74 lines
2.3 KiB
C++
74 lines
2.3 KiB
C++
// Copyright (c) 2015 Amanieu d'Antras
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//
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// Permission is hereby granted, free of charge, to any person obtaining a copy
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// of this software and associated documentation files (the "Software"), to deal
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// in the Software without restriction, including without limitation the rights
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// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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// copies of the Software, and to permit persons to whom the Software is
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// furnished to do so, subject to the following conditions:
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//
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// The above copyright notice and this permission notice shall be included in
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// all copies or substantial portions of the Software.
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//
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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// THE SOFTWARE.
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namespace async {
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namespace detail {
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// Thread-safe singleton wrapper class
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#ifdef HAVE_THREAD_SAFE_STATIC
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// C++11 guarantees thread safety for static initialization
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template<typename T>
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class singleton {
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public:
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static T& get_instance()
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{
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static T instance;
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return instance;
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}
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};
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#else
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// Some compilers don't support thread-safe static initialization, so emulate it
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template<typename T>
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class singleton {
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std::mutex lock;
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std::atomic<bool> init_flag;
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alignas(T) std::uint8_t storage[sizeof(T)];
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static singleton instance;
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// Use a destructor instead of atexit() because the latter does not work
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// properly when the singleton is in a library that is unloaded.
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~singleton()
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{
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if (init_flag.load(std::memory_order_acquire))
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reinterpret_cast<T*>(&storage)->~T();
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}
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public:
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static T& get_instance()
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{
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T* ptr = reinterpret_cast<T*>(&instance.storage);
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if (!instance.init_flag.load(std::memory_order_acquire)) {
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std::lock_guard<std::mutex> locked(instance.lock);
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if (!instance.init_flag.load(std::memory_order_relaxed)) {
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new(ptr) T;
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instance.init_flag.store(true, std::memory_order_release);
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}
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}
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return *ptr;
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}
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};
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template<typename T> singleton<T> singleton<T>::instance;
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#endif
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} // namespace detail
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} // namespace async
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