gecko/xpcom/glue/AutoRestore.h

194 lines
7.3 KiB
C
Raw Normal View History

/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
/* ***** BEGIN LICENSE BLOCK *****
* Version: MPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
* http://www.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is nsAutoRestore.h.
*
* The Initial Developer of the Original Code is the Mozilla Foundation.
* Portions created by the Initial Developer are Copyright (C) 2009
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
* L. David Baron <dbaron@dbaron.org>, Mozilla Corporation (original author)
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the MPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the MPL, the GPL or the LGPL.
*
* ***** END LICENSE BLOCK ***** */
/* functions for restoring saved values at the end of a C++ scope */
#ifndef mozilla_AutoRestore_h_
#define mozilla_AutoRestore_h_
#include "prtypes.h"
#include "nsDebug.h"
namespace mozilla {
/**
* The following classes are designed to cause assertions to detect
* inadvertent use of guard objects as temporaries. In other words,
* when we have a guard object whose only purpose is its constructor and
* destructor (and is never otherwise referenced), the intended use
* might be:
* AutoRestore savePainting(mIsPainting);
* but is is easy to accidentally write:
* AutoRestore(mIsPainting);
* which compiles just fine, but runs the destructor well before the
* intended time.
*
* They work by adding (#ifdef DEBUG) an additional parameter to the
* guard object's constructor, with a default value, so that users of
* the guard object's API do not need to do anything. The default value
* of this parameter is a temporary object. C++ (ISO/IEC 14882:1998),
* section 12.2 [class.temporary], clauses 4 and 5 seem to assume a
* guarantee that temporaries are destroyed in the reverse of their
* construction order, but I actually can't find a statement that that
* is true in the general case (beyond the two specific cases mentioned
* there). However, it seems to be true.
*
* These classes are intended to be used only via the macros immediately
* below them:
* MOZILLA_DECL_USE_GUARD_OBJECT_NOTIFIER declares (ifdef DEBUG) a member
* variable, and should be put where a declaration of a private
* member variable would be placed.
* MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM should be placed at the end of the
* parameters to each constructor of the guard object; it declares
* (ifdef DEBUG) an additional parameter. (But use the *_ONLY_PARAM
* variant for constructors that take no other parameters.)
* MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL should likewise be used in
* the implementation of such constructors when they are not inline.
* MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT should be used in
* the implementation of such constructors to pass the parameter to
* a base class that also uses these macros
* MOZILLA_GUARD_OBJECT_NOTIFIER_INIT is a statement that belongs in each
* constructor. It uses the parameter declared by
* MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM.
*
* For more details, and examples of using these macros, see
* https://developer.mozilla.org/en/Using_RAII_classes_in_Mozilla
*/
#ifdef DEBUG
class GuardObjectNotifier
{
private:
bool* mStatementDone;
public:
GuardObjectNotifier() : mStatementDone(NULL) {}
~GuardObjectNotifier() {
*mStatementDone = true;
}
void SetStatementDone(bool *aStatementDone) {
mStatementDone = aStatementDone;
}
};
class GuardObjectNotificationReceiver
{
private:
bool mStatementDone;
public:
GuardObjectNotificationReceiver() : mStatementDone(false) {}
~GuardObjectNotificationReceiver() {
/*
* Assert that the guard object was not used as a temporary.
* (Note that this assert might also fire if Init is not called
* because the guard object's implementation is not using the
* above macros correctly.)
*/
NS_ABORT_IF_FALSE(mStatementDone,
"guard object used as temporary");
}
void Init(const GuardObjectNotifier &aNotifier) {
/*
* aNotifier is passed as a const reference so that we can pass a
* temporary, but we really intend it as non-const
*/
const_cast<GuardObjectNotifier&>(aNotifier).
SetStatementDone(&mStatementDone);
}
};
#define MOZILLA_DECL_USE_GUARD_OBJECT_NOTIFIER \
mozilla::GuardObjectNotificationReceiver _mCheckNotUsedAsTemporary;
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM \
, const mozilla::GuardObjectNotifier& _notifier = \
mozilla::GuardObjectNotifier()
#define MOZILLA_GUARD_OBJECT_NOTIFIER_ONLY_PARAM \
const mozilla::GuardObjectNotifier& _notifier = \
mozilla::GuardObjectNotifier()
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL \
, const mozilla::GuardObjectNotifier& _notifier
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT \
, _notifier
#define MOZILLA_GUARD_OBJECT_NOTIFIER_INIT \
PR_BEGIN_MACRO _mCheckNotUsedAsTemporary.Init(_notifier); PR_END_MACRO
#else /* defined(DEBUG) */
#define MOZILLA_DECL_USE_GUARD_OBJECT_NOTIFIER
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM
#define MOZILLA_GUARD_OBJECT_NOTIFIER_ONLY_PARAM
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL
#define MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT
#define MOZILLA_GUARD_OBJECT_NOTIFIER_INIT PR_BEGIN_MACRO PR_END_MACRO
#endif /* !defined(DEBUG) */
/**
* Save the current value of a variable and restore it when the object
* goes out of scope. For example:
* {
* AutoRestore<PRBool> savePainting(mIsPainting);
* mIsPainting = PR_TRUE;
*
* // ... your code here ...
*
* // mIsPainting is reset to its old value at the end of this block
* }
*/
template <class T>
class NS_STACK_CLASS AutoRestore
{
private:
T& mLocation;
T mValue;
MOZILLA_DECL_USE_GUARD_OBJECT_NOTIFIER
public:
AutoRestore(T& aValue MOZILLA_GUARD_OBJECT_NOTIFIER_PARAM)
: mLocation(aValue), mValue(aValue)
{
MOZILLA_GUARD_OBJECT_NOTIFIER_INIT;
}
~AutoRestore() { mLocation = mValue; }
};
}
#endif /* !defined(mozilla_AutoRestore_h_) */