Re: Confused about a thread-safe singleton example.

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 3 Dec 2008 13:25:30 -0800 (PST)

Message-ID:

<65a9b416-c9e5-4a05-9d07-9d4c5ab48c01@u14g2000yqg.googlegroups.com>

On 3 Dec, 18:12, Noah Roberts <u...@example.net> wrote:

Maxim Yegorushkin wrote:

On Dec 3, 4:59 pm, Noah Roberts <u...@example.net> wrote:

Maxim Yegorushkin wrote:

On Dec 3, 9:43 am, James Kanze <james.ka...@gmail.com> wrote:

1. Don't use singletons. Ever. Pretty much all of the value
of the GoF Design Patterns book is negated by the fact that
they chose to legitimize Singleton as a design pattern.
Singleton is just a fancy name for global variable. We should
call it the Global Variable (anti)Pattern.

And there are valid reasons for using it.

What are those reasons?

Some things must be inherently unique in a process.

This is what global variables are for.

Globals do not offer any protection against further instantiation. =

something must be unique then it should only be possible to create a
single variable of that type. How are you going to offer that with =

unprotected variable, a comment saying, "Don't make any of these..."?

[]

Using abstract classes (interfaces) and global references you can
achieve the desired effect.

That method does not work.

[]

Here is a proof that it works. Example of a type you can't instantiate
and a global variable of that type:

    // header.h begin
    struct X
    {
        virtual void foo() = 0;
    };
    extern X& global_x;
    // header.h end

    // source.cc begin
    #include "header.h"
    namespace {
    struct XImpl : X
    {
        void foo() {}
    } x;
    }
    X& global_x = x;
    // source.cc end

And look, you've just implemented a singleton. This isn't 'just' a glo=

bal.

A global variable by definition is a singleton, is it not?

Singleton is an idea that there is only one object of some thing. In C+
+ it can be represented as global object, as SomeThing::instance(),
same pimpl / monostate, and likely there are more ways of representing
this idea.

Singleton is undeniably a useful idea. My point is that representing
singletons as regular global objects looks to me more natural and
elegant than SomeThing::instance(). May be because C++ is considered a
multi-paradigm language and SomeThing::instance() looks coming from
the object-oriented paradigm of it, whereas global variables feel
natural when using C++ both as a procedural programming language and
OO-language: just a regular variable with a large scope.

--
Max