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Programming
Pure Virtual Function in C++
Post 302120069 by porter on Sunday 3rd of June 2007 04:11:07 PM
06-03-2007
History. 
If I have a function pointer in C
typedef int (*foo_ptr)(void);
foo_ptr my_foo;
I can either
(a) leave it to be undefined and a danger to all who touch it
(b) set it to point to an implementation of foo()
(c) set it to NULL or (foo_ptr)0 so people can check it's validity.
C++ is derived from C and setting it to 0 is an indication that this class does not provide an implementation of this function.
There are other places where a non-zero initialisation of a pointer is special such as the SIG_IGN/SIG_DFL and _XtInherit.
Unless you are either (a) implementing a C++ compiler (b) answering a homework question, the reason why is not actually important.
If I have a function pointer in C
typedef int (*foo_ptr)(void);
foo_ptr my_foo;
I can either
(a) leave it to be undefined and a danger to all who touch it
(b) set it to point to an implementation of foo()
(c) set it to NULL or (foo_ptr)0 so people can check it's validity.
C++ is derived from C and setting it to 0 is an indication that this class does not provide an implementation of this function.
There are other places where a non-zero initialisation of a pointer is special such as the SIG_IGN/SIG_DFL and _XtInherit.
Unless you are either (a) implementing a C++ compiler (b) answering a homework question, the reason why is not actually important.
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LEARN ABOUT DEBIAN
pure-pw
pure-pw(8) Pure-FTPd pure-pw(8) NAME
pure-pw - Manage virtual users files for Pure-FTPd SYNTAX
pure-pw useradd login [-f passwd_file] [-F puredb_file] -u uid [-g gid] -D/-d home_directory [-c gecos] [-t download_bandwidth] [-T upload_bandwidth] [-n max number_of_files] [-N max_Mbytes] [-q upload_ratio] [-Q download_ratio] [-r <allow client host>/<mask>[,<ip>/<mask>]...] [-R <deny client host>/<mask>[,<ip>/<mask>]...] [-i <allow local host>/<mask>[,<ip>/<mask>]...] [-I <deny local host>/<mask>[,<ip>/<mask>]...] [-y <max number of concurrent sessions>] [-z <hhmm>-<hhmm>] [-m] pure-pw usermod login [-f passwd_file] [-F puredb_file] [-u uid] [-g gid] -D/-d home_directory -[c gecos] [-t download_bandwidth] [-T upload_bandwidth] [-n max_number_of_files] [-N max_Mbytes] [-q upload_ratio] [-Q download_ratio] [-r <allow client host>/<mask>[,<ip>/<mask>]...] [-R <deny client host>/<mask>[,<ip>/<mask>]...] [-i <allow local host>/<mask>[,<ip>/<mask>]...] [-I <deny local host>/<mask>[,<ip>/<mask>]...] [-y <max number of concurrent sessions>] [-z <hhmm>-<hhmm>] [-m] pure-pw userdel login [-f passwd_file] [-F puredb_file] [-m] pure-pw passwd login [-f passwd_file] [-F puredb_file] [-m] pure-pw show login [-f passwd_file] [-m] pure-pw mkdb [<puredb_database_file> [-f passwd_file]] [-F puredb_file] pure-pw list [-f passwd_file] DESCRIPTION
Virtual users is a simple mechanism to store a list of users, with their password, name, uid, directory, etc. It's just like /etc/passwd. But it's not /etc/passwd. It's a different file, only for FTP. It means that you can easily create FTP-only accounts without messing your system accounts. Additionnaly, virtual users files can store individual quotas, ratios, bandwidth, etc. System accounts can't do this. Thousands of virtual users can share the same system user, as long as they all are chrooted, and they have their own home directory. FILES
/etc/pure-ftpd/pureftpd.passwd /etc/pure-ftpd/pureftpd.pdb ENVIRONMENT VARIABLES
PURE_PASSWDFILE If this variable is defined, this is the default value for the text password file. Without this variable, /etc/pure- ftpd/pureftpd.passwd is assumed. PURE_DBFILE If this variable is defined, this is the default value for the PureDB password file. Without this variable, /etc/pure- ftpd/pureftpd.pdb is assumed. EXAMPLES
Please read http://download.pureftpd.org/pub/pure-ftpd/doc/README.Virtual-Users AUTHORS
Frank DENIS <j at pureftpd dot org> SEE ALSO
ftp(1), pure-ftpd(8) pure-ftpwho(8) pure-mrtginfo(8) pure-uploadscript(8) pure-statsdecode(8) pure-pw(8) pure-quotacheck(8) pure-authd(8) RFC 959, RFC 2228, RFC 2389 and RFC 2428. Pure-FTPd team 1.0.36 pure-pw(8)