迈耶斯对单例的实现如何实际上是单例

2021-12-22 00:00:00 design-patterns singleton c++ c++11

我已经阅读了很多关于单身人士，何时应该和不应该使用它们，以及如何安全地实现它们.我正在用 C++11 编写，并且遇到了 Meyer 对单例的惰性初始化实现，如 这个问题.

I have been reading a lot about Singletons, when they should and shouldn't be used, and how to implement them safely. I am writing in C++11, and have come across the Meyer's lazy initialized implementation of a singleton, as seen in this question.

这个实现是:

static Singleton& instance()
{
     static Singleton s;
     return s;
}

我从 SO 上的其他问题中了解到这是线程安全的，但我不明白这实际上是一个单例模式.我已经用其他语言实现了单例，这些总是像来自 维基百科的这个例子一样结束:

I understand how this is thread safe from other questions here on SO, but what I don't understand is how this is actually a singleton pattern. I have implemented singletons in other languages, and these always end up something like this example from Wikipedia:

public class SingletonDemo {
        private static volatile SingletonDemo instance = null;

        private SingletonDemo() {       }

        public static SingletonDemo getInstance() {
                if (instance == null) {
                        synchronized (SingletonDemo .class){
                                if (instance == null) {
                                        instance = new SingletonDemo ();
                                }
                      }
                }
                return instance;
        }
}

当我查看第二个示例时，非常直观地看出这是一个单例，因为该类持有对自身的一个实例的引用，并且只返回该实例.但是，在第一个示例中，我不明白这如何防止对象的两个实例存在.所以我的问题是:

When I look at this second example, it is very intuitive how this is a singleton, since the class holds a reference to one instance of itself, and only ever returns that instance. However, in the first example, I don't understand how this prevents there ever existing two instances of the object. So my questions are:

  1. 第一个实现如何强制执行单例模式?我认为这与 static 关键字有关，但我希望有人能向我深入解释幕后发生的事情.
  2. 在这两种实现方式之间，一种优于另一种吗?利弊是什么?

感谢您的帮助，

推荐答案

这是一个单例，因为函数 local 的 static 存储持续时间意味着程序中只存在该 local 的一个实例.

This is a singleton because static storage duration for a function local means that only one instance of that local exists in the program.

在幕后，这可以粗略地认为等同于以下 C++98(甚至可能被编译器像这样模糊地实现):

Under the hood, this can very roughly be considered to be equivalent to the following C++98 (and might even be implemented vaguely like this by a compiler):

static bool __guard = false;
static char __storage[sizeof(Singleton)]; // also align it

Singleton& Instance() {
  if (!__guard ) {
    __guard = true;
    new (__storage) Singleton();
  }
  return *reinterpret_cast<Singleton*>(__storage);
}

// called automatically when the process exits
void __destruct() {
  if (__guard)
    reinterpret_cast<Singleton*>(__storage)->~Singleton();
}

线程安全位使它变得有点复杂，但本质上是一样的.

The thread safety bits make it get a bit more complicated, but it's essentially the same thing.

查看 C++11 的实际实现，每个静态(如上面的布尔值)都有一个保护变量，它也用于屏障和线程.查看 Clang 的 AMD64 输出:

Looking at an actual implementation for C++11, there is a guard variable for each static (like the boolean above), which is also used for barriers and threads. Look at Clang's AMD64 output for:

Singleton& instance() {
   static Singleton instance;
   return instance;
}

instance 的 AMD64 程序集，来自 Ubuntu's Clang 3.0 on AMD64 at -O1(由 http://gcc 提供).Godbolt.org/ 是:

The AMD64 assembly for instance from Ubuntu's Clang 3.0 on AMD64 at -O1 (courtesy of http://gcc.godbolt.org/ is:

instance():                           # @instance()
  pushq %rbp
  movq  %rsp, %rbp
  movb  guard variable for instance()::instance(%rip), %al
  testb %al, %al
  jne   .LBB0_3
  movl  guard variable for instance()::instance, %edi
  callq __cxa_guard_acquire
  testl %eax, %eax
  je    .LBB0_3
  movl  instance()::instance, %edi
  callq Singleton::Singleton()
  movl  guard variable for instance()::instance, %edi
  callq __cxa_guard_release
.LBB0_3:
  movl  instance()::instance, %eax
  popq  %rbp
  ret

可以看到它引用了一个全局守卫来查看是否需要初始化，使用__cxa_guard_acquire，再次测试初始化??，等等.除了使用 AMD64 程序集和 中指定的符号/布局外，几乎所有方面都与您从维基百科发布的版本类似安腾 ABI.

You can see that it references a global guard to see if initialization is required, uses __cxa_guard_acquire, tests the initialization again, and so on. Exactly in almost every way like version you posted from Wikipedia, except using AMD64 assembly and the symbols/layout specified in the Itanium ABI.

请注意，如果您运行该测试，您应该为 Singleton 提供一个非平凡的构造函数，因此它不是 POD，否则优化器将意识到执行所有这些保护/锁定工作毫无意义.

Note that if you run that test you should give Singleton a non-trivial constructor so it's not a POD, otherwise the optimizer will realize that there's no point to doing all that guard/locking work.

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