Re: [boost] [uuid] uuid::create() function

From: Jos Hickson (jos.hickson_at_[hidden])
Date: 2007-06-14 04:19:57

On 13/06/07, Andy <atompkins_at_[hidden]> wrote:
> "Jos Hickson" <jos.hickson_at_[hidden]> wrote in
> news:fc03d05a0706120226n717d457fueca21627444a90d3_at_[hidden]:
>
> < snip >
>
> >> Really what we need is a Boost.Seed library to compliment
> >> Boost.Random so that anybody like yourself using the latter doesn't
> >> have to spend all this time wondering how to seed things generically.
>
> I think that the seed class would be a good addition to Boost.Random.
>
> < snip >
>
> >> >
> >> > template <typename Engine=boost::mt19937>
> >> > class uuid_generator {
> >> > public:
> >> >   uuid_generator() {
> >> >     boost::seed_rgn e;
> >> >     boost::generator_iterator<boost::seed_rng> begin(&e);
> >> >     boost::generator_iterator<boost::seed_rnd> end;
> >> >     en_.seed(begin, end);
> >> >   }
> >> >
> >> >   boost::uuid operator()() {
> >> >     // get random data from en_
> >> >     // return uuid based on this data
> >> >   }
> >> > private:
> >> >   Engine en_;
> >> > };
> >> >
> >> > Now this works fine, the slow seed_rng is used to fully seed the
> >> > random number generator used.  It can be specialized for
> >> > boost::random_device since boost::random_device does not have a
> >> > seed function.
> >> >
> >> > Andy.
> >> >
> >>
> So now I have this, and I like it, but I have been unable to use
> boost::enable_if, and boost::disable_if to enable/disable the constructors.
>
> template <typename Engine = boost::mt19937>
> class basic_uuid_generator {
> public:
>
>   // only enable if engine can not be seeded
>   // boost::random_device does not need seeding
>   basic_uuid_generator(/*enable only if Engine=boost::random_device*/)
>   {}
>
>   // enable if engine can be seeded
>   // seed engine
>   basic_uuid_generator(/*enable if Engine != boost::random_device*/)
>   {
>     boost::seed_rgn seed;
>     boost::generator_iterator<boost::seed_rng> begin(&seed);
>     boost::generator_iterator<boost::seed_rng> end;
>     en_.seed(begin, end);
>   }
>
>   // assume they have already seeded e and make a copy
>   // maybe we should store a reference to e?
>   explicit uuid_generator(Engine const& e)
>     : en_(e)
>   {}
>
>   boost::uuid operator()() { ... }
>
> priavte:
>   Engine en_;
> };
>
> typedef basic_uuid_generator<> uuid_generator;
>
I might be wrong here but I believe that as enable_if relies on SFINAE
then the template parameter it depends on needs to be a template
parameter of the constructor or method itself rather than of the
enclosing class.  As the class can be instantiated with any template
parameter then both versions of the constructor will be available.  I
think you would need to specialise the class itself for
boost::random_device.

Jos