constexpr basic_endpoint() noexcept;
Postconditions: this->address() == ip::address() and port() == 0.
constexpr basic_endpoint(const protocol_type& proto,
port_type port_num) noexcept;
Requires: proto == protocol_type::v4() || proto == protocol_type::v6().
constexpr basic_endpoint(const ip::address& addr,
port_type port_num) noexcept;
Postconditions: this->address() == addr and port() == port_num.