访问单例时避免未定义行为的技术

#pragma once #include <set> class Payload { public: Payload() = default; ~Payload() = default; Payload(const Payload &) = delete; Payload(Payload &&) = delete; Payload& operator=(const Payload &) = delete; Payload& operator=(Payload &&) = delete; void add(int value) { m_data.emplace(value); } private: std::set<int> m_data; }; 

 #pragma once #include <iostream> template<typename T> class SingletonClassic : public T { public: ~SingletonClassic() { std::cout << "~SingletonClassic()" << std::endl; } SingletonClassic(const SingletonClassic &) = delete; SingletonClassic(SingletonClassic &&) = delete; SingletonClassic& operator=(const SingletonClassic &) = delete; SingletonClassic& operator=(SingletonClassic &&) = delete; static SingletonClassic& instance() { std::cout << "instance()" << std::endl; static SingletonClassic inst; return inst; } private: SingletonClassic() { std::cout << "SingletonClassic()" << std::endl; } }; 

 #include "SingletonClassic.h" #include "Payload.h" #include <memory> class ClassicSingleThreadedUtility { public: ClassicSingleThreadedUtility() { // To ensure that singleton will be constucted before utility SingletonClassic<Payload>::instance(); } ~ClassicSingleThreadedUtility() { auto &instance = SingletonClassic<Payload>::instance(); for ( int i = 0; i < 100; ++i ) instance.add(i); } }; // 1. Create an empty unique_ptr // 2. Create singleton (because of modified ClassicSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<ClassicSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<ClassicSingleThreadedUtility>(); // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order is correct int main() { return 0; } 

 #include "SingletonClassic.h" #include "Payload.h" #include <memory> class ClassicSingleThreadedUtility { public: ClassicSingleThreadedUtility() { // To ensure that singleton will be constucted before utility SingletonClassic<Payload>::instance(); } ~ClassicSingleThreadedUtility() { auto &instance = SingletonClassic<Payload>::instance(); for ( int i = 0; i < 100; ++i ) instance.add(i); } }; // 1. Create an empty unique_ptr // 2. Create singleton (because of modified ClassicSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<ClassicSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<ClassicSingleThreadedUtility>(); // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order seems to be correct ... int main() { // ... but user swaps unique_ptrs emptyUnique.swap(utilityUnique); // Guaranteed destruction order is still the same: // - utilityUnique; // - singleton; // - emptyUnique, // but now utilityUnique is empty, and emptyUnique is filled, // so destruction order is incorrect return 0; } 

 #include "SingletonClassic.h" #include "Payload.h" #include <memory> #include <iostream> class ClassicSingleThreadedUtility { public: ClassicSingleThreadedUtility() { thread_local auto flag_strong = std::make_shared<char>(0); m_flag_weak = flag_strong; SingletonClassic<Payload>::instance(); } ~ClassicSingleThreadedUtility() { if ( !m_flag_weak.expired() ) { auto &instance = SingletonClassic<Payload>::instance(); for ( int i = 0; i < 100; ++i ) instance.add(i); } } private: std::weak_ptr<char> m_flag_weak; }; // 1. Create an empty unique_ptr // 2. Create singleton (because of modified ClassicSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<ClassicSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<ClassicSingleThreadedUtility>(); // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order seems to be correct ... int main() { // ... but user swaps unique_ptrs emptyUnique.swap(utilityUnique); { // To demonstrate normal processing before application ends auto utility = ClassicSingleThreadedUtility(); } // Guaranteed destruction order is still the same: // - utilityUnique; // - singleton; // - emptyUnique, // but now utilityUnique is empty, and emptyUnique is filled, // so destruction order is incorrect ... // ... but utility uses a variable with thread storage duration to detect thread termination. return 0; } 

 #pragma once #include <memory> #include <iostream> template<typename T> class SingletonShared : public T { public: ~SingletonShared() { std::cout << "~SingletonShared()" << std::endl; } SingletonShared(const SingletonShared &) = delete; SingletonShared(SingletonShared &&) = delete; SingletonShared& operator=(const SingletonShared &) = delete; SingletonShared& operator=(SingletonShared &&) = delete; static std::shared_ptr<SingletonShared> instance() { std::cout << "instance()" << std::endl; // "new" and no std::make_shared because of private c-tor static auto inst = std::shared_ptr<SingletonShared>(new SingletonShared); return inst; } private: SingletonShared() { std::cout << "SingletonShared()" << std::endl; } }; 

 #include "SingletonShared.h" #include <Payload.h> #include <memory> class SharedSingleThreadedUtility { public: SharedSingleThreadedUtility() { // To ensure that singleton will be constucted before utility SingletonShared<Payload>::instance(); } ~SharedSingleThreadedUtility() { if ( auto instance = SingletonShared<Payload>::instance() ) for ( int i = 0; i < 100; ++i ) instance->add(i); } }; // 1. Create an empty unique_ptr // 2. Create singleton (because of modified SharedSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<SharedSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<SharedSingleThreadedUtility>(); // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order is correct int main() { return 0; } 

