Is there a `shared_lock_guard` and if not, what wo

Question

I wanted to use a std::mutex in my class, and noticed that it isn't copyable. I'm at the bottom level of my library here, so it seems like a terrible idea to have this behaviour.

I used std::lock_guard on the std::mutex, but there doesn't seem to be a shared_lock_guard, which would be preferable to provide write-locks-exclusively behaviour. Is this an oversight or trivial to implement myself?

Answer 1

With C++14 You can use a std::shared_lock and a std::unique_lock to implement read/write locking:

class lockable
{
public:
    using mutex_type = std::shared_timed_mutex;
    using read_lock  = std::shared_lock<mutex_type>;
    using write_lock = std::unique_lock<mutex_type>;

private:
    mutable mutex_type mtx;

    int data = 0;

public:

    // returns a scoped lock that allows multiple
    // readers but excludes writers
    read_lock lock_for_reading() { return read_lock(mtx); }

    // returns a scoped lock that allows only
    // one writer and no one else
    write_lock lock_for_writing() { return write_lock(mtx); }

    int read_data() const { return data; }
    void write_data(int data) { this->data = data; }
};

int main()
{
    lockable obj;

    {
        // reading here
        auto lock = obj.lock_for_reading(); // scoped lock
        std::cout << obj.read_data() << '\n';
    }

    {
        // writing here
        auto lock = obj.lock_for_writing(); // scoped lock
        obj.write_data(7);
    }
}

Note: If you have C++17, then you can use std::shared_mutex for mutex_type.

Answer 2

It’s not part of C++ standard yet, but you can find implementation example in boost.

template<typename SharedMutex>
class shared_lock_guard
{
private:
    SharedMutex& m;

public:
    typedef SharedMutex mutex_type;
    explicit shared_lock_guard(SharedMutex& m_):
        m(m_)
    {
        m.lock_shared();
    }
    shared_lock_guard(SharedMutex& m_,adopt_lock_t):
        m(m_)
    {}
    ~shared_lock_guard()
    {
        m.unlock_shared();
    }
};

It requires mutex class conforming to SharedMutex concept though; std::shared_mutex is part of proposed C++17 standard and boost had one already for some time: boost::shared_mutex.