从std :: unique_ptr的STL容器中查找find（）的安全性

Question

Example code.

class Obj
{
    public:
    void doSome(void)
    {
        std::cout << "Hello World!" << std::endl;
    }
};

std::unordered_map<int, std::unique_ptr<Obj>> map;

// insert -- done with single thread and before find()
map[123] = std::move( std::unique_ptr<Obj>(new Obj) );

// find -- run from multiple threads
auto search = map.find(123);  // <=== (Q)
if (search != map.end())      
{
  search->second->doSome();
}

(Q)

How about the thread safty if there are multiple threads running //find section with map.find(123) ?

will map.find(123) always find the obj in every thread ? as long as the search->second not assigned to someone else?

Answer 1

When more than one thread accesses the same variable and at least one of them writes to it you have a data race. That's not the case here, where everyone is reading the same data. That's okay. There's another issue, though, which isn't addressed in this code: depending on when the data is stored into the map object, some threads might not see the updated version of the map object. The simplest way to deal with this synchronization problem is to set up the map object before creating any of the reader threads.

Answer 2

Neither find() , nor any other method in the unordered map is thread safe. If it's possible for one execution thread to call find() while any other thread calls any unordered map method that modifies it, this results in undefined behavior.

If multiple execution threads are calling find() with the same key, provided that there is no undefined behavior all execution threads will get the same value for that key.