Just like a shared mutex, except acquiring a shared lock while a thread is waiting to acquire a unique lock will block.

unique_lock_prioritizing_shared_mutex.hpp

template<typename SharedMutex>
class unique_lock_prioritizing_shared_mutex {
public:
    void lock() {
        std::lock_guard<decltype(control)> lock(control);
        mutex.lock();
    }

    void unlock() {
        mutex.unlock();
    }

    bool try_lock() {
        return mutex.try_lock();
    }

    void lock_shared() {
        std::lock_guard<decltype(control)> lock(control);
        mutex.lock_shared();
    }

    bool try_lock_shared() {
        return mutex.try_lock_shared();
    }

    void unlock_shared() {
        mutex.unlock_shared();
    }

private:
    std::mutex control;
    SharedMutex mutex;
};