Threadsafe Vector class for C++

Question

Does anyone know a quick and dirty threadsafe vector class for c++? I am multithreading some code, and I believe the problem I have is related to the way the vectors are used. I plan to rewrite the code, but before I go crazy redoing the code, I would like to test it with a threadsafe vector to be sure. I also figure if such a thing is out there, it would be much easier than writing my own version.

Answer 1

This is difficult because of algorithms.

Suppose you wrapped vector so that all its member functions are serialised using a mutex, like Java synchronized methods. Then concurrent calls to std::remove on that vector still wouldn't be safe, because they rely on looking at the vector and changing it based on what they see.

So your LockingVector would need to specialize every template in the standard algorithms, to lock around the whole thing. But then other algorithms like std::remove_if would be calling user-defined code under the lock. Doing this silently behind the scenes is a recipe for locking inversion as soon as someone starts creating vectors of objects which themselves internally take locks around all their methods.

In answer to your actual question: sorry, no, I don't know of one. For a quick test of the kind you need, I recommend that you start out with:

template <typename T>
class LockedVector {
    private:
    SomeKindOfLock lock;
    std::vector<T> vec;
};

Then drop it in as a replacement container, and start implementing member functions (and member typedefs, and operators) until it compiles. You'll notice pretty quickly if any of your code is using iterators on the vector in a way which simply cannot be made thread-safe from the inside out, and if need be you can temporarily change the calling code in those cases to lock the vector via public methods.

Answer 2

You can check out TBB (like concurrent_vector). I've never used it though, honestly, I find putting the scope guard objects around access easier (especially if the vector is properly encapsulated).

Answer 3

I think you'll find it far easier to continue to use std::vector, but to protect concurrent access using some kind of mutex or other operating system synchronization object. You'll also definitely want to use RAII if you are using a mutex.