ceph/src/common/Initialize.h

   1 // -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
   2 /*
   3  * Ceph - scalable distributed file system
   4  *
   5  * Copyright (C) 2014 UnitedStack <haomai@unitedstack.com>
   6  *
   7  * Author: Haomai Wang <haomaiwang@gmail.com>
   8  *
   9  * This is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public
  11  * License version 2.1, as published by the Free Software
  12  * Foundation.  See file COPYING.
  13  *
  14  */
  15 /* Copyright (c) 2011 Stanford University
  16  *
  17  * Permission to use, copy, modify, and distribute this software for any
  18  * purpose with or without fee is hereby granted, provided that the above
  19  * copyright notice and this permission notice appear in all copies.
  20  *
  21  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR(S) DISCLAIM ALL WARRANTIES
  22  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  23  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL AUTHORS BE LIABLE FOR
  24  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  25  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  26  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  27  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  28  */
  29
  30 #ifndef CEPH_INITIALIZE_H
  31 #define CEPH_INITIALIZE_H
  32
  33 /**
  34  * This class is used to manage once-only initialization that should occur
  35  * before main() is invoked, such as the creation of static variables.  It
  36  * also provides a mechanism for handling dependencies (where one class
  37  * needs to perform its once-only initialization before another).
  38  *
  39  * The simplest way to use an Initialize object is to define a static
  40  * initialization method for a class, say Foo::init().  Then, declare
  41  * a static Initialize object in the class:
  42  * "static Initialize initializer(Foo::init);".
  43  * The result is that Foo::init will be invoked when the object is
  44  * constructed (before main() is invoked).  Foo::init can create static
  45  * objects and perform any other once-only initialization needed by the
  46  * class.  Furthermore, if some other class needs to ensure that Foo has
  47  * been initialized (e.g. as part of its own initialization) it can invoke
  48  * Foo::init directly (Foo::init should contain an internal guard so that
  49  * it only performs its functions once, even if invoked several times).
  50  *
  51  * There is also a second form of constructor for Initialize that causes a
  52  * new object to be dynamically allocated and assigned to a pointer, instead
  53  * of invoking a function. This form allows for the creation of static objects
  54  * that are never destructed (thereby avoiding issues with the order of
  55  * destruction).
  56  */
  57 class Initialize {
  58  public:
  59   /**
  60    * This form of constructor causes its function argument to be invoked
  61    * when the object is constructed.  When used with a static Initialize
  62    * object, this will cause \p func to run before main() runs, so that
  63    * \p func can perform once-only initialization.
  64    *
  65    * \param func
  66    *      This function is invoked with no arguments when the object is
  67    *      constructed.  Typically the function will create static
  68    *      objects and/or invoke other initialization functions.  The
  69    *      function should normally contain an internal guard so that it
  70    *      only performs its initialization the first time it is invoked.
  71    */
  72   explicit Initialize(void (*func)()) {
  73     (*func)();
  74   }
  75
  76   /**
  77    * This form of constructor causes a new object of a particular class
  78    * to be constructed with a no-argument constructor and assigned to a
  79    * given pointer.  This form is typically used with a static Initialize
  80    * object: the result is that the object will be created and assigned
  81    * to the pointer before main() runs.
  82    *
  83    * \param p
  84    *      Pointer to an object of any type. If the pointer is NULL then
  85    *      it is replaced with a pointer to a newly allocated object of
  86    *      the given type.
  87    */
  88   template<typename T>
  89   explicit Initialize(T*& p) {
  90     if (p == NULL) {
  91       p = new T;
  92     }
  93   }
  94 };
  95
  96 #endif  // CEPH_INITIALIZE_H