The most elegant way to encapsulate WinAPI callbacks inside a class

Question

Hello.

I am thinking about elegant way to encapsulate WinAPI callbacks inside a class. Suppose I am making a class handling asynchronous I/O. All Windows callbacks should be stdcall functions, not class methods (I need to pass their addresses to ReadFileEx WinAPI function for example). So, I cannot just pass method addresses as a callback routines to WinAPI functions.

What is the most elegant way to encapsulate functionality of this type inside a class so that the class have events OnReadCompleted and OnWriteCompleted (I am using Delphi as a primary language, but I guess the situation must be the same in C++ because class methods are different from simple methods by the fact, that the first hidden parameter of them is this link. Of course this class is not a singleton and there can be many of them created by app at the same time.

What do you think would be the good way to implement this?

Answer 1

I doubt this is in any way elegant but, IMO, the easiest is to convert the address of a method of a class to a procedure address and pass it to the winapi. Sure, it's a hack, but the VCL does the very same with classes.MakeObjectInstance, if only for a specific construct.. See this question for a source for this kind of implementaion and some other, more OO ways to handle the situation.

Answer 2

You can use static keyword for that. But it's available only in new Delphi versions.

Like this:

type
  TMyThread = class
  private
    // ...

    class function ThreadProc(Param: Pointer): DWord; stdcall; static; // <- WinAPI call back

    function Execute: DWord; // <- actual callback
  public
    constructor Create;
    // ...
  end;

{ TMyThread }

constructor TMyThread.Create;
begin
  // ...
  FHandle := CreateThread(nil, 0, @ThreadProc, Self, 0, FID);
end;

class function TMyThread.ThreadProc(Param: Pointer): DWord;
begin
  Result := TMyThread(Param).Execute;
end;

function TMyThread.Execute: DWord;
begin
  MessageBox(0, 'Hello from thread', 'Information', MB_OK or MB_ICONINFORMATION);
  Result := 0;
end;

Here: ThreadProc is WinAPI callback routine. It requires to have some form of custom argument, where you can pass Self. It can not access instance members. That's why it's just a wrapper for real callback (Execute), which is part of class and can access its fields and methods.