如何使 function 线程安全

Question

This is the code where i would be inserting values in a unordererd map and would also query those values at regular intervals.

class MemoryMap
{
private:
    std::unordered_map<std::string, std::string> maps_;
    std::mutex mutex_;
    


public:
    void AddMap(std::string key, std::string value);
    std::string GetMap(std::string key);
    void PrintMemoryMap(std::string key);

};



void MemoryMap::AddMap(std::string key, std::string value)
{
    std::unique_lock<std::mutex> lock(mutex_);
    maps_[key] = value;
    
}

std::string MemoryMap::GetMap(std::string key)
{
    std::unique_lock<std::mutex> lock(mutex_);
    if (maps_.find(key) == maps_.end())
        return "";
    
        return maps_.at(key);
}

I would be using this object in two different threads and i want when insertion would be happening through AddMap function than GetMap function should wait for the insertion to finish. Also GetMap function would be called concurrently.

Is my current code sufficient to address this issue?

Answer 1

It is sufficient. The mutex lock guarantees at most one thread get call get or set at the same time.

However, your code might be not optimized if you want to achieve concurrent reads. In C++, unordered_map is a container, which has thread safety like this: https://en.cppreference.com/w/cpp/container#Thread_safety Two threads can safely call get at the same time because it is a constant function, if no thread is modifying the container.