Should I use lock_guard, scoped_lock or unique_lock in this situation?

Question

I have read many of the questions already answered that relate to this but none of them gave me a clear understanding of which I should be using when I have multiple writers but a single reader. The code below is a contrived example of what I'm talking about.

struct StateInfo {
    long wd{};
    uint32_t perc{};
};

class Blah 
{
    const int numDevices = getDevices();
    std::shared_mutex sharedMutexSI_;
    vector<StateInfo> stateInfo;
public:

    Blah() : stateInfo(numDevices){};

    void writers(StateInfo &newSi, const int i)
    {
        std::shared_lock<std::shared_mutex> _MULTIPLE(sharedMutexSI_);
        stateInfo[i] = newSi;
    }

    StateInfo reader(const int i)
    {
        std::lock_guard<std::shared_mutex> _EXCLUSIVE(sharedMutexSI_);
        return stateInfo[i];
    }
};

The situation is that multiple writers may simultaneously update the stateInfo vector but never the same item in the vector as i is unique to each thread. A single reader thread may at any time try to read any of the vector items.

1. Is the above code correct in avoiding race conditions?

2. Is lock_guard the right one to use or should I use scoped_lock or unique_lock?

Answer 1

To summarize what was already written in comments:

Yes, the code is correct. However, it may be inefficient, because it disallows reading from any array element while writing to another array element. You might want to do more fine-grained synchronization by using a mutex for each array element.

class Blah 
{
    const int numDevices = getDevices();
    std::vector<std::mutex> mutexes;
    std::vector<StateInfo> stateInfo;
public:

    Blah() : mutexes(numDevices), stateInfo(numDevices){}

    void writers(StateInfo &newSi, const int i)
    {
        std::lock_guard<std::mutex> guard(mutexes[i]);
        stateInfo[i] = newSi;
    }

    StateInfo reader(const int i)
    {
        std::lock_guard<std::mutex> guard(mutexes[i]);
        return stateInfo[i];
    }
};