difference between std::mutex and std::shared_mutex

Question

I came across an std::shared_mutex in C++17. what exactly is std::shared_mutex and how it is different from std::mutex?

Answer 1

std::shared_mutex can be useful especially in cases where data structure (like DNS cache) gets rarely updated. Using a std::mutex to protect the data structure could be overly pessimistic, because it eliminates the possible concurrency in reading the data structure when it isn’t undergoing modification. Multiple threads can have a shared lock on the same std::shared_mutex at the same time.

One such example from Anthony Williams book:

class dns_cache
{
    std::map entries;
    mutable boost::shared_mutex entry_mutex;

public:

    dns_entry find_entry(std::string const& domain) const
    {
        boost::shared_lock lk(entry_mutex);
        std::map::const_iterator const it = entries.find(domain);
        return (it==entries.end()) ? dns_entry() : it->second;
    }

    void update_or_add_entry(std::string const& domain,
                            dns_entry const& dns_details)
    {
        std::lock_guard lk(entry_mutex);
        entries[domain] = dns_details;
    }
};

Here, function find_entry basically does the Read operation, whereas update_or_add_entry performs the Write operation.

So, it can said that std::shared_mutex is a typical reader-writer mutex, because it allows for two different kinds of usage: exclusive access by a single “writer” thread or shared, concurrent access by multiple “reader” threads.