndNil(hdl.obj);
end;
end.
How to use it
procedure ExtractionResultTests.ObjectHandlerTest;
var
a: TRow;
ah: TObjectHandler;
begin
a := TRow.Create;
ah.obj := a;
end;
In this case the destructor of the TRow object is called when the handler goes out of scope. The same idea could be used for other resources like TMutex / TCriticalSection / anything else along those lines.