18.18.txt

since the swap function defined in the standard library is a template function,
argument dependent lookup doesn't occur (no specific type, can accept any)

the using declaration for std::swap makes the std::swap template function visible in function (local) scope,
as a "failsafe" for all calls for swap, essentially the template function is either
an overload for any swap calls with two parameters that has the lowest priority,
or the only function that can be called, it depends on the types it is called with as arguments
if the types have a swap non-member function, it will call that instead of std::swap in the std namespace

argument dependent lookup (ADL) will ensure that the swap we call will be correctly identified
via the argument types, and the compiler will look in the arguments' namespaces to find
a more specialized function that should have greater performance

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if mem1 is a string, the string version of swap is called
	the using declaration places the std::swap template function in scope as an alias (swap)
	since there is a function named swap that takes two strings, all calls for swap with two
	string arguments will call that function instead of the template one via ADL

if mem1 is an int, then the library version of swap is called (std::swap)
	the using declaration places the std::swap template function in scope as an alias (swap)
	there are no functions named swap for ints, as it is a built-in type,
	only function that we can call via an unqualified swap is the one in the library