C++学习笔记-Singletons

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#include <iostream>

class Random
{
public:
    Random(const Random&) = delete;

    static Random& GetInstance()
    {
        static Random instance;
        return instance;
    }

    /* 这样你可以直接Random::Float()的方式调用，而不需要调用GetInstance() */
    static float Float() { return GetInstance().FloatImpl(); }
    
private:
    Random() {};
    float FloatImpl() { return 0.5f; } // 模拟返回一个float随机数
};

void BasicSingletonDemo()
{
    std::cout << "=== Basic Singleton Demo ===" << std::endl;
    
    // Random instance = Random::GetInstance(); // 删除了拷贝构造函数，所以这么赋值会报错为拷贝赋值相当于拷贝了一个实例，这样就不是单例了。
    float value = Random::Float();
    std::cout << "Random value: " << value << std::endl;
    
    // 验证单例性质
    Random& instance1 = Random::GetInstance();
    Random& instance2 = Random::GetInstance();
    
    std::cout << "Instance1 address: " << &instance1 << std::endl;
    std::cout << "Instance2 address: " << &instance2 << std::endl;
    std::cout << "Same instance: " << (&instance1 == &instance2 ? "Yes" : "No") << std::endl;
}

int main()
{
    BasicSingletonDemo();
    return 0;
}

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#include <iostream>
#include <memory>
#include <mutex>

// 1. 懒汉式单例（线程不安全）
class LazySingleton
{
private:
    static LazySingleton* instance;
    LazySingleton() { std::cout << "LazySingleton created" << std::endl; }
    
public:
    static LazySingleton* GetInstance()
    {
        if (instance == nullptr)
        {
            instance = new LazySingleton();
        }
        return instance;
    }
    
    void DoSomething() { std::cout << "LazySingleton doing something" << std::endl; }
    
    // 注意：这个实现没有处理内存释放
};

LazySingleton* LazySingleton::instance = nullptr;

// 2. 饿汉式单例（线程安全）
class EagerSingleton
{
private:
    static EagerSingleton instance;
    EagerSingleton() { std::cout << "EagerSingleton created" << std::endl; }
    
public:
    static EagerSingleton& GetInstance()
    {
        return instance;
    }
    
    void DoSomething() { std::cout << "EagerSingleton doing something" << std::endl; }
};

EagerSingleton EagerSingleton::instance; // 程序启动时创建

// 3. 线程安全的懒汉式单例
class ThreadSafeSingleton
{
private:
    static ThreadSafeSingleton* instance;
    static std::mutex mutex_;
    ThreadSafeSingleton() { std::cout << "ThreadSafeSingleton created" << std::endl; }
    
public:
    static ThreadSafeSingleton* GetInstance()
    {
        std::lock_guard<std::mutex> lock(mutex_);
        if (instance == nullptr)
        {
            instance = new ThreadSafeSingleton();
        }
        return instance;
    }
    
    void DoSomething() { std::cout << "ThreadSafeSingleton doing something" << std::endl; }
};

ThreadSafeSingleton* ThreadSafeSingleton::instance = nullptr;
std::mutex ThreadSafeSingleton::mutex_;

// 4. 双重检查锁定（Double-Checked Locking）
class DoubleCheckedSingleton
{
private:
    static DoubleCheckedSingleton* instance;
    static std::mutex mutex_;
    DoubleCheckedSingleton() { std::cout << "DoubleCheckedSingleton created" << std::endl; }
    
public:
    static DoubleCheckedSingleton* GetInstance()
    {
        if (instance == nullptr)
        {
            std::lock_guard<std::mutex> lock(mutex_);
            if (instance == nullptr)
            {
                instance = new DoubleCheckedSingleton();
            }
        }
        return instance;
    }
    
    void DoSomething() { std::cout << "DoubleCheckedSingleton doing something" << std::endl; }
};

DoubleCheckedSingleton* DoubleCheckedSingleton::instance = nullptr;
std::mutex DoubleCheckedSingleton::mutex_;