 #include "SingletonShared.h" #include "Payload.h" #include <memory> class SharedSingleThreadedUtility { public: SharedSingleThreadedUtility() { // To ensure that singleton will be constucted before utility SingletonShared<Payload>::instance(); } ~SharedSingleThreadedUtility() { // Sometimes this check may result as "false" even for destroyed singleton // preventing from visual effects of undefined behaviour ... //if ( auto instance = SingletonShared::instance() ) // for ( int i = 0; i < 100; ++i ) // instance->add(i); // ... so this code will demonstrate UB in colour auto instance = SingletonShared<Payload>::instance(); for ( int i = 0; i < 100; ++i ) instance->add(i); } }; // 1. Create an empty unique_ptr // 2. Create singleton (because of modified SharedSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<SharedSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<SharedSingleThreadedUtility>(); // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order seems to be correct ... int main() { // ... but user swaps unique_ptrs emptyUnique.swap(utilityUnique); // Guaranteed destruction order is the same: // - utilityUnique; // - singleton; // - emptyUnique, // but now utilityUnique is empty, and emptyUnique is filled, // so destruction order is incorrect return 0; } 

 #include "SingletonShared.h" #include "Payload.h" #include <memory> class SharedSingleThreadedUtility { public: SharedSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_singleton(SingletonShared<Payload>::instance()) { } ~SharedSingleThreadedUtility() { // Sometimes this check may result as "false" even for destroyed singleton // preventing from visual effects of undefined behaviour ... //if ( m_singleton ) // for ( int i = 0; i < 100; ++i ) // m_singleton->add(i); // ... so this code will allow to demonstrate UB in colour for ( int i = 0; i < 100; ++i ) m_singleton->add(i); } private: // A copy of smart pointer, not a reference std::shared_ptr<SingletonShared<Payload>> m_singleton; }; // 1. Create an empty unique_ptr // 2. Create singleton (because of SharedSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<SharedSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<SharedSingleThreadedUtility>(); int main() { // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order is correct ... // ... but user swaps unique_ptrs emptyUnique.swap(utilityUnique); // Guaranteed destruction order is the same: // - utilityUnique; // - singleton; // - emptyUnique, // but now utilityUnique is empty, and emptyUnique is filled, // so destruction order is incorrect... // ... but utility have made a copy of shared_ptr when it was available, // so it's correct again. return 0; } 

 #pragma once #include <functional> class CallbackPayload { public: CallbackPayload() = default; ~CallbackPayload() = default; CallbackPayload(const CallbackPayload &) = delete; CallbackPayload(CallbackPayload &&) = delete; CallbackPayload& operator=(const CallbackPayload &) = delete; CallbackPayload& operator=(CallbackPayload &&) = delete; void setCallback(std::function<void()> &&fn) { m_callbackFn = std::move(fn); } private: std::function<void()> m_callbackFn; }; 

 #pragma once #include <iostream> class SomethingWithVeryImportantDestructor { public: SomethingWithVeryImportantDestructor() { std::cout << "SomethingWithVeryImportantDestructor()" << std::endl; } ~SomethingWithVeryImportantDestructor() { std::cout << "~SomethingWithVeryImportantDestructor()" << std::endl; } SomethingWithVeryImportantDestructor(const SomethingWithVeryImportantDestructor &) = delete; SomethingWithVeryImportantDestructor(SomethingWithVeryImportantDestructor &&) = delete; SomethingWithVeryImportantDestructor& operator=(const SomethingWithVeryImportantDestructor &) = delete; SomethingWithVeryImportantDestructor& operator=(SomethingWithVeryImportantDestructor &&) = delete; }; 

 #include "SingletonShared.h" #include "CallbackPayload.h" #include "SomethingWithVeryImportantDestructor.h" class SharedSingleThreadedUtility { public: SharedSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_singleton(SingletonShared<CallbackPayload>::instance()) { std::cout << "SharedSingleThreadedUtility()" << std::endl; } ~SharedSingleThreadedUtility() { std::cout << "~SharedSingleThreadedUtility()" << std::endl; } void setCallback(std::function<void()> &&fn) { if ( m_singleton ) m_singleton->setCallback(std::move(fn)); } private: // A copy of smart pointer, not a reference std::shared_ptr<SingletonShared<CallbackPayload>> m_singleton; }; int main() { auto utility = std::make_shared<SharedSingleThreadedUtility>(); auto something = std::make_shared<SomethingWithVeryImportantDestructor>(); // lambda with "utility" and "something" captured utility->setCallback( [utility, something](){} ); return 0; } 

 #pragma once #include <memory> #include <iostream> template<typename T> class SingletonWeak : public T { public: ~SingletonWeak() { std::cout << "~SingletonWeak()" << std::endl; } SingletonWeak(const SingletonWeak &) = delete; SingletonWeak(SingletonWeak &&) = delete; SingletonWeak& operator=(const SingletonWeak &) = delete; SingletonWeak& operator=(SingletonWeak &&) = delete; static std::weak_ptr<SingletonWeak> instance() { std::cout << "instance()" << std::endl; // "new" and no std::make_shared because of private c-tor static auto inst = std::shared_ptr<SingletonWeak>(new SingletonWeak); return inst; } private: SingletonWeak() { std::cout << "SingletonWeak()" << std::endl; } }; 

