Re: [boost] Re: New Singleton Release

From: Jason Hise (chaos_at_[hidden])
Date: 2005-03-05 18:03:03

Martin wrote:

>>Keep at least one global or static pointer to the singleton.  The global 
>>pointer is different in concept than a real global instance, as it is 
>>being used to control the moment of creation, rather than to provide a 
>>global point of access.  You can hide the global pointer in an unnamed 
>>namepace in some obscure source file if you want to make it really clear 
>>that it isn't the access mechanism.
>>    
>>
>
>The problem is that I don't have a source files to put a global variable in. 
>
>My library is a template that lives entierly in header files. At the moment I 
>got a single cpp file that only contains the global variable. It works but it 
>forces the library users to include the source file in their project so I 
>would like to get rid of it.
>
>The singleton seem to solve the problem but since the global variable isn't 
>created until it is used it could happen that two threads create it at the 
>same time. The global variable is constant so it is only the creation that 
>needs locking. I could use a locking policy but it would be easier if I could 
>just force creation at startup. 
>  
>
Well, I guess I don't see any problem with declaring the global pointer 
in the header... maybe just put it in a detail namespace.  An extra 
pointer will exist in each translation unit, but that shouldn't cause 
any problems.

>>Disabling of the singleton's default copy constructor was intentional, 
>>you can ignore or disable this warning.
>>    
>>
>
>Why does a singleton need a copy contructor?
>
It doesn't... which is why I disabled it by declaring it privately and 
not defining it.  If I had ignored it a default copy ctor would have 
been generated.  The error is saying that a class deriving from the 
singleton was unable to generate its own automatic copy constructor 
because the derived class couldn't access the base's copy constructor, 
which I had made private and disabled.  This is intentional.

-Jason