C++/WinRT implementation extension points: abi_guard, abi_enter, abi_exit, and final_release

Raymond Chen

C++/WinRT provides a few extension points for implementations to customize default behavior of inspectable objects.

When the last reference to an object is released, the object is destroyed. However, you may need to do some special cleanup while the object is still alive. The classic example of this is COM objects suffering double-destruction due to a temporary refcount, and the standard solution is to artificially bump the reference count during destruction.

The C++/WinRT library takes the standard solution and goes a step further: An implementation class can optionally implement a final_release method. If you provide such a method, then instead of destructing the object immediately upon the release of the final client reference, the C++/WinRT library calls your final_release method with the last remaining reference to the object, in the form of a unique_ptr. The object is still alive (it has not started destructing), so you can do normal things with it, like pass it to another method that may temporarily bump its reference count. You can even co_await in your final_release if you need to do some asynchronous work before letting the object finally disappear.

Normally, the object will destruct when the unique_ptr destructs, but you can hasten its death by calling unique_ptr.reset(), or you can postpone the inevitable by saving the unique_ptr somewhere. You can read Kenny Kerr’s discussion of final_release for more details.

The less commonly-used extension point is the abi_guard and its close friends abi_enter and abi_exit.

If your implementation defines a method named abi_enter, then it will be called at the entry to every projected interface method (not counting the methods of IInspectable). Similarly, if you define a method named abi_exit, it will be called at the exit from every such method, but will not be called if abi_enter throws an exception. (It will be called if an exception is thrown by the method itself.)

The calls to abi_enter and abi_exit are made with no parameters, and the return value is discarded.

You might use abi_enter to, say, throw an invalid_state_error exception if a client tries to use an object after it has been put into an unusable state, say, after a Shut­Down or Disconnect method. The C++/WinRT iterator classes use this feature to throw a invalid_state_error exception in the abi_enter method if the underlying collection has changed.

If the simple abi_enter and abi_exit methods aren’t fancy enough for you, you can define a nested class named abi_guard, in which case an instance of the abi_guard will be created on entry to each non-IInspectable projected interface method with a reference to the object as its constructor parameter. The abi_guard is destructed on exit from the method. You can put whatever extra state you like into the abi_guard class.

Basically, the deal is that the default abi_guard calls abi_enter at construction and calls abi_exit at destruction. And the default abi_enter methods do nothing. You can therefore plug in either at the abi_enter/abi_exit level, or at the abi_guard level.

Note that these guards are used only if you invoke the methods via the projected interface. If you invoke the methods directly on the implementation object, then those calls go straight to the implementation without any guards.

struct Thing : ThingT<Thing, IClosable>
  void abi_enter();
  void abi_exit();

  void Close();

void example1()
  auto thing = make<Thing>();
  thing.Close(); // calls abi_enter and abi_exit

void example2()
  auto thing = make_self<Thing>();
  thing->Close(); // does not call abi_enter or abi_exit

Note also that the guards are used only for the duration of the method call. If the method is a coroutine, the guard applies only until the IAsyncXxx is returned, not until the coroutine completes.

IAsyncAction CloseAsync()
  // guard is active here

  // guard becomes inactive once co_await starts,
  // at which point CloseAsync returns an IAsyncAction.
  co_await DoSomethingElseAsync();

  // guard is not active here

Guards are useful for specific cases like the “object that is no longer usable”, but their applicability in general is somewhat limited because they don’t know what method is being invoked. So you can’t do things like “If the object is not connected, then reject all method calls except for Connect.”



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