 #include "SingletonWeak.h" #include "Payload.h" #include <memory> class WeakSingleThreadedUtility { public: WeakSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_weak(SingletonWeak<Payload>::instance()) { } ~WeakSingleThreadedUtility() { // Sometimes this check may result as "false" even in case of incorrect usage, // and there's no way to guarantee a demonstration of undefined behaviour in colour if ( auto strong = m_weak.lock() ) for ( int i = 0; i < 100; ++i ) strong->add(i); } private: // A weak copy of smart pointer, not a reference std::weak_ptr<SingletonWeak<Payload>> m_weak; }; // 1. Create an empty unique_ptr // 2. Create singleton (because of WeakSingleThreadedUtility c-tor) // 3. Create utility std::unique_ptr<WeakSingleThreadedUtility> emptyUnique; auto utilityUnique = std::make_unique<WeakSingleThreadedUtility>(); int main() { // This guarantee destruction in order: // - utilityUnique; // - singleton; // - emptyUnique. // This order is correct ... // ... but user swaps unique_ptrs emptyUnique.swap(utilityUnique); // Guaranteed destruction order is the same: // - utilityUnique; // - singleton; // - emptyUnique, // but now utilityUnique is empty, and emptyUnique is filled, // so destruction order is incorrect... // ... but utility have made a weak copy of shared_ptr when it was available, // so it's correct again. return 0; } 

 #include "SingletonWeak.h" #include "CallbackPayload.h" #include "SomethingWithVeryImportantDestructor.h" class WeakSingleThreadedUtility { public: WeakSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_weak(SingletonWeak<CallbackPayload>::instance()) { std::cout << "WeakSingleThreadedUtility()" << std::endl; } ~WeakSingleThreadedUtility() { std::cout << "~WeakSingleThreadedUtility()" << std::endl; } void setCallback(std::function<void()> &&fn) { if ( auto strong = m_weak.lock() ) strong->setCallback(std::move(fn)); } private: // A weak copy of smart pointer, not a reference std::weak_ptr<SingletonWeak<CallbackPayload>> m_weak; }; int main() { auto utility = std::make_shared<WeakSingleThreadedUtility>(); auto something = std::make_shared<SomethingWithVeryImportantDestructor>(); // lambda with "utility" and "something" captured utility->setCallback( [utility, something](){} ); return 0; } 

 #include "SingletonShared.h" #include "Payload.h" #include <memory> #include <cstdlib> class SharedSingleThreadedUtility { public: SharedSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_singleton(SingletonShared<Payload>::instance()) { } ~SharedSingleThreadedUtility() { // Sometimes this check may result as "false" even for destroyed singleton // preventing from visual effects of undefined behaviour ... //if ( m_singleton ) // for ( int i = 0; i < 100; ++i ) // m_singleton->add(i); // ... so this code will allow to demonstrate UB in colour for ( int i = 0; i < 100; ++i ) m_singleton->add(i); } private: // A copy of smart pointer, not a reference std::shared_ptr<SingletonShared<Payload>> m_singleton; }; void cracker() { SharedSingleThreadedUtility(); } // 1. Register cracker() using std::atexit // 2. Create singleton // 3. Create utility auto reg = [](){ std::atexit(&cracker); return 0; }(); auto utility = SharedSingleThreadedUtility(); // This guarantee destruction in order: // - utility; // - singleton. // This order is correct. // Additionally, there's a copy of shared_ptr in the class instance... // ... but there was std::atexit registered before singleton, // so cracker() will be invoked after destruction of utility and singleton. // There's second try to create a singleton - and it's incorrect. int main() { return 0; } 

 #include "SingletonWeak.h" #include "Payload.h" #include <memory> #include <cstdlib> class WeakSingleThreadedUtility { public: WeakSingleThreadedUtility() // To ensure that singleton will be constucted before utility : m_weak(SingletonWeak<Payload>::instance()) { } ~WeakSingleThreadedUtility() { // Sometimes this check may result as "false" even in case of incorrect usage, // and there's no way to guarantee a demonstration of undefined behaviour in colour if ( auto strong = m_weak.lock() ) for ( int i = 0; i < 100; ++i ) strong->add(i); } private: // A weak copy of smart pointer, not a reference std::weak_ptr<SingletonWeak<Payload>> m_weak; }; void cracker() { WeakSingleThreadedUtility(); } // 1. Register cracker() using std::atexit // 2. Create singleton // 3. Create utility auto reg = [](){ std::atexit(&cracker); return 0; }(); auto utility = WeakSingleThreadedUtility(); // This guarantee destruction in order: // - utility; // - singleton. // This order is correct. // Additionally, there's a copy of shared_ptr in the class instance... // ... but there was std::atexit registered before singleton, // so cracker() will be invoked after destruction of utility and singleton. // There's second try to create a singleton - and it's incorrect. int main() { return 0; }