C++多线程环境下的单例类对象创建

news2024/9/17 8:27:39

使用C++无锁编程实现多线程下的单例模式

贺志国
2023.8.1

一、尺寸较小的类单例对象创建

如果待创建的单例类SingletonForMultithread内包含的成员变量较少，整个类占用的内存空间较小，则可以使用如下方法来创建单例对象（如果类的尺寸较大则不允许,64位Linux系统默认栈的最大空间为8 MB，64位Windows系统默认栈的最大空间为1 MB）：

class SmallSingletonForMultithread {
 public:
  static SmallSingletonForMultithread& GetInstance() {
    static SmallSingletonForMultithread instance;
    return instance;
  }
  
 private:
  SmallSingletonForMultithread() = default;
  ~SmallSingletonForMultithread() = default;

  SmallSingletonForMultithread(const SmallSingletonForMultithread&) = delete;
  SmallSingletonForMultithread& operator=(const SmallSingletonForMultithread&) = delete;
  SmallSingletonForMultithread(SmallSingletonForMultithread&&) = delete;
  SmallSingletonForMultithread& operator=(SmallSingletonForMultithread&&) = delete;
};

二、尺寸较大的类单例对象创建（要求显式调用销毁函数来避免内存泄漏）

在实际工作中，由于某些单例类的尺寸较大，静态变量存储栈区无法容纳该单例对象，因此无法使用上述方法来创建单例对象，这时需要使用new在堆区动态创建单例对象。为了避免多线程环境下对于单例对象的抢夺，可使用C++无锁编程来实现。需要付出的代价就是，最后一个调用者需要显式地调用销毁函数DestoryInstance来避免内存泄漏，示例代码如下所示：

#include <atomic>
#include <cassert>
#include <mutex>

class SingletonForMultithread {
 public:
  static SingletonForMultithread* GetInstance() {
    if (!instance_.load(std::memory_order_acquire)) {
      instance_.store(new SingletonForMultithread, std::memory_order_release);
    }

    return instance_.load(std::memory_order_relaxed);
  }

  static void DestoryInstance() {
    if (instance_.load(std::memory_order_acquire)) {
      auto* ptr = instance_.exchange(nullptr, std::memory_order_release);
      delete ptr;
      ptr = nullptr;
    }
  }

 private:
  SingletonForMultithread() = default;
  ~SingletonForMultithread() = default;

  SingletonForMultithread(const SingletonForMultithread&) = delete;
  SingletonForMultithread& operator=(const SingletonForMultithread&) = delete;
  SingletonForMultithread(SingletonForMultithread&&) = delete;
  SingletonForMultithread& operator=(SingletonForMultithread&&) = delete;

 private:
  static std::atomic<SingletonForMultithread*> instance_;
};

std::atomic<SingletonForMultithread*> SingletonForMultithread::instance_;

int main() {
  auto* singleton = SingletonForMultithread::GetInstance();
  assert(singleton != nullptr);

  singleton->DestoryInstance();

  return 0;
}

三、尺寸较大的类单例对象创建（不用显式调用销毁函数来避免内存泄漏）

很多时候，我们无法显式地调用销毁函数来避免内存泄漏，这时就可借助std::unique_ptr<T>和std::call_once来实现，示例代码如下：

#include <cassert>
#include <memory>
#include <mutex>

class SingletonForMultithread {
 public:
  ~SingletonForMultithread() = default;

  static SingletonForMultithread* GetInstance() {
    static std::unique_ptr<SingletonForMultithread> instance;
    static std::once_flag only_once;

    std::call_once(only_once,
                   []() { instance.reset(new SingletonForMultithread); });

    return instance.get();
  }

 private:
  SingletonForMultithread() = default;

  SingletonForMultithread(const SingletonForMultithread&) = delete;
  SingletonForMultithread& operator=(const SingletonForMultithread&) = delete;
  SingletonForMultithread(SingletonForMultithread&&) = delete;
  SingletonForMultithread& operator=(SingletonForMultithread&&) = delete;
};

int main() {
  auto* singleton = SingletonForMultithread::GetInstance();
  assert(singleton != nullptr);

  return 0;
}