Singleton实例声明为GetInstance方法的静态变量 [英] Singleton instance declared as static variable of GetInstance method

问题描述

我见过Singleton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;
}

我看到以下积极的方法：

I see following positive sides of this approach:

• 代码更简单，因为编译器只有在第一次调用GetInstance时才负责创建此对象。


• 代码更安全，因为没有其他方法可以引用实例，但是使用GetInstance方法，没有其他方法来更改实例，但是在GetInstance方法内。

这种方法的负面是什么（除了这不是很OOP-ish）？

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如果控制在变量初始化时并发执行声明，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：

• 线程安全。
  
  但是这是因为g ++明确添加了代码来保证它。

你有两个单身，他们在建造和破坏期间尝试和使用彼此。

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

阅读：
查找C ++静态初始化顺序问题

是如果从全局变量的析构函数访问单例。在这种情况下，singleton已经被销毁了，但是get方法仍然会返回对被销毁的对象的引用。

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.

这里有一些方法，值得做。只要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.

一个更安全的定义，但是很丑陋：

我相信你可以添加一些合适的宏整理这个

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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