如何以线程安全的方式迭代容器?

How to iterate over a container in a thread-safe way?(如何以线程安全的方式迭代容器?)

问题描述

我有一个容器 (C++)，我需要从不同的线程以两种方式对其进行操作:1) 添加和删除元素，以及 2) 迭代其成员.显然，在迭代发生时删除元素 = 灾难.代码如下所示:

I have a container (C++) on which I need to operate in two ways, from different threads: 1) Add and remove elements, and 2) iterate through its members. Clearly, remove element while iteration is happening = disaster. The code looks something like this:

class A
{
public:
   ...
   void AddItem(const T& item, int index) { /*Put item into my_stuff at index*/ }
   void RemoveItem(const T& item) { /*Take item out of m_stuff*/ }
   const list<T>& MyStuff() { return my_stuff; } //*Hate* this, but see class C
private:
   Mutex mutex; //Goes in the *Item methods, but is largely worthless in MyStuff()
   list<T> my_stuff; //Just as well a vector or deque
};
extern A a; //defined in the .cpp file

class B
{
   ...
   void SomeFunction() { ... a.RemoveItem(item); }
};

class C
{
   ...
   void IterateOverStuff()
   {
      const list<T>& my_stuff(a.MyStuff());
      for (list<T>::const_iterator it=my_stuff.begin(); it!=my_stuff.end(); ++it)
      {
          ...
      }
   }
};

再次，B::SomeFunction() 和 C::IterateOverStuff() 被异步调用.我可以使用什么数据结构来确保在迭代期间，my_stuff 受到保护"，不受添加或删除操作的影响?

Again, B::SomeFunction() and C::IterateOverStuff() are getting called asynchronously. What's a data structure I can use to ensure that during the iteration, my_stuff is 'protected' from add or remove operations?

推荐答案

听起来像一个 reader/writer需要锁.基本上，这个想法是您可能有 1 个或多个读者或单个作者.永远不能同时拥有读写锁.

sounds like a reader/writer lock is needed. Basically, the idea is that you may have 1 or more readers OR a single writer. Never can you have a read and write lock at the same time.

我认为适合您的设计的使用示例涉及进行小改动.将迭代"函数添加到拥有列表的类并使其模板化，以便您可以传递函数/仿函数来定义对每个节点执行的操作.像这样的东西(快速而肮脏的伪代码，但你明白了......):

An example of usage which I think fits your design involves making a small change. Add an "iterate" function to the class which owns the list and make it templated so you can pass a function/functor to define what to do for each node. Something like this (quick and dirty pseudo code, but you get the point...):

class A {
public:
    ...
    void AddItem(const T& item, int index) {
        rwlock.lock_write();
        // add the item
        rwlock.unlock_write();
    }

    void RemoveItem(const T& item) {
        rwlock.lock_write();
        // remove the item
        rwlock.unlock_write();
    }

    template <class P>
    void iterate_list(P pred) {
        rwlock.lock_read();
        std::for_each(my_stuff.begin(), my_stuff.end(), pred);
        rwlock.unlock_read();
    }

private:
    rwlock_t rwlock;
    list<T> my_stuff; //Just as well a vector or deque
};


extern A a; //defined in the .cpp file

class B {
    ...
    void SomeFunction() { ... a.RemoveItem(item); }
};

class C {
    ...

    void read_node(const T &element) { ... }

    void IterateOverStuff() {
        a.iterate_list(boost::bind(&C::read_node, this));
   }
};

另一个选项是让读写器锁公开访问，并让调用者负责正确使用锁.但这更容易出错.

Another Option would be to make the reader/writer lock publicly accessible and have the caller responsible for correctly using the lock. But that's more error prone.

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