单例实例声明为 GetInstance 方法的静态变量，它是线程安全的吗?

2021-12-22 00:00:00 design-patterns singleton c++

我见过单例模式的实现，其中实例变量在 GetInstance 方法中被声明为静态变量.像这样:

I've seen implementations of Singleton patterns where instance variable was declared as static variable in GetInstance method. Like this:

SomeBaseClass &SomeClass::GetInstance()
{
   static SomeClass instance;
   return instance;
}

我认为这种方法有以下积极方面:

I see following positive sides of this approach:

  • 代码更简单，因为只有在第一次调用 GetInstance 时，才由编译器负责创建此对象.
  • 代码更安全，因为没有其他方法可以获取对实例的引用，而是使用 GetInstance 方法并且没有其他方法可以更改实例，而是在 GetInstance 方法中.

这种方法的缺点是什么(除了这不是非常面向对象编程)?这是线程安全的吗?

What are the negative sides of this approach (except that this is not very OOP-ish) ? Is this thread-safe?

推荐答案

在 C++11 中它是线程安全的:

In C++11 it is thread safe:

§6.7 [stmt.dcl] p4 如果在初始化变量的同时控制进入声明，则并发执行将等待初始化完成.

§6.7 [stmt.dcl] p4 If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.

在 C++03 中:

  • 在 g++ 下，它是线程安全的.
    但这是因为 g++ 明确添加了代码来保证它.

一个问题是，如果您有两个单例，并且它们在构造和销毁过程中尝试互相使用.

One problem is that if you have two singletons and they try and use each other during construction and destruction.

请阅读:查找 C++ 静态初始化顺序问题

这个问题的一个变体是从全局变量的析构函数访问单例.在这种情况下，单例肯定已经被销毁了，但是 get 方法仍然会返回对被销毁对象的引用.

A variation on this problem is if the singleton is accessed from the destructor of a global variable. In this situation the singleton has definitely been destroyed, but the get method will still return a reference to the destroyed object.

有很多方法可以解决这个问题，但它们很乱，不值得做.只是不要从全局变量的析构函数中访问单例.

There are ways around this but they are messy and not worth doing. Just don't access a singleton from the destructor of a global variable.

更安全但丑陋的定义:
我相信你可以添加一些适当的宏来整理这个

A Safer definition but ugly:
I am sure you can add some appropriate macros to tidy this up

SomeBaseClass &SomeClass::GetInstance()
{
#ifdef _WIN32 
Start Critical Section Here
#elif  defined(__GNUC__) && (__GNUC__ > 3)
// You are OK
#else
#error Add Critical Section for your platform
#endif

    static SomeClass instance;

#ifdef _WIN32
END Critical Section Here
#endif 

    return instance;
}

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