java - Java 中的单例与 C++ 中的单例

Question

在 Java 中，我可以像这样创建一个单例(只要它不作为异常抛出):

private static Singleton m_this = new Singleton();

这非常方便，因为它本质上是线程安全的。

我可以用 C++ 做类似的事情吗？

Answer 1

一种使用线程安全初始化创建单例的方法，由 C++11 的标准保证。是:

class SomeSingleton {
  public:
    static SomeSingleton& instance() {
      static SomeSingleton instance_;
      return instance_;
    }

  private:
  SomeSingleton() {
    ...
  }
};

这是线程安全的，因为 local static variable initialization is thread-safe in C++11 .相关标准文件，N3485 , 在第 6.7.4 节中说:

such a variable is initialized the first time control passes through its declaration; such a variable is considered initialized upon the completion of its initialization. [...] If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.

带脚注:

The implementation must not introduce any deadlock around execution of the initializer.

您可以使用 CRTP 抽象成一个漂亮的模板基类:

//Singleton template definition
template <typename TDerived>
class Singleton {
  static_assert(is_base_of<Singleton, TDerived>::value, "Singleton: unexpected Derived template parameter");
  public:
    static TDerived& instance() {
      static TDerived instance_;
      return instance_;
    }

  protected:
  Singleton() {
  }
};

// SomeSingleton definition, using the Singleton template
class SomeSingleton : public Singleton<SomeSingleton> {
  ...
};