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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
//
// File: COMDependentHandle.h
//
//
// FCall's for the DependentHandle class
//
#ifndef __COMDEPENDENTHANDLE_H__
#define __COMDEPENDENTHANDLE_H__
#include "fcall.h"
// A dependent handle is conceputally a tuple containing two object reference
//
// * A Primary object (think key)
// * A Secondary Object (think value)
//
// The reference to both the primary object is (long) weak (will not keep the object alive). However the
// reference to the secondary object is (long) weak if the primary object is dead, and strong if the primary
// object is alive. (Hence it is a 'Dependent' handle since the strength of the secondary reference depends
// on the primary).
//
// The effect of this semantics is that it seems that while the DependentHandle exists, the system behaves as
// if there was a normal strong reference from the primary object to the secondary one.
//
// The usefulness of a DependentHandle is to allow other objects to be 'attached' to a given object. By
// having a hash table where the entries are dependent handles you can attach arbitrary objects to another
// object.
//
// If you attmpted to do this with an ordinary table if the value would have to be a strong reference, which
// if it points back to the key, will form a loop that the GC will not be able to break DependentHandle is
// effecively a way of informing the GC about the dedendent relationsship between the key and the value so
// such cycles can be broken.
//
// Almost all the interesting for DependentHandle is in code:Ref_ScanDependentHandles
class DependentHandle
{
public:
static FCDECL3(VOID, nInitialize, Object *primary, Object *secondary, OBJECTHANDLE *outHandle);
static FCDECL2(VOID, nGetPrimary, OBJECTHANDLE handle, Object **outPrimary);
static FCDECL3(VOID, nGetPrimaryAndSecondary, OBJECTHANDLE handle, Object **outPrimary, Object **outSecondary);
static FCDECL1(VOID, nFree, OBJECTHANDLE handle);
static FCDECL2(VOID, nSetPrimary, OBJECTHANDLE handle, Object *primary);
static FCDECL2(VOID, nSetSecondary, OBJECTHANDLE handle, Object *secondary);
};
#endif
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