Implementing an O-O view of the stack code; C, C++, ...

[Andrew Cagney <cagney at gnu dot org>]

As a way of, perhaps, making the C++ or O-O discussion a little more 
concrete, I'd like to consider GDB's unwinder.  Here's a very abstract 
depiction of the current design:

                StackFactory
                     .
                     . (uses or creates)
                     .
                   Frame
                     |
                     + SignalFrame
                     + DummyFrame
                     + DwarfFrame
                     + ...

and here's a possible transformation that adds virtual frames:

                       Frame
                         |
      .------------------+--------------.
      |                                 |
 FrameDecorator                      AbiFrame
      |                                 |
 VirtualFrame                           +-- SignalFrame
                                        +-- DummyFrame
                                        +-- DwarfFrame

(Virtual inlined instances would refer to or "decorate" an ABI frame; 
and both virtual and abi frame chains would be available.  A 
VirtualFrame's is-signal method, for instance, could return the 
decorated AbiFrame's is-signal method.)

The implementation of this would involve both virtual methods and simple 
inheritance.

Perhaps the proponents of each language - C (?), C++ (Tom?) (even Ada? 
Robert?) - would be interested in posting stubbed code illustrating how 
this can be implemented so we can better consider the pros and cons.

As a starting point, here's some [buggy] pseudo code:

	class Frame
		abstract Frame getOuter();
		abstract boolean isSignal();
		...

	class FrameDecorator extends Frame
		private Frame decorated;
                FrameDecorator(Frame decorated) {
			this.decorated = decorated;
		}
		boolean isSignal() {
			return this.decorated.isSignal();

	class VirtualFrame extends FrameDecorator
		private Frame outer;
		Frame getOuter() {
			if (outer != null)
				return outer;
			else
				outerAbi = "decorated/abi".getOuter();
				... construct the inline frames ...
				return outerVirtual;

	class AbiFrame extends Frame
		private Frame outer;
		Frame getOuter() {
			if (outer == null)
				return outer;
			else
				... choose outer using sniffer ...
				return outer;
		}
	
	class SignalFrame extends AbiFrame
		boolean isSignal() {
			return true;
		}