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/**
* @file llhandle.h
* @brief "Handle" to an object (usually a floater) whose lifetime you don't
* control.
*
* $LicenseInfo:firstyear=2001&license=viewerlgpl$
* Second Life Viewer Source Code
* Copyright (C) 2010, Linden Research, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation;
* version 2.1 of the License only.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
* Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
* $/LicenseInfo$
*/
#ifndef LLHANDLE_H
#define LLHANDLE_H
#include "llpointer.h"
#include <boost/type_traits/is_convertible.hpp>
#include <boost/utility/enable_if.hpp>
/**
* Helper object for LLHandle. Don't instantiate these directly, used
* exclusively by LLHandle.
*/
class LLTombStone : public LLRefCount
{
public:
LLTombStone(void* target = NULL) : mTarget(target) {}
void setTarget(void* target) { mTarget = target; }
void* getTarget() const { return mTarget; }
private:
mutable void* mTarget;
};
/**
* LLHandles are used to refer to objects whose lifetime you do not control or influence.
* Calling get() on a handle will return a pointer to the referenced object or NULL,
* if the object no longer exists. Note that during the lifetime of the returned pointer,
* you are assuming that the object will not be deleted by any action you perform,
* or any other thread, as normal when using pointers, so avoid using that pointer outside of
* the local code block.
*
* https://wiki.lindenlab.com/mediawiki/index.php?title=LLHandle&oldid=79669
*
* The implementation is like some "weak pointer" implementations. When we
* can't control the lifespan of the referenced object of interest, we can
* still instantiate a proxy object whose lifespan we DO control, and store in
* the proxy object a dumb pointer to the actual target. Then we just have to
* ensure that on destruction of the target object, the proxy's dumb pointer
* is set NULL.
*
* LLTombStone is our proxy object. LLHandle contains an LLPointer to the
* LLTombStone, so every copy of an LLHandle increments the LLTombStone's ref
* count as usual.
*
* One copy of the LLHandle, specifically the LLRootHandle, must be stored in
* the referenced object. Destroying the LLRootHandle is what NULLs the
* proxy's target pointer.
*
* Minor optimization: we want LLHandle's mTombStone to always be a valid
* LLPointer, saving some conditionals in dereferencing. That's the
* getDefaultTombStone() mechanism. The default LLTombStone object's target
* pointer is always NULL, so it's semantically identical to allowing
* mTombStone to be invalid.
*/
template <typename T>
class LLHandle
{
template <typename U> friend class LLHandle;
template <typename U> friend class LLHandleProvider;
public:
LLHandle() : mTombStone(getDefaultTombStone()) {}
template<typename U>
LLHandle(const LLHandle<U>& other, typename boost::enable_if< typename boost::is_convertible<U*, T*> >::type* dummy = 0)
: mTombStone(other.mTombStone)
{}
bool isDead() const
{
return mTombStone->getTarget() == NULL;
}
void markDead()
{
mTombStone = getDefaultTombStone();
}
T* get() const
{
return reinterpret_cast<T*>(mTombStone->getTarget());
}
friend bool operator== (const LLHandle<T>& lhs, const LLHandle<T>& rhs)
{
return lhs.mTombStone == rhs.mTombStone;
}
friend bool operator!= (const LLHandle<T>& lhs, const LLHandle<T>& rhs)
{
return !(lhs == rhs);
}
friend bool operator< (const LLHandle<T>& lhs, const LLHandle<T>& rhs)
{
return lhs.mTombStone < rhs.mTombStone;
}
friend bool operator> (const LLHandle<T>& lhs, const LLHandle<T>& rhs)
{
return lhs.mTombStone > rhs.mTombStone;
}
protected:
LLPointer<LLTombStone> mTombStone;
private:
typedef T* pointer_t;
static LLPointer<LLTombStone>& getDefaultTombStone()
{
static LLPointer<LLTombStone> sDefaultTombStone = new LLTombStone;
return sDefaultTombStone;
}
};
/**
* LLRootHandle isa LLHandle which must be stored in the referenced object.
* You can either store it directly and explicitly bind(this), or derive from
* LLHandleProvider (q.v.) which automates that for you. The essential point
* is that destroying the LLRootHandle (as a consequence of destroying the
* referenced object) calls unbind(), setting the LLTombStone's target pointer
* NULL.
*/
template <typename T>
class LLRootHandle : public LLHandle<T>
{
public:
typedef LLRootHandle<T> self_t;
typedef LLHandle<T> base_t;
LLRootHandle(T* object) { bind(object); }
LLRootHandle() {};
~LLRootHandle() { unbind(); }
// this is redundant, since an LLRootHandle *is* an LLHandle
//LLHandle<T> getHandle() { return LLHandle<T>(*this); }
void bind(T* object)
{
// unbind existing tombstone
if (LLHandle<T>::mTombStone.notNull())
{
if (LLHandle<T>::mTombStone->getTarget() == (void*)object) return;
LLHandle<T>::mTombStone->setTarget(NULL);
}
// tombstone reference counted, so no paired delete
LLHandle<T>::mTombStone = new LLTombStone((void*)object);
}
void unbind()
{
LLHandle<T>::mTombStone->setTarget(NULL);
}
//don't allow copying of root handles, since there should only be one
private:
LLRootHandle(const LLRootHandle& other) {};
};
/**
* Use this as a mixin for simple classes that need handles and when you don't
* want handles at multiple points of the inheritance hierarchy
*/
template <typename T>
class LLHandleProvider
{
public:
LLHandle<T> getHandle() const
{
// perform lazy binding to avoid small tombstone allocations for handle
// providers whose handles are never referenced
mHandle.bind(static_cast<T*>(const_cast<LLHandleProvider<T>* >(this)));
return mHandle;
}
template <typename U>
LLHandle<U> getDerivedHandle(typename boost::enable_if< typename boost::is_convertible<U*, T*> >::type* dummy = 0) const
{
LLHandle<U> downcast_handle;
downcast_handle.mTombStone = getHandle().mTombStone;
return downcast_handle;
}
protected:
typedef LLHandle<T> handle_type_t;
LLHandleProvider()
{
// provided here to enforce T deriving from LLHandleProvider<T>
}
private:
mutable LLRootHandle<T> mHandle;
};
#endif
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