106 lines
3.2 KiB
C++
106 lines
3.2 KiB
C++
///////////////////////////////////////////////////////////////////////////////
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// MuldeR's Utilities for Qt
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// Copyright (C) 2004-2019 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 <QThread>
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#include <QAtomicPointer>
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#include <QAtomicInt>
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//CRT
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#include <functional>
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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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* The lazy-initialized value of type T 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 `initializer` lambda-function that was passed to the constructor. The return value of the `initializer` lambda-function 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. The *first* thread trying to access the value will invoke the `initializer` lambda-function; concurrent threads may need to busy-wait until the initialization is completed. The `initializer` lambda-function is invoked at most once.
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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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Lazy(std::function<T*(void)> &&initializer) : m_initializer(initializer) { }
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T& operator*(void)
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{
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return (*getValue());
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}
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T* operator->(void)
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{
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return getValue();
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}
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bool initialized()
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{
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return (m_state > 1);
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}
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~Lazy(void)
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{
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if(T *const value = m_value)
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{
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delete value;
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}
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}
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protected:
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__forceinline T* getValue()
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{
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T *value;
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while (!(value = m_value))
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{
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if (m_state.testAndSetOrdered(0, 1))
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{
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if (value = m_initializer())
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{
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m_value.fetchAndStoreOrdered(value);
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m_state.fetchAndStoreOrdered(2);
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break; /*success*/
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}
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m_state.fetchAndStoreOrdered(0);
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MUTILS_THROW("Initializer returned NULL pointer!");
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}
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QThread::yieldCurrentThread();
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}
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return value;
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}
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private:
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QAtomicPointer<T> m_value;
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QAtomicInt m_state;
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const std::function<T*(void)> m_initializer;
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};
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}
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