gecko/js/src/xpconnect/public/nsAutoJSValHolder.h

/* -*- Mode: c++; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*- */
/* ***** 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 worker threads.
 *
 * The Initial Developer of the Original Code is
 *   Mozilla Corporation.
 * Portions created by the Initial Developer are Copyright (C) 2008
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Ben Turner <bent.mozilla@gmail.com> (Original Author)
 *   Ben Newman <b{enjamn,newman}@mozilla.com>
 *
 * 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
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 * the provisions above, a recipient may use your version of this file under
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 * ***** END LICENSE BLOCK ***** */

#ifndef __NSAUTOJSVALHOLDER_H__
#define __NSAUTOJSVALHOLDER_H__

#include "jsapi.h"

/**
 * Simple class that looks and acts like a jsval except that it unroots
 * itself automatically if Root() is ever called. Designed to be rooted on the
 * context or runtime (but not both!).
 */
class nsAutoJSValHolder
{
public:

  nsAutoJSValHolder()
    : mRt(NULL)
    , mVal(JSVAL_NULL)
    , mGCThing(NULL)
    , mHeld(JS_FALSE)
  {
    // nothing to do
  }

  /**
   * Always release on destruction.
   */
  virtual ~nsAutoJSValHolder() {
    Release();
  }

  /**
   * Hold by rooting on the context's runtime.
   */
  JSBool Hold(JSContext* aCx) {
    return Hold(JS_GetRuntime(aCx));
  }

  /**
   * Hold by rooting on the runtime.
   * Note that mGCThing may be JSVAL_NULL, which is not a problem.
   */
  JSBool Hold(JSRuntime* aRt) {
    if (!mHeld) {
      if (JS_AddNamedRootRT(aRt, &mGCThing, "nsAutoJSValHolder")) {
        mRt = aRt;
        mHeld = JS_TRUE;
      } else {
        Release(); // out of memory
      }
    }
    return mHeld;
  }

  /**
   * Manually release, nullifying mVal, mGCThing, and mRt, but returning
   * the original jsval.
   */
  jsval Release() {
    NS_ASSERTION(!mHeld || mRt, "Bad!");

    jsval oldval = mVal;

    if (mHeld) {
      JS_RemoveRootRT(mRt, &mGCThing); // infallible
      mHeld = JS_FALSE;
    }

    mVal = JSVAL_NULL;
    mGCThing = NULL;
    mRt = NULL;

    return oldval;
  }

  /**
   * Determine if Hold has been called.
   */
  JSBool IsHeld() {
    return mHeld;
  }

  /**
   * Pretend to be a JSObject*.
   */
  operator JSObject*() const {
    return JSVAL_IS_OBJECT(mVal)
         ? JSVAL_TO_OBJECT(mVal)
         : JSVAL_NULL;
  }

  /**
   * Pretend to be a jsval.
   */
  operator jsval() const { return mVal; }

  nsAutoJSValHolder &operator=(JSObject* aOther) {
#ifdef DEBUG
    if (aOther) {
      NS_ASSERTION(mHeld, "Not rooted!");
    }
#endif
    return *this = OBJECT_TO_JSVAL(aOther);
  }

  nsAutoJSValHolder &operator=(jsval aOther) {
#ifdef DEBUG
    if (aOther) {
      NS_ASSERTION(mHeld, "Not rooted!");
    }
#endif
    mVal = aOther;
    mGCThing = JSVAL_IS_GCTHING(aOther)
             ? JSVAL_TO_GCTHING(aOther)
             : NULL;
    return *this;
  }

private:
  JSRuntime* mRt;
  jsval mVal;
  void* mGCThing;
  JSBool mHeld;
};

#endif /* __NSAUTOJSVALHOLDER_H__ */