argument-dependent lookup

When arguments of a function belong to a specific namespace, C++ searches for the function in the arguments’ namespace instead of the global namespace.

#include <algorithm>
 
using std::swap;
 
namespace wat {
	struct test_t {};
	void swap(test_t &a, test_t &b) {
		test_t c = a;
		a = b;
		b = c;
	}
}
 
int main() {
	wat::test_t a, b;
	swap(a, b); // calls wat::swap
	return 0;
}