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.
  
• A simple virtual method table is generated for the management operators. Thanks to this it is possible to combine management operators with all 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.

Managements 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 after memory allocation for a record and called after the internal compiler call to recordrtti(data,typeinfo,@int_initialize); It allows automatic initialization for a record. A simple example is:

{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}{$macro on}

program TestInitialize;

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 and called before the internal call to recordrtti(data,typeinfo,@int_finalize); It is useful for automatic custom finalization code. A simple example looks like:

{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}{$macro on}

program testfinalize;

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 has been duplicated by copying the contents byte by byte and is called *after* FPC internal call recordrtti(data,typeinfo,@int_addref); By itself it does not any lifetime management, but you can use it to implement it. See also Copy. example:

{$if FPC_FULLVERSION < 30101}
{$ERROR this demo needs version 3.1.1}
{$endif}
{$mode delphi}{$macro on}

program testaddref;

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 simple example! There is a (complex) example in /tests/test/tmoperator8.pas

A Simple example of use

Simple resource handler

 unit UResourceHandlers;  

 {$if FPC_FULLVERSION < 30101}{$ERROR this demo needs version 3.1.1}{$endif}  
 {$mode delphi} 
 {$modeswitch advancedrecords}  

 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 or TCriticalSeccions i guess.