// 5. Meyer's Singleton（推荐）
class MeyersSingleton
{
private:
    MeyersSingleton() { std::cout << "MeyersSingleton created" << std::endl; }
    
public:
    static MeyersSingleton& GetInstance()
    {
        static MeyersSingleton instance; // C++11保证线程安全
        return instance;
    }
    
    // 禁止拷贝和赋值
    MeyersSingleton(const MeyersSingleton&) = delete;
    MeyersSingleton& operator=(const MeyersSingleton&) = delete;
    
    void DoSomething() { std::cout << "MeyersSingleton doing something" << std::endl; }
};

// 6. 使用智能指针的单例
class SmartPtrSingleton
{
private:
    static std::unique_ptr<SmartPtrSingleton> instance;
    static std::once_flag initialized;
    SmartPtrSingleton() { std::cout << "SmartPtrSingleton created" << std::endl; }
    
public:
    static SmartPtrSingleton& GetInstance()
    {
        std::call_once(initialized, []() {
            instance = std::unique_ptr<SmartPtrSingleton>(new SmartPtrSingleton());
        });
        return *instance;
    }
    
    SmartPtrSingleton(const SmartPtrSingleton&) = delete;
    SmartPtrSingleton& operator=(const SmartPtrSingleton&) = delete;
    
    void DoSomething() { std::cout << "SmartPtrSingleton doing something" << std::endl; }
};

std::unique_ptr<SmartPtrSingleton> SmartPtrSingleton::instance = nullptr;
std::once_flag SmartPtrSingleton::initialized;

void SingletonImplementationsDemo()
{
    std::cout << "=== Singleton Implementations Demo ===" << std::endl;
    
    std::cout << "\n1. Lazy Singleton:" << std::endl;
    LazySingleton::GetInstance()->DoSomething();
    
    std::cout << "\n2. Eager Singleton:" << std::endl;
    EagerSingleton::GetInstance().DoSomething();
    
    std::cout << "\n3. Thread Safe Singleton:" << std::endl;
    ThreadSafeSingleton::GetInstance()->DoSomething();
    
    std::cout << "\n4. Double Checked Singleton:" << std::endl;
    DoubleCheckedSingleton::GetInstance()->DoSomething();
    
    std::cout << "\n5. Meyer's Singleton:" << std::endl;
    MeyersSingleton::GetInstance().DoSomething();
    
    std::cout << "\n6. Smart Pointer Singleton:" << std::endl;
    SmartPtrSingleton::GetInstance().DoSomething();
}

int main()
{
    SingletonImplementationsDemo();
    return 0;
}

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#include <iostream>
#include <string>
#include <fstream>
#include <vector>
#include <map>
#include <chrono>
#include <iomanip>
#include <sstream>

// 日志管理器单例
class Logger
{
public:
    enum class Level { DEBUG, INFO, WARNING, ERROR };
    
private:
    std::ofstream logFile;
    Level currentLevel;
    
    Logger() : currentLevel(Level::INFO)
    {
        logFile.open("application.log", std::ios::app);
    }
    
public:
    static Logger& GetInstance()
    {
        static Logger instance;
        return instance;
    }
    
    Logger(const Logger&) = delete;
    Logger& operator=(const Logger&) = delete;
    
    ~Logger()
    {
        if (logFile.is_open())
        {
            logFile.close();
        }
    }
    
    void SetLevel(Level level) { currentLevel = level; }
    
    void Log(Level level, const std::string& message)
    {
        if (level >= currentLevel)
        {
            auto now = std::chrono::system_clock::now();
            auto time_t = std::chrono::system_clock::to_time_t(now);
            
            std::stringstream ss;
            ss << std::put_time(std::localtime(&time_t), "%Y-%m-%d %H:%M:%S");
            
            std::string levelStr = LevelToString(level);
            std::string logEntry = "[" + ss.str() + "] " + levelStr + ": " + message;
            
            std::cout << logEntry << std::endl;
            if (logFile.is_open())
            {
                logFile << logEntry << std::endl;
                logFile.flush();
            }
        }
    }
    
    void Debug(const std::string& message) { Log(Level::DEBUG, message); }
    void Info(const std::string& message) { Log(Level::INFO, message); }
    void Warning(const std::string& message) { Log(Level::WARNING, message); }
    void Error(const std::string& message) { Log(Level::ERROR, message); }
    
private:
    std::string LevelToString(Level level)
    {
        switch (level)
        {
            case Level::DEBUG: return "DEBUG";
            case Level::INFO: return "INFO";
            case Level::WARNING: return "WARNING";
            case Level::ERROR: return "ERROR";
            default: return "UNKNOWN";
        }
    }
};

