Empty class in std::tuple

Question

The size of any object or member subobject is required to be at least 1 even if the type is an empty class type [...], in order to be able to guarantee that the addresses of distinct objects of the same type are always distinct.

_{cppreference quote}

This I knew. What I just found out is that some library types, like std::tuple don't use any size for contained empty classes. Is this true? If yes, how is that ok?

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Edit: after reading @bolov's final note on his answer I still one question: since Empty is POD it is safe to memcpy to it. But if you would memcpy to a "phantom" address (see @bolov's answer) you would effectively write inside an int element (sizoef(Empty) is 1). That doesn't seem ok.

Answer 1

The size of an object must be greater than zero. The size of a subobject does not have that constraint. This leads to the Empty Base Optimization (EBO), in which an empty base class does not take up space (compilers started implementing this nearly 20 years ago). And that, in turn, leads to the usual implementation of std::tuple as an inheritance chain, in which empty base classes don't take up space.