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.
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 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.
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 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:
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, and is called *after* the internal compiler call to "recordrtti(data,typeinfo,@int_addref);"
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.