One-liner RAII in Delphi

1
Barry Kelly Barry Kelly 11 years ago in anonymous methods, Delphi, RAII 0

Some C++ aficionados point at the ability, in C++, to create an object such that the creation allocates some kind of resource, and the destruction frees the resource, with much glee. The idiom is RAII: Resource Acquisition Is Initialization. The technique can be a useful, albeit somewhat opaque, way of acquiring a resource for the duration of a block of code and relinquishing the resource upon exiting the block. It's C++'s equivalent of Delphi / Java / C# / SEH's try / finally blocks (though of course C#'s "using" statement is an even closer analogue).

Delphi has a similar mechanism in the form of deterministic reference counting for interfaces. This can be (ab)used to functionally implement almost exactly the same technique as C++. The degree to which it is useful, however, is moderated by readability issues; RAII doesn't necessarily employ a visible nesting, and rather depends on the implied nesting of the scope in which it's used.

Anyway, here's a way of implementing RAII in Delphi, for people to whom the technique may not have occurred yet. It relies on defining a general-purpose class which implements an interface, and whose death will be triggered by scope exit:

type
  TScopeExitNotifier = class(TInterfacedObject)
  private
    FProc: TProc;
  public
    constructor Create(const AProc: TProc);
    destructor Destroy; override;
  end;

constructor TScopeExitNotifier.Create(const AProc: TProc);
begin
  FProc := AProc;
end;

destructor TScopeExitNotifier.Destroy;
begin
  if Assigned(FProc) then
    FProc;
  inherited;
end;

function MakeScopeExitNotifier(const AProc: TProc): IInterface;
begin
  Result := TScopeExitNotifier.Create(AProc);
end;

I would generally recommend hiding the actual class in the implementation-section of a unit - the functional interface of MakeScopeExitNotifier ought to be sufficient. Using this function, we can now implement a one-liner RAII equivalent of the following code:

  Writeln('Disable controls on ', {ASurface});
  try
    // (user code)
  finally
    Writeln('Enable controls on ', {ASurface});
  end;

Of course, I'm using strings etc. for simplicity, so that the demo translation is compilable. Here is the definition of the function which will be called to form the one-liner:

function DisableEnableControls(const ASurface: string): IInterface;
begin
  Writeln('Disable controls on ', ASurface);
  Result := MakeScopeExitNotifier(procedure
  begin
    Writeln('Enable controls on ', ASurface);
  end);
end;

And here it is in action:

procedure P;
begin
  DisableEnableControls('Foo');
  DisableEnableControls('Bar');
  Writeln('Controls on Foo and Bar are disabled now');
end;

Because of the way Delphi's deterministic reference counting and destruction works, Foo and Bar will be released in reverse order of construction, as expected for RAII. Functions that return an interface value in Delphi are implemented behind the scenes as procedures that take an out Result parameter; these are in effect hidden variables that keep the return values alive until the scope is exited.

The output of the program is as follows:

Disable controls on Foo
Disable controls on Bar
Controls on Foo and Bar are disabled now
Enable controls on Bar
Enable controls on Foo

Of course, it's not necessarily clear what DisableEnableControls(string) would do, and how it protects the scope. That's why I wouldn't necessarily recommend using this technique, except to counter C++-ite arguments. Using anonymous methods directly would seem to be a more readable approach in general, something along these lines:

procedure DisableEnableControls(const ASurface: string; const ACB: TProc);
begin
  Writeln('Disable controls on ', ASurface);
  try
    if Assigned(ACB) then
      ACB;
  finally
    Writeln('Enable controls on ', ASurface);
  end;
end;

procedure P;
begin
  DisableEnableControls('Foo', procedure
  begin
    DisableEnableControls('Bar', procedure
    begin
      Writeln('Controls are disabled now.');
    end);
  end);
end;

Output is the same as earlier. The extra complexity / verbosity of this abstraction over and above a literal try/finally would of course be more justified if the logic in the place of Enable and Disable sections were more involved, such as possibly setting up and rolling back transactions, etc.

However, the functionality of MakeScopeExitNotifier() routine is actually useful in the context of anonymous methods, because by capturing a variable containing the return value, you can get informed as to when the scope that created your anonymous method dies. This can be useful to e.g. free any objects that were allocated when the anonymous method was created:

function FreeOnExit(const Args: array of TObject): IInterface;
var
  toFree: TArray<TObject>;
begin
  // copy open array to toFree because open arrays can't be captured
  SetLength(toFree, Length(Args));
  Move(Args[0], toFree[0], Length(Args) * SizeOf(Args[0]));
  
  Result := MakeScopeExitNotifier(procedure
  var
    i: Integer;
  begin
    Writeln('FreeOnExit is freeing objects');
    for i := 0 to High(toFree) do
      toFree[i].Free;
  end);
end;

procedure Use(const x: IInterface);
begin
end;

function Q: TProc;
var
  onDie: IInterface;
  obj: TObject;
begin
  obj := nil;
  onDie := FreeOnExit([obj]);
  Writeln('Allocating an object');
  obj := TObject.Create;
  Result := procedure
  begin
    Use(onDie); // cause onDie to be captured
    Writeln(Format(
      'Inside callback, obj still alive: %p: %s',
      [Pointer(obj), obj.ToString]));
  end;
end;

procedure P;
var
  r: TProc;
begin
  Writeln('Before Q');
  r := Q();
  Writeln('Invoking result of Q');
  r;
  Writeln('Clearing result of Q');
  r := nil;
  Writeln('Result is cleared');
end;

The output is something like:

Before Q
Allocating an object
Invoking result of Q
Inside callback, obj still alive: 00970D20: TObject
Clearing result of Q
FreeOnExit is freeing objects
Result is cleared

This technique, capturing a variable of an interface or other deterministically lifetime-managed type, is essential for flexibility in cleaning up any resources that an anonymous method needs to for its lifetime. Note, however, that there can be a gotcha: the following alternative, using MakeScopeExitNotifier directly, will not work:

// <BROKEN CODE DO NOT USE>
function Q: TProc;
var
  onDie: IInterface;
begin
  onDie := MakeScopeExitNotifier(procedure
  begin
    Writeln('died!');
  end);
  Result := procedure
  begin
    Use(onDie);
  end;
end;
// </BROKEN CODE DO NOT USE>

The reason it's broken is that the anonymous method that's handed off to MakeScopeExitNotifier will itself keep the scope alive. So in order to make use of MakeScopeExitNotifier for this particular purpose, the anonymous method passed to it needs to be in a different scope.