initial code repo

[stor4nfv.git] / src / ceph / src / common / Initialize.h

diff --git a/src/ceph/src/common/Initialize.h b/src/ceph/src/common/Initialize.h
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- 
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2014 UnitedStack <haomai@unitedstack.com>
+ *
+ * Author: Haomai Wang <haomaiwang@gmail.com>
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ *
+ */
+/* Copyright (c) 2011 Stanford University
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR(S) DISCLAIM ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL AUTHORS BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef CEPH_INITIALIZE_H
+#define CEPH_INITIALIZE_H
+
+/**
+ * This class is used to manage once-only initialization that should occur
+ * before main() is invoked, such as the creation of static variables.  It
+ * also provides a mechanism for handling dependencies (where one class
+ * needs to perform its once-only initialization before another).
+ * 
+ * The simplest way to use an Initialize object is to define a static
+ * initialization method for a class, say Foo::init().  Then, declare
+ * a static Initialize object in the class:
+ * "static Initialize initializer(Foo::init);".
+ * The result is that Foo::init will be invoked when the object is
+ * constructed (before main() is invoked).  Foo::init can create static
+ * objects and perform any other once-only initialization needed by the
+ * class.  Furthermore, if some other class needs to ensure that Foo has
+ * been initialized (e.g. as part of its own initialization) it can invoke
+ * Foo::init directly (Foo::init should contain an internal guard so that
+ * it only performs its functions once, even if invoked several times).
+ *
+ * There is also a second form of constructor for Initialize that causes a
+ * new object to be dynamically allocated and assigned to a pointer, instead
+ * of invoking a function. This form allows for the creation of static objects
+ * that are never destructed (thereby avoiding issues with the order of
+ * destruction).
+ */
+class Initialize {
+ public:
+  /**
+   * This form of constructor causes its function argument to be invoked
+   * when the object is constructed.  When used with a static Initialize
+   * object, this will cause \p func to run before main() runs, so that
+   * \p func can perform once-only initialization.
+   *
+   * \param func
+   *      This function is invoked with no arguments when the object is
+   *      constructed.  Typically the function will create static
+   *      objects and/or invoke other initialization functions.  The
+   *      function should normally contain an internal guard so that it
+   *      only performs its initialization the first time it is invoked.
+   */
+  explicit Initialize(void (*func)()) {
+    (*func)();
+  }
+
+  /**
+   * This form of constructor causes a new object of a particular class
+   * to be constructed with a no-argument constructor and assigned to a
+   * given pointer.  This form is typically used with a static Initialize
+   * object: the result is that the object will be created and assigned
+   * to the pointer before main() runs.
+   *
+   * \param p
+   *      Pointer to an object of any type. If the pointer is NULL then
+   *      it is replaced with a pointer to a newly allocated object of
+   *      the given type.
+   */
+  template<typename T>
+  explicit Initialize(T*& p) {
+    if (p == NULL) {
+      p = new T;
+    }
+  }
+};
+
+#endif  // CEPH_INITIALIZE_H