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Unified Diff: src/plugin/Placeholder.h

Issue 29323611: Issue #1234, #2058 - Rewrite log facility, improving thread implementation
Patch Set: address comments Created Feb. 4, 2016, 8:59 p.m.
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Index: src/plugin/Placeholder.h
===================================================================
new file mode 100644
--- /dev/null
+++ b/src/plugin/Placeholder.h
@@ -0,0 +1,78 @@
+/*
+ * 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]);
+ }
+};
+
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