75 lines
2.8 KiB
C++
75 lines
2.8 KiB
C++
///////////////////////////////////////////////////////////////////////////////
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// MuldeR's Utilities for Qt
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// Copyright (C) 2004-2017 LoRd_MuldeR <MuldeR2@GMX.de>
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//
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// This library is free software; you can redistribute it and/or
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// modify it under the terms of the GNU Lesser General Public
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// License as published by the Free Software Foundation; either
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// version 2.1 of the License, or (at your option) any later version.
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//
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public
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// License along with this library; if not, write to the Free Software
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// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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//
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// http://www.gnu.org/licenses/lgpl-2.1.txt
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//////////////////////////////////////////////////////////////////////////////////
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/**
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* @file
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* @brief This file contains a template class for lazy initialization
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*/
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#pragma once
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//MUtils
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#include <MUtils/Global.h>
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#include <MUtils/Exception.h>
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//Qt
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#include <QScopedPointer>
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#include <QAtomicPointer>
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namespace MUtils
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{
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/**
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* \brief Lazy initialization template class
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*
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* In order to create your own "lazy" initializer, inherit from the `Lazy<T>` class an implement the create() function. The lazy-initialized value can be obtained from a `Lazy<T>` instance by using the `operator*()`. Initialization of the value happens when the `operator*()` is called for the very first time, by invoking the concrete create() function. The return value of create() is then stored internally, so that any subsequent call to the `operator*()` immediately returns the previously created value.
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*
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* **Note on thread-saftey:** This class is thread-safe in the sense that all calls to `operator*()` on the same `Lazy<T>` instance, regardless from which thread, are guaranteed to return the exactly same value/object. Still, if the value has *not* been initialized yet **and** if multiple threads happen to call `operator*()` at the same time, then the concrete create() function *may* be invoked more than once (concurrently and by different threads). In that case, all but one return value of create() are discarded, and all threads eventually receive the same value/object.
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*/
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template<typename T> class Lazy
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{
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public:
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T& operator*(void)
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{
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while (!m_data)
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{
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if (T *const initializer = create())
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{
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if (m_data.testAndSetOrdered(NULL, initializer))
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{
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return *initializer;
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}
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}
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else
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{
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MUTILS_THROW("Initializer function returned NULL!");
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}
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}
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return *m_data;
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}
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protected:
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virtual T *create() = 0;
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private:
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QAtomicPointer<T> m_data;
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};
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}
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