Difference between pointer and reference as thread parameter

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The constructor of std::thread deduces argument types and stores copies of them by value. This is needed to ensure the lifetime of the argument object is at least the same as that of the thread.

C++ template function argument type deduction mechanism deduces type T from an argument of type T&. All arguments to std::thread are copied and then passed to the thread function so that f1() and f2() always use that copy.

If you insist on using a reference, wrap the argument using boost::ref() or std::ref():

thread t1(f1, boost::ref(ret));

Or, if you prefer simplicity, pass a pointer. This is what boost::ref() or std::ref() do for you behind the scene.

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