Question

In my C++ code I’m keeping a pointer to an object
which should be created lazily, i.e., created only upon request.
I have following code, which is clearly not thread-safe.

LAZY* get_lazy()
{
    if (0 == _lazy)
        _lazy = create_lazy();
    return _lazy;
}

I wonder what kind of synchronization should I use here?
I know Boost.thread provides supports for one-time initialization.
But I’m hoping that there is a simple solution using TBB + C++ only.
I should also note that…

• I cannot create _lazy as a static object (I actually want to keep an unbounded array of such lazily created objects)
• Such LAZY objects cannot be over-allocated (creation is very expensive)

Answer 1

You need a local mutex (tbb::mutex), to be sure you create your lazy object only once.

#include <tbb/mutex.h>

tbb::atomic<LAZY*> _lazy;
tbb::mutex myMutex;

LAZY* GetLazy()
{
  if (0 == _lazy)
  {
    myMutex.lock();
    if (0 == _lazy)
        _lazy = create_lazy();
    myMutex.unlock();
  }
  return _lazy;
}