C++ singleton Post: 302727763

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Top Forums Programming C++ singleton Post 302727763 by DreamWarrior on Tuesday 6th of November 2012 09:51:53 PM

11-06-2012

Registered User

I'm fond of this method....

Code:

class Single
{
public:
  static Single &getSingle()
  {
    static Single theSingle;
    return theSingle;
  }

private:
  Single();  // private constructor to enforce singleton

//public:  // (1) if you uncomment this line then (2) below
           //       can be made to compile (and crash!).
  ~Single();
};

Single::Single()
{
}

Single::~Single()
{
}

int main()
{
  Single &s = Single::getSingle();
//  delete &s;  // (2) won't compile if you uncomment unless you
                //     uncomment (1) above; but, who would do this?
  return 0;
}

This has the benefit that the reference can't be freed (though, as I show, the caller can jump through hoops to free it, so the private destructor is still good practice). However, I'd suspect that most people would see a reference and know that deleting it is probably bad.

Buyer beware, however, that the standard is pretty hush-hush on its thread safety. Though, most practical compilers (g++) wrap the local static member ("theSingle") with locks to insure it does not get constructed more than once. In my opinion, this is preferable to roll-your-own. However, if you must, a pthread_once_t control is probably the way to do it as here:

Code:

#include <pthread.h>

class Single
{
public:
  static Single &getSingle()
  {
     pthread_once(&onceCtl, BuildTheSingle);
     return *theSingle;
  }

private:
  static Single *theSingle;
  static pthread_once_t onceCtl;
  static void BuildTheSingle();

  Single();
  ~Single();
};

pthread_once_t Single::onceCtl = PTHREAD_ONCE_INIT;
Single *Single::theSingle = NULL;

void Single::BuildTheSingle()
{
  theSingle = new Single();
}

Single::Single()
{
}

Single::~Single()
{
}

Which uses the same reference return to, IMO, instruct the caller that it's not theirs and they shouldn't be free'ing it.

I suppose you could also use auto pointers or something, too, but...this is good enough for me, lol.

edit: for the record, the former implementation is thread-safe in C++0x, as such, in any newer code compiled with C++0x compliant compiler, I'd recommend it to rid all the boilerplate crap and distill it right to only that code which is both sensible and required. Just one man's opinion....

Last edited by DreamWarrior; 11-06-2012 at 11:11 PM..

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net::server::multitype

Net::Server::MultiType(3pm)				User Contributed Perl Documentation			       Net::Server::MultiType(3pm)

NAME

       Net::Server::MultiType - Net::Server personality

SYNOPSIS

	   use base qw(Net::Server::MultiType);

	   sub process_request {
	       #...code...
	   }

	   my @types = qw(PreFork Fork Single);

	   Net::Server::MultiType->run(server_type => @types);

DESCRIPTION

       Please read the pod on Net::Server first.  This module is a personality, or extension, or sub class, of the Net::Server module.

       This personality is intended to allow for easy use of multiple Net::Server personalities.  Given a list of server types,
       Net::Server::MultiType will require one at a time until it finds one that is installed on the system.  It then adds that package to its
       @ISA, thus inheriting the methods of that personality.

ARGUMENTS

       In addition to the command line arguments of the Net::Server base class, Net::Server::MultiType contains one other configurable parameter.

	   Key		     Value	      Default
	   server_type	     'server_type'    'Single'

       server_type
	   May be called many times to build up an array or possible server_types.  At execution, Net::Server::MultiType will find the first
	   available one and then inherit the methods of that personality

CONFIGURATION FILE

       "Net::Server::MultiType" allows for the use of a configuration file to read in server parameters.  The format of this conf file is simple
       key value pairs.  Comments and white space are ignored.

	   #-------------- file test.conf --------------

	   ### multi type info
	   ### try PreFork first, then go to Single
	   server_type PreFork
	   server_type Single

	   ### server information
	   min_servers	 20
	   max_servers	 80
	   spare_servers 10

	   max_requests  1000

	   ### user and group to become
	   user        somebody
	   group       everybody

	   ### logging ?
	   log_file    /var/log/server.log
	   log_level   3
	   pid_file    /tmp/server.pid

	   ### access control
	   allow       .+.(net|com)
	   allow       domain.com
	   deny        a.+

	   ### background the process?
	   background  1

	   ### ports to bind
	   host        127.0.0.1
	   port        localhost:20204
	   port        20205

	   ### reverse lookups ?
	   # reverse_lookups on

	   #-------------- file test.conf --------------

PROCESS FLOW

       See Net::Server

HOOKS

       There are no additional hooks in Net::Server::MultiType.

TO DO

       See Net::Server

AUTHOR

       Paul T. Seamons paul@seamons.com

SEE ALSO

       Please see also Net::Server::Fork, Net::Server::INET, Net::Server::PreFork, Net::Server::MultiType, Net::Server::Single

perl v5.14.2							    2012-06-05					       Net::Server::MultiType(3pm)