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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.

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