// 配置管理器单例
class ConfigManager
{
private:
    std::map<std::string, std::string> config;
    
    ConfigManager()
    {
        LoadConfig();
    }
    
    void LoadConfig()
    {
        // 模拟从文件加载配置
        config["database_host"] = "localhost";
        config["database_port"] = "5432";
        config["max_connections"] = "100";
        config["log_level"] = "INFO";
        config["cache_size"] = "1024";
    }
    
public:
    static ConfigManager& GetInstance()
    {
        static ConfigManager instance;
        return instance;
    }
    
    ConfigManager(const ConfigManager&) = delete;
    ConfigManager& operator=(const ConfigManager&) = delete;
    
    std::string GetString(const std::string& key, const std::string& defaultValue = "")
    {
        auto it = config.find(key);
        return (it != config.end()) ? it->second : defaultValue;
    }
    
    int GetInt(const std::string& key, int defaultValue = 0)
    {
        auto value = GetString(key);
        return value.empty() ? defaultValue : std::stoi(value);
    }
    
    void SetString(const std::string& key, const std::string& value)
    {
        config[key] = value;
    }
    
    void SetInt(const std::string& key, int value)
    {
        config[key] = std::to_string(value);
    }
    
    void PrintAll()
    {
        std::cout << "Configuration:" << std::endl;
        for (const auto& [key, value] : config)
        {
            std::cout << "  " << key << " = " << value << std::endl;
        }
    }
};

// 数据库连接池单例
class DatabasePool
{
private:
    std::vector<std::string> availableConnections;
    std::vector<std::string> usedConnections;
    int maxConnections;
    
    DatabasePool()
    {
        maxConnections = ConfigManager::GetInstance().GetInt("max_connections", 10);
        InitializePool();
    }
    
    void InitializePool()
    {
        for (int i = 0; i < maxConnections; ++i)
        {
            availableConnections.push_back("Connection_" + std::to_string(i));
        }
        Logger::GetInstance().Info("Database pool initialized with " + std::to_string(maxConnections) + " connections");
    }
    
public:
    static DatabasePool& GetInstance()
    {
        static DatabasePool instance;
        return instance;
    }
    
    DatabasePool(const DatabasePool&) = delete;
    DatabasePool& operator=(const DatabasePool&) = delete;
    
    std::string GetConnection()
    {
        if (availableConnections.empty())
        {
            Logger::GetInstance().Warning("No available database connections");
            return "";
        }
        
        std::string connection = availableConnections.back();
        availableConnections.pop_back();
        usedConnections.push_back(connection);
        
        Logger::GetInstance().Debug("Acquired connection: " + connection);
        return connection;
    }
    
    void ReleaseConnection(const std::string& connection)
    {
        auto it = std::find(usedConnections.begin(), usedConnections.end(), connection);
        if (it != usedConnections.end())
        {
            usedConnections.erase(it);
            availableConnections.push_back(connection);
            Logger::GetInstance().Debug("Released connection: " + connection);
        }
    }
    
    void PrintStatus()
    {
        std::cout << "Database Pool Status:" << std::endl;
        std::cout << "  Available: " << availableConnections.size() << std::endl;
        std::cout << "  In use: " << usedConnections.size() << std::endl;
        std::cout << "  Total: " << maxConnections << std::endl;
    }
};

// 缓存管理器单例
template<typename Key, typename Value>
class CacheManager
{
private:
    std::map<Key, Value> cache;
    size_t maxSize;
    
