线程安全单例实现在C ++中

问题描述

以下是C ++中公知的单例模式的实现。

但是，我不完全确定它的线程安全性。

根据类似问题的答案以前在这里问过，它似乎是线程安全的。

是这样吗？

  //奇怪的循环模板模式
 //将类与其Singleton-ness（几乎）分开。 
 #include< iostream> 
使用命名空间std; 
 
模板< class T> Singleton课程{
 Singleton（const Singleton&）; 
 Singleton& operator =（const Singleton&）; 
 protected：
 Singleton（）{} 
 virtual〜Singleton（）{} 
 public：
 static T& instance（）{
 static T theInstance; 
 return theInstance; 
} 
}; 
 
 //要创建为Singleton的样本类
 class MyClass：public Singleton< MyClass> {
 int x; 
 protected：
 friend class Singleton< MyClass> ;; 
 MyClass（）{x = 0; } 
 public：
 void setValue（int n）{x = n; } 
 int getValue（）const {return x; } 
}; 
  

解决方案

不，这不是线程安全的，因为静态本地没有任何保护。默认情况下，静态本地不是线程安全的。这意味着您可能遇到以下问题

• 单身份的构造方法多次运行


• 对static的赋值不能保证是原子的，因此您可以在多线程方案中看到部分赋值。


• 可能还有一些我失踪了。

这是Raymond Chen的详细博客条目，为什么默认情况下C ++静态不是线程安全的。

The following is a well known implementation of singleton pattern in C++.
However, I'm not entirely sure whether its thread-safe.
Based upon answers to similar question asked here previously, it seems it is thread safe.
Is that so?

//Curiously Recurring Template Pattern
//Separates a class from its Singleton-ness (almost).
#include <iostream>
using namespace std;

template<class T> class Singleton {
  Singleton(const Singleton&);
  Singleton& operator=(const Singleton&);
protected:
  Singleton() {}
  virtual ~Singleton() {}
public:
  static T& instance() {
    static T theInstance;
    return theInstance;
  }
};

// A sample class to be made into a Singleton
class MyClass : public Singleton<MyClass> {
 int x;
protected:
  friend class Singleton<MyClass>;
  MyClass() { x = 0; }
public:
 void setValue(int n) { x = n; }
  int getValue() const { return x; }
};

解决方案

No, this is not thread safe because the static local is not guarded in any way. By default a static local is not thread safe. This means you could run into the following issues

• Constructor for the singleton runs more than once
• The assignment to the static is not guaranteed to be atomic hence you could see a partial assignment in multi-threaded scenarios
• Probably a few more that I'm missing.

Here is a detailed blog entry by Raymond Chen on why C++ statics are not thread safe by default.

• http://blogs.msdn.com/oldnewthing/archive/2004/03/08/85901.aspx

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