state_test.cpp

#include <thread>
#include <mutex>
#include <atomic>

class State
{
    public:
        std::mutex mMutex;
};

std::atomic<State*> curr;
std::atomic<int> threadState;

void loop()
{
    threadState = 1;

    while(threadState != 2)
    {
        State* current = curr;

        if(current && current->mMutex.try_lock())
        {
            std::this_thread::sleep_for(std::chrono::milliseconds(1));

            current->mMutex.unlock();
        }
    }
}

State states[3];

State* getFreeState()
{
    while(true)
    {
        for(size_t i = 0; i < 3; i++)
        {
            if(&states[i] != curr && states[i].mMutex.try_lock())
            {
                return &states[i];
            }
        }
    }

    return NULL;
}

void setCurrent(State* current)
{
    current->mMutex.unlock();
    curr = current;
}

int main(int argc, char** argv)
{
    // initialisation
    curr = nullptr;
    threadState = 0;

    std::thread t(loop);

    while(threadState != 1)
        std::this_thread::yield();

    for(size_t i = 0; i < 100; i++)
    {
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
        State* s = getFreeState();
        setCurrent(s);
    }

    threadState = 2;
    t.join();

    return 0;
}