    CacheManager()
    {
        maxSize = ConfigManager::GetInstance().GetInt("cache_size", 100);
    }
    
public:
    static CacheManager& GetInstance()
    {
        static CacheManager instance;
        return instance;
    }
    
    CacheManager(const CacheManager&) = delete;
    CacheManager& operator=(const CacheManager&) = delete;
    
    void Put(const Key& key, const Value& value)
    {
        if (cache.size() >= maxSize)
        {
            // 简单的LRU：删除第一个元素
            cache.erase(cache.begin());
        }
        cache[key] = value;
        Logger::GetInstance().Debug("Cache put: key added");
    }
    
    bool Get(const Key& key, Value& value)
    {
        auto it = cache.find(key);
        if (it != cache.end())
        {
            value = it->second;
            Logger::GetInstance().Debug("Cache hit: key found");
            return true;
        }
        Logger::GetInstance().Debug("Cache miss: key not found");
        return false;
    }
    
    void Clear()
    {
        cache.clear();
        Logger::GetInstance().Info("Cache cleared");
    }
    
    size_t Size() const { return cache.size(); }
    size_t MaxSize() const { return maxSize; }
};

void PracticalApplicationsDemo()
{
    std::cout << "=== Practical Applications Demo ===" << std::endl;
    
    // 配置管理
    std::cout << "\n1. Configuration Manager:" << std::endl;
    auto& config = ConfigManager::GetInstance();
    config.PrintAll();
    
    // 日志管理
    std::cout << "\n2. Logger:" << std::endl;
    auto& logger = Logger::GetInstance();
    logger.SetLevel(Logger::Level::DEBUG);
    logger.Info("Application started");
    logger.Debug("Debug information");
    logger.Warning("This is a warning");
    logger.Error("An error occurred");
    
    // 数据库连接池
    std::cout << "\n3. Database Pool:" << std::endl;
    auto& dbPool = DatabasePool::GetInstance();
    dbPool.PrintStatus();
    
    auto conn1 = dbPool.GetConnection();
    auto conn2 = dbPool.GetConnection();
    dbPool.PrintStatus();
    
    dbPool.ReleaseConnection(conn1);
    dbPool.PrintStatus();
    
    // 缓存管理
    std::cout << "\n4. Cache Manager:" << std::endl;
    auto& cache = CacheManager<std::string, std::string>::GetInstance();
    
    cache.Put("user:123", "John Doe");
    cache.Put("user:456", "Jane Smith");
    
    std::string value;
    if (cache.Get("user:123", value))
    {
        std::cout << "Found in cache: " << value << std::endl;
    }
    
    if (!cache.Get("user:789", value))
    {
        std::cout << "Not found in cache: user:789" << std::endl;
    }
    
    std::cout << "Cache size: " << cache.Size() << "/" << cache.MaxSize() << std::endl;
}

int main()
{
    PracticalApplicationsDemo();
    return 0;
}

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#include <iostream>
#include <memory>
#include <functional>

// 单例的问题演示
class ProblematicSingleton
{
private:
    static int instanceCount;
    
public:
    ProblematicSingleton() { instanceCount++; }
    static ProblematicSingleton& GetInstance()
    {
        static ProblematicSingleton instance;
        return instance;
    }
    
    static int GetInstanceCount() { return instanceCount; }
    void DoSomething() { std::cout << "Doing something..." << std::endl; }
};

int ProblematicSingleton::instanceCount = 0;

// 替代方案1：依赖注入
class Service
{
public:
    virtual ~Service() = default;
    virtual void DoWork() = 0;
};

class ConcreteService : public Service
{
public:
    void DoWork() override
    {
        std::cout << "ConcreteService doing work" << std::endl;
    }
};

class Client
{
private:
    std::shared_ptr<Service> service;
    
public:
    Client(std::shared_ptr<Service> svc) : service(svc) {}
    
    void Execute()
    {
        service->DoWork();
    }
};

// 替代方案2：静态类
class StaticUtility
{
public:
    static void DoUtilityWork()
    {
        std::cout << "Static utility doing work" << std::endl;
    }
    
    static int Calculate(int a, int b)
    {
        return a + b;
    }
    
private:
    StaticUtility() = delete; // 禁止实例化
};

