Issue #1234, #2058 - Rewrite log facility, improving thread implementation

src/plugin/Placeholder.h

+/*
+ * This file is part of Adblock Plus <https://adblockplus.org/>,
+ * Copyright (C) 2006-2015 Eyeo GmbH
+ *
+ * Adblock Plus is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * Adblock Plus is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Allows construction of an object in one block and usage in another.
+ *
+ * Splitting construction and usage is not necessary in all cases,
+ *   but there are many when it is.
+ * In any relevant case it's necessary to define something in a context that contains both blocks.
+ * One is when the class is not default-constructible.
+ * You might be able to construct an initial value if you know the class,
+ *   but that's not the case in general for templates.
+ * Another is when there's no copy assignment available,
+ *   such as std::unique_ptr.
+ *
+ * A 'Placeholder' can be initialized with either a copy or move constructor.
+ * We can't use assignment to initialize, because (in general) assignment assumes
+ *   that the lvalue target is already initialized.
+ */
+template<class T>
+class Placeholder
+{
+  /**
+   * We 'allocate' memory for the object in the instance itself;
+   *   there's no use of the heap.
+   *
+   * Note that it's a normative standard that sizeof(char)==1 
+   */
+  char storage[sizeof(T)];
+
+public:
+  /**
+   * Initialize by copying a glvalue into our private storage.
+   */
+  void Construct(const T& x)
+  {
+    /*
+     * We use placement-new syntax to use our private storage.
+     * The constructor expression invokes the copy constructor.
+     */
+    new (&storage[0]) T(x);
+  }
+
+  /**
+   * Initialize by moving an rvalue into our private storage.
+   */
+  void Construct(T&& x)
+  {
+    /*
+     * We use placement-new syntax to use our private storage.
+     * We explicitly cast to an rvalue to ensure use of the move constructor.
+     */
+    new (&storage[0]) T(std::move(x));
+  }
+
+  /**
+   * Access the object in storage.
+   */
+  T& Object()
+  {
+    return *reinterpret_cast<T *>(&storage[0]);
+  }
+};
+