How to prevent multiple initializations of a singleton in C++ with threading?
Quick question that's been bugging me - I'm working on a C++ application using C++17 and I've implemented a singleton pattern for a configuration manager... The class is intended to load configurations from a file, but I'm running into issues when multiple threads attempt to access the singleton instance simultaneously. I get a segmentation fault when the instance is being initialized while another thread tries to access it. Here’s my current implementation: ```cpp #include <iostream> #include <mutex> #include <memory> class ConfigManager { private: static std::unique_ptr<ConfigManager> instance; static std::mutex mutex; std::string configValue; ConfigManager() { // Simulate loading configuration from a file configValue = "Loaded Configuration"; std::cout << "ConfigManager initialized." << std::endl; } public: static ConfigManager* getInstance() { if (!instance) { std::lock_guard<std::mutex> lock(mutex); if (!instance) { instance.reset(new ConfigManager()); } } return instance.get(); } std::string getConfigValue() { return configValue; } }; std::unique_ptr<ConfigManager> ConfigManager::instance = nullptr; std::mutex ConfigManager::mutex; void threadFunction() { ConfigManager* config = ConfigManager::getInstance(); std::cout << config->getConfigValue() << std::endl; } int main() { std::thread t1(threadFunction); std::thread t2(threadFunction); t1.join(); t2.join(); return 0; } ``` I've tried using a double-checked locking mechanism to ensure that the instance is created only once, but I'm still encountering issues. The segmentation fault occurs occasionally, which makes me suspect that the mutex is not being applied correctly or that there's a race condition. I've also checked that the instance is not being accessed before it’s initialized, but the error persists. What would be the best way to ensure that my singleton instance is thread-safe during initialization? Are there any common pitfalls with using unique_ptr in this context? Any suggestions would be appreciated! This is part of a larger API I'm building.