Re: understanding singleton design pattern

From:

=?ISO-8859-1?Q?Marcel_M=FCller?= <news.5.maazl@spamgourmet.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 17 Jan 2009 17:45:08 +0100

Message-ID:

<49720b15$0$30223$9b4e6d93@newsspool1.arcor-online.net>

subramanian100in@yahoo.com, India wrote:

Suppose Test is class. Suppose the application which defines the Test
class, needs one, and only one, instance of Test class. Additionally,
lazy initialization and global access are necessary.(I have borrowed
these requirements of singleton design pattern from
http://www.vincehuston.org/dp/singleton.html)

Suppose I write a global(free) function:

Test *getTestInstance()
{
    static Test *instance = 0;

    if ( ! instance )
        instance = new Test;

    return instance;
}

I want to know the following:

Instead of the singleton design pattern, if the above function is
used, what are the drawbacks of using the above function ? What are
the advantages of singleton design pattern over the above function ?

The constructor of Test must be public for the above to work. So the
compiler would not reject, when a second instance of Test is created. It
is only up to the programmer whether to use Test as a singleton or not.
Of course alternatively you could make getTestInstance a friend of Test.
But making it a static member of Test is more clear.

Furthermore if the application should provide a defined behaviour at
shutdown, you must delete Test somewhere.
static boost::scoped_ptr<Test> instance;
will do this job for you. If you do not have boost, std::auto_ptr is
fine too.

Marcel