Re: [boost] singleton usage

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-02-14 14:38:19

Jorge Lodos wrote:
>
>> as a "singleton" to solve the initialization order problem (the only
>> legitimate use of the "singleton pattern", in my opinion).
>>
>
> Could you please elaborate? Why do you consider this is the only
> legitimate use for this pattern? Thanks in advance.

A singleton is simply a glorified wrapper over several global variables and 
functions. There is little difference between

Singleton::instance().f();

and

Singleton::f();

where the second Singleton is a namespace. The global variables don't become 
any less "evil" when they are made members of a singleton class.

When a class represents an out-of-program entity that, by its nature, has a 
single instance, this class can be flattened into functions.

This isn't black and white, of course; today's code may treat the video card 
as a singleton, but tomorrow's computers may add multiple video card 
support. In this case I still prefer something along the lines of

VideoCard & get_video_card( int id ); // noncopyable

or maybe even

video_card_ref get_video_card( int id ); // reference copy semantics

instead of the "multisingleton" way of

VideoCard::instance( id ).

But "true" singletons are either crippled ordinary classes or collections of 
global functions (under the assumption that there are no initialization 
order issues, of course).