Re: [boost] none.hpp

From: Martin Wille (mw8329_at_[hidden])
Date: 2006-07-25 02:57:54

Joe Gottman wrote:
> In order to avoid having an unnamed namespace inside a header file, none.hpp 
> has been rewritten as
> 
>     namespace boost {
>         none_t const none = ((none_t)0) ;
>     } // namespace
> 
> But if two source files contain none.hpp won't this violate the 
> one-definition rule?

none would have internal linkage according to 7.1.5.1/2. So it wouldn't
violate the ODR. But a pointer to none from one translation unit would
differ from a pointer to none from a different TU.

>                       What's wrong with defining none_t and none as follows:
> 
> namespace boost {
>     enum none_t {none = 0};
> }
> 
> This allows us to have a unique value boost::none of type boost::none_t that 
> can be included in any number of source files without violating the ODR.

It would also avoid the pointer-to-none problem mentioned above, simply
because you can't create a pointer to an enumerator.

However, none_t instances of enumeration type would be implicitely
convertible to int, while the member pointer version of none_t isn't.

Regards,
m

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