C++11 Atomics. Why does this compile, but not link?

Question

I'm attempting to write a multi-producer, multi-consumer queue. I'm using G++ 4.6 on Arch Linux, it breaks on G++ 4.7 also.

#include <atomic>
#include <condition_variable>
#include <memory>
#include <mutex>
#include <vector>
#include <iostream>
#include <string>
#include <sstream>

template <typename T> class concurrent_queue
{
  public:
  concurrent_queue(size_t n) : items(n), item_states(n), producer_index(0), consumer_index(0) {
  }
  virtual ~concurrent_queue () {
  }
  T * consume() {
    auto index = consumer_index;
    T * item = nullptr;
    state_t state = USED;

    // Extract item and block location.
    while (!item_states[index].compare_exchange_strong(state, BLOCKED, std::memory_order_acquire, std::memory_order_acquire)) {
      // Wait for an item to become available
      std::unique_lock<std::mutex> lock(item_mutex);
      has_items.wait(lock);

      // Move to the newly available item (might already have been taken by another consumer).
      index = consumer_index;
    }

    // Tell consumers to move to next location, wrap to beginning of circular buffer if necessary.
    ++consumer_index;
    consumer_index %= items.size();

    // Unblock index so that producers can write to it.
    items[index] = nullptr;
    return item;
  }
  void produce(T * value) {
    items[producer_index] = value;
    ++producer_index;
    producer_index %= items.size();
    has_items.notify_one();
  }
private:
  typedef enum {
    EMPTY,
    USED,
    BLOCKED
  } state_t;

  // Used as a buffer of items
  std::vector<T* > items;
  std::vector<std::atomic<state_t> > item_states;
  size_t producer_index;
  size_t consumer_index;
  std::mutex item_mutex;
  std::condition_variable has_items;
};

// Test code
using namespace std;

template <typename T> void pop_n_print(concurrent_queue<T> & queue) {
  stringstream message;
  message << "popped " << *queue.consume() << endl;
  cout << message.str();
}

int main ()
{
  concurrent_queue<int> ints(5);
  ints.produce(new int(1));
  ints.produce(new int(2));

  pop_n_print(ints);
  pop_n_print(ints);

  return 0;
}

I compile this code with g++ --std=c++0x queue.cc -o test_queue, but I get this error message:

/tmp/ccqCjADk.o: In function `concurrent_queue<int>::consume()':
queue.cc:(.text._ZN16concurrent_queueIiE7consumeEv[concurrent_queue<int>::consume()]+0x9f): undefined reference to `std::atomic<concurrent_queue<int>::state_t>::compare_exchange_strong(concurrent_queue<int>::state_t&, concurrent_queue<int>::state_t, std::memory_order, std::memory_order)'
collect2: ld returned 1 exit status

I'm not sure why this is occurring, but it seems to suggest that I need to link against something. Any ideas on where I'm going wrong? Any pointers are greatly appreciated.

Answer 1

Because std::atomic exist only for certain Ts, mainly numeric types.