Objective-C blocks and C++ objects

I have a method that is being executed on a background thread. From that method I'm trying to dispatch_async a block on the main thread. The block uses a local C++ object which is supposed to be copy constructed according to the Apple reference. I'm getting a segmentation fault and from the trace I see that something very sketchy is going on. Here's the simplified version of my code.

struct A
{
    A() { printf("0x%08x: A::A()\n", this); }
    A(A const &that) { printf("0x%08x: A::A(A const &%p)\n", this, &that); }
    ~A() { printf("0x%08x: A::~A()\n", this); }
    void p() const { printf("0x%08x: A::p()\n", this); }
};

- (void)runs_on_a_background_thread
{
    A a;
    a.p();
    dispatch_async(dispatch_get_main_queue(), ^{
        printf("block begins\n");
        a.p();
        printf("block ends\n");
    });
}

And this is the output:

0xbfffc2af: A::A()
0xbfffc2af: A::p()
0xbfffc2a8: A::A(A const &0xbfffc2af)
0x057ae6b4: A::A(A const &0xbfffc2a8)
0xbfffc2a8: A::~A()
0xbfffc2af: A::~A()
0xbfffdfcf: A::A(A const &0x57ae6b4)
0xbfffdfcf: A::~A()
block begins
0xbfffdfcf: A::p()
block ends
0x057ae6b4: A::~A()

There are two things that I don't understand. The first one is why by the time it gets to 0xbfffdfcf: A::p() the destructor on that object has been called already.

The second thing I'm struggling with is why there are so many copy constructors being called. I expect one. That should happen when a copy of a is created to be captured by the block.

I'm using Xcode 3.2.5 with GCC. I experience the same behavior on the simulator and on the device.