学步园

Strongly-typed smart pointers are now possible in Delphi, leveraging the work on generics. Here's a simple smart pointer type:

  TSmartPointer<T: class> = record
  strict private
    FValue: T;
    FLifetime: IInterface;
  public
    constructor Create(const AValue: T); overload;
    class operator Implicit(const AValue: T): TSmartPointer<T>;
    property Value: T read FValue;
  end;

Here it is in action, where TLifetimeWatcher is a little class that executes some code when it dies:

procedure UseIt;
var
  x: TSmartPointer<TLifetimeWatcher>;
begin
  x := TLifetimeWatcher.Create(procedure
  begin
    Writeln('I died.');
  end);
end;

Here's the full project code that defines TSmartPointer<>, TLifetimeWatcher, and the above test routine:

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  TLifetimeWatcher = class(TInterfacedObject)
  private
    FWhenDone: TProc;
  public
    constructor Create(const AWhenDone: TProc);
    destructor Destroy; override;
  end;

{ TLifetimeWatcher }

constructor TLifetimeWatcher.Create(const AWhenDone: TProc);
begin
  FWhenDone := AWhenDone;
end;

destructor TLifetimeWatcher.Destroy;
begin
  if Assigned(FWhenDone) then
    FWhenDone;
  inherited;
end;

type
  TSmartPointer<T: class> = record
  strict private
    FValue: T;
    FLifetime: IInterface;
  public
    constructor Create(const AValue: T); overload;
    class operator Implicit(const AValue: T): TSmartPointer<T>;
    property Value: T read FValue;
  end;

{ TSmartPointer<T> }

constructor TSmartPointer<T>.Create(const AValue: T);
begin
  FValue := AValue;
  FLifetime := TLifetimeWatcher.Create(procedure
  begin
    AValue.Free;
  end);
end;

class operator TSmartPointer<T>.Implicit(const AValue: T): TSmartPointer<T>;
begin
  Result := TSmartPointer<T>.Create(AValue);
end;

procedure UseIt;
var
  x: TSmartPointer<TLifetimeWatcher>;
begin
  x := TLifetimeWatcher.Create(procedure
  begin
    Writeln('I died.');
  end);
end;

begin
  try
    UseIt;
    Readln;
  except
    on E:Exception do
      Writeln(E.Classname, ': ', E.Message);
  end;
end.

 

Update: Fixed capture - was capturing a location in the structure, which of course will be freely copied etc.

 

原帖地址： http://barrkel.blogspot.com/2008/09/smart-pointers-in-delphi.html