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+/*
+ * Copyright (c) 2003-2007, John Wiegley.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * - Neither the name of New Artisans LLC nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * @file   scopevar.h
+ * @author John Wiegley
+ * @date   Sun May  6 20:10:52 2007
+ * 
+ * @brief  Adds a facility to C++ for handling "scoped executions".
+ * 
+ * There are sometimes cases where you would like to guarantee that
+ * something happens at the end of a scope, such as calling a function
+ * to close a resource for you.
+ *
+ * The common idiom for this has become to write a helper class whose
+ * destructor will call that function.  Of course, it must then be
+ * passed information from the calling scope to hold onto as state
+ * information, since the code within the class itself has no access
+ * to its points of use.
+ *
+ * This type of solution is cumbersome enough that it's sometimes
+ * avoided.  Take calling pthread_mutex_unlock(pthread_mutex_t *) for
+ * example.  A typical snippet of safe C++ code might look like this:
+ *
+ * @code
+ *   void foo(pthread_mutex_t * mutex) {
+ *     if (pthread_mutex_lock(mutex) == 0) {
+ *       try {
+ *         // Do work that requires the mutex to be locked; then...
+ *         pthread_mutex_unlock(mutex);
+ *       }
+ *       catch (std::logic_error& exc) {
+ *         // This is an exception we actually handle, and then exit
+ *         pthread_mutex_unlock(mutex);
+ *       }
+ *       catch (...) {
+ *         // These are exceptions we do not handle, but still the
+ *         // mutex must be unlocked
+ *         pthread_mutex_unlock(mutex);
+ *         throw;
+ *       }
+ *     }
+ *   }
+ * @endcode
+ *
+ * The alternative to this, as mentioned above, is to create a helper
+ * class named pthread_scoped_lock, which might look like this:
+ *
+ * @code
+ *   class pthread_scoped_lock : public boost::noncopyable {
+ *     pthread_mutex_t * mutex;
+ *   public:
+ *     explicit pthread_scoped_lock(pthread_mutex_t * locked_mutex)
+ *       : mutex(locked_mutex) {}
+ *     ~pthread_scoped_lock() {
+ *       pthread_mutex_unlock(mutex);
+ *     }
+ *   };
+ * @endcode
+ *
+ * Although this helper class is just as much work as writing the code
+ * above, it only needs to be written once.  Now the access code can
+ * look like this:
+ *
+ * @code
+ *   void foo(pthread_mutex_t * mutex) {
+ *     if (pthread_mutex_lock(mutex) == 0) {
+ *       pthread_scoped_lock(mutex);
+ *       try {
+ *         // Do work that requires the mutex to be locked
+ *       }
+ *       catch (std::logic_error& exc) {
+ *         // This is an exception we actually handle, and then exit
+ *       }
+ *     }
+ *   }
+ * @endcode
+ *
+ * But what if it could be even easier?  That is what this file is
+ * for, to provide a scopevar<> class which guarantees execution
+ * of arbtirary code after a scope has terminated, without having to
+ * resort to custom utility classes.  It relies on boost::bind to
+ * declare pending function calls.  Here it what the above would look
+ * like:
+ *
+ * @code
+ *   void foo(pthread_mutex_t * mutex) {
+ *     if (pthread_mutex_lock(mutex) == 0) {
+ *       scopevar<void> unlock_mutex
+ *         (boost::bind(pthread_mutex_unlock, mutex));
+ *       try {
+ *         // Do work that requires the mutex to be locked
+ *       }
+ *       catch (std::logic_error& exc) {
+ *         // This is an exception we actually handle, and then exit
+ *       }
+ *     }
+ *   }
+ * @endcode
+ *
+ * The advantage here is that no helper class ever needs to created,
+ * and hence no bugs from such helper classes can creep into the code.
+ * The single call to boost::bind creates a closure binding that will
+ * be invoked once the containing scope has terminated.
+ *
+ * Another kind of scopevar is useful for setting the values of
+ * variables to a predetermined value upon completion of a scope.
+ * Consider this example:
+ *
+ * @code
+ *   bool foo_was_run;
+ *
+ *   void foo() {
+ *     scopevar<bool&> set_success((_1 = true), foo_was_run);
+ *     // do some code, and make sure foo_was_run is set to true
+ *     // once the scope is exited -- however this happens.
+ *   }
+ * @endcode
+ *
+ * In this case, the Boost.Lambda library is used to create an
+ * anonymous functor whose job is to set the global variable
+ * `foo_was_run' to a predetermined value.
+ *
+ * Lastly, there is another helper class, `scoped_variable' whose job
+ * is solely to return variables to the value they had at the moment
+ * the scoped_variable class was instantiated.  For example, let's say
+ * you have a `bar' variable that you want to work on, but you want to
+ * guarantee that its value is restored upon exiting the scope.  This
+ * can be useful in recursion, for "pushing" and "popping" variable
+ * values during execution, for example:
+ *
+ * @code
+ *   std::string bar = "Hello";
+ *   void foo() {
+ *     scoped_variable<std::string> restore_bar(bar);
+ *     bar = "Goodbye";
+ *     // do work with the changed bar; it gets restored upon exit
+ *   }
+ * @endcode
+ *
+ * As a shortcut, you can specify the new value for the pushed
+ * variable as a second constructor argument:
+ *
+ * @code
+ *   std::string bar = "Hello";
+ *   void foo() {
+ *     scoped_variable<std::string> restore_bar(bar, "Goodbye");
+ *     // do work with the changed bar; it gets restored upon exit
+ *   }
+ * @endcode
+ *
+ * Finally, you can stop a scopevar or scoped_variable from
+ * invoking its completion code by calling the `clear' method on the
+ * object instance.  Once `clear' is called, the scoped execution
+ * becomes inert and will do nothing when the enclosing scope is
+ * exited.
+ */
+
+#ifndef _SCOPEVAR_H
+#define _SCOPEVAR_H
+
+template <typename T>
+class push_variable : public boost::noncopyable
+{
+  T&   var;
+  T    prev;
+  bool enabled;
+
+public:
+  explicit push_variable(T& _var)
+    : var(_var), prev(var), enabled(true) {}
+  explicit push_variable(T& _var, const T& value)
+    : var(_var), prev(var), enabled(true) {
+    var = value;
+  }
+  ~push_variable() {
+    if (enabled)
+      var = prev;
+  }
+
+  void clear() {
+    enabled = false;
+  }
+};
+
+#endif // _SCOPEVAR_H