Impossible to be const-correct when combining data and it's lock?

Question

I've been looking at ways to combine a piece of data which will be accessed by multiple threads alongside the lock provisioned for thread-safety. I think I've got to a point where I don't think its possible to do this whilst maintaining const-correctness.

Take the following class for example:

template <typename TType, typename TMutex>
class basic_lockable_type
{

public:
    typedef TMutex lock_type;

public:
    template <typename... TArgs>
    explicit basic_lockable_type(TArgs&&... args)
        : TType(std::forward<TArgs...>(args)...) {}

    TType& data() { return data_; }
    const TType& data() const { return data_; }

    void lock() { mutex_.lock(); }
    void unlock() { mutex_.unlock(); }

private:
    TType           data_;
    mutable TMutex  mutex_;

};

typedef basic_lockable_type<std::vector<int>, std::mutex> vector_with_lock;

In this I try to combine the data and lock, marking mutex_ as mutable. Unfortunately this isn't enough as I see it because when used, vector_with_lock would have to be marked as mutable in order for a read operation to be performed from a const function which isn't entirely correct (data_ should be mutable from a const).

void print_values() const
{
    std::lock_guard<vector_with_lock> lock(values_);
    for(const int val : values_)
    {
        std::cout << val << std::endl;
    }
} 

vector_with_lock values_;

Can anyone see anyway around this such that const-correctness is maintained whilst combining data and lock? Also, have I made any incorrect assumptions here?

Answer 1

Personally, I'd prefer a design where you don't have to lock manually, and the data is properly encapsulated in a way that you cannot actually access it without locking first.

One option is to have a friend function apply or something that does the locking, grabs the encapsulated data and passes it to a function object that is run with the lock held within it.

//! Applies a function to the contents of a locker_box
/*! Returns the function's result, if any */
template <typename Fun, typename T, typename BasicLockable>
ResultOf<Fun(T&)> apply(Fun&& fun, locker_box<T, BasicLockable>& box) {
    std::lock_guard<BasicLockable> lock(box.lock);
    return std::forward<Fun>(fun)(box.data);
}
//! Applies a function to the contents of a locker_box
/*! Returns the function's result, if any */
template <typename Fun, typename T, typename BasicLockable>
ResultOf<Fun(T const&)> apply(Fun&& fun, locker_box<T, BasicLockable> const& box) {
    std::lock_guard<BasicLockable> lock(box.lock);
    return std::forward<Fun>(fun)(box.data);
}

Usage then becomes:

void print_values() const
{
    apply([](std::vector<int> const& the_vector) {
        for(const int val : the_vector) {
            std::cout << val << std::endl;
        }
    }, values_);
} 

Alternatively, you can abuse range-based for loop to properly scope the lock and extract the value as a "single" operation. All that is needed is the proper set of iterators¹:

 for(auto&& the_vector : box.open()) {
    // lock is held in this scope
    // do our stuff normally
    for(const int val : the_vector) {
        std::cout << val << std::endl;
    }
 }

I think an explanation is in order. The general idea is that open() returns a RAII handle that acquires the lock on construction and releases it upon destruction. The range-based for loop will ensure this temporary lives for as long as that loop executes. This gives the proper lock scope.

That RAII handle also provides begin() and end() iterators for a range with the single contained value. This is how we can get at the protected data. The range-based loop takes care of doing the dereferencing for us and binding it to the loop variable. Since the range is a singleton, the "loop" will actually always run exactly once.

The box should not provide any other way to get at the data, so that it actually enforces interlocked access.

Of course one can stow away a reference to the data once the box is open, in a way that the reference is available after the box closes. But this is for protecting against Murphy, not Machiavelli.

The construct looks weird, so I wouldn't blame anyone for not wanting it. One one hand I want to use this because the semantics are perfect, but on the other hand I don't want to because this is not what range-based for is for. On the gripping hand this range-RAII hybrid technique is rather generic and can be easily abused for other ends, but I will leave that to your imagination/nightmares ;) Use at your own discretion.

---

¹ Left as an exercise for the reader, but a short example of such a set of iterators can be found in my own locker_box implementation.

Answer 2

What do you understand by "const correct"? Generally, I think that there is a consensus for logical const, which means that if the mutex isn't part of the logical (or observable) state of your object, there's nothing wrong with declaring it mutable, and using it even in const functions.