+ :param ptr: pointer to initialized `memcached_st` struct
+ :param group_key: key namespace
+ :param group_key_length: length of the `group_key` without any terminating zero
+ :param key: the key to delete
+ :param key_length: the length og the `key` without any terminating zero
+ :param expiration: obsolete since :manpage:`memcached(1)` version 1.4
+ :returns: `memcached_return_t` indicating success