boost function对象 (三)

boost::overloaded_function<
const std::string& (const std::string&)
, int (int)
, double (double)
> identity(identity_s, identity_i, identity_d);

// All calls via single `identity` function.
BOOST_TEST(identity("abc") == "abc");
BOOST_TEST(identity(123) == 123);
BOOST_TEST(identity(1.23) == 1.23);