// 替代方案3：命名空间
namespace UtilityNamespace
{
    void DoWork()
    {
        std::cout << "Namespace utility doing work" << std::endl;
    }
    
    int Calculate(int a, int b)
    {
        return a * b;
    }
}

// 替代方案4：工厂模式
class Factory
{
public:
    static std::unique_ptr<Service> CreateService()
    {
        return std::make_unique<ConcreteService>();
    }
};

// 替代方案5：全局对象（谨慎使用）
class GlobalService
{
public:
    void DoWork() { std::cout << "Global service doing work" << std::endl; }
};

GlobalService g_globalService; // 全局实例

void SingletonProblemsDemo()
{
    std::cout << "=== Singleton Problems Demo ===" << std::endl;
    
    std::cout << "Singleton problems:" << std::endl;
    std::cout << "1. Global state - makes testing difficult" << std::endl;
    std::cout << "2. Hidden dependencies - not clear from function signatures" << std::endl;
    std::cout << "3. Tight coupling - hard to replace or mock" << std::endl;
    std::cout << "4. Thread safety concerns" << std::endl;
    std::cout << "5. Destruction order issues" << std::endl;
    std::cout << "6. Violates Single Responsibility Principle" << std::endl;
    
    // 演示隐藏依赖问题
    auto& singleton = ProblematicSingleton::GetInstance();
    singleton.DoSomething(); // 不清楚这个函数依赖什么
}

void AlternativesDemo()
{
    std::cout << "\n=== Alternatives Demo ===" << std::endl;
    
    // 1. 依赖注入
    std::cout << "1. Dependency Injection:" << std::endl;
    auto service = std::make_shared<ConcreteService>();
    Client client(service);
    client.Execute(); // 依赖关系明确
    
    // 2. 静态类
    std::cout << "\n2. Static Class:" << std::endl;
    StaticUtility::DoUtilityWork();
    int result = StaticUtility::Calculate(5, 3);
    std::cout << "Static calculation result: " << result << std::endl;
    
    // 3. 命名空间
    std::cout << "\n3. Namespace:" << std::endl;
    UtilityNamespace::DoWork();
    int result2 = UtilityNamespace::Calculate(5, 3);
    std::cout << "Namespace calculation result: " << result2 << std::endl;
    
    // 4. 工厂模式
    std::cout << "\n4. Factory Pattern:" << std::endl;
    auto factoryService = Factory::CreateService();
    factoryService->DoWork();
    
    // 5. 全局对象
    std::cout << "\n5. Global Object:" << std::endl;
    g_globalService.DoWork();
}

// 何时使用单例的指导原则
void SingletonGuidelinesDemo()
{
    std::cout << "\n=== Singleton Guidelines ===" << std::endl;
    
    std::cout << "When to use Singleton:" << std::endl;
    std::cout << "1. Truly need only one instance (e.g., hardware interface)" << std::endl;
    std::cout << "2. Global access point is necessary" << std::endl;
    std::cout << "3. Lazy initialization is important" << std::endl;
    std::cout << "4. Thread safety is handled properly" << std::endl;
    
    std::cout << "\nWhen NOT to use Singleton:" << std::endl;
    std::cout << "1. Just for convenience (use dependency injection)" << std::endl;
    std::cout << "2. When you need multiple instances in tests" << std::endl;
    std::cout << "3. When the class has multiple responsibilities" << std::endl;
    std::cout << "4. When global state is not actually needed" << std::endl;
    
    std::cout << "\nBetter alternatives:" << std::endl;
    std::cout << "1. Dependency injection for testability" << std::endl;
    std::cout << "2. Static functions for stateless utilities" << std::endl;
    std::cout << "3. Factory pattern for object creation" << std::endl;
    std::cout << "4. Service locator pattern for service discovery" << std::endl;
}

int main()
{
    SingletonProblemsDemo();
    AlternativesDemo();
    SingletonGuidelinesDemo();
    
    return 0;
}