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Subject: Re: [boost] [Opaque] Request for interest in Opaque typedefs library emulation
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-07 13:54:53

vicente.botet wrote:
> The library include two class templates (CRTP) to define opaque types:
> * public_opaque_type<OT,UT> : implicit conversion from OT to UT
> * private_opaque_type<OT,UT> : no conversion from OT to UT

Why not name them typedef_public and typedef_private to correlate with the proposed syntax?

> To define a new OT with this mixin classes
> struct serial_number: public_opaque_type<serial_number, unsigned>
> {
> };
> struct game_score: private_opaque_type<game_score, unsigned>
> {
> };

Can you imagine any use case in which someone would want to add anything else to such a type? They won't be able to if opaque types ever become part of the standard, will they?

> In addition, defines macros that make this simpler
> BOOST_OPAQUE_PUBLIC_TYPEDEF(unsigned, serial_number);
> BOOST_OPAQUE_PRIVATE_TYPEDEF(unsigned, game_score);


> Next follows the definition of the public_opaque_type
> template <typename Final, typename T>
> class public_opaque_type
> : boost::totally_ordered< Final
> , boost::integer_arithmetic< Final
> , boost::bitwise< Final
> , boost::unit_steppable< Final
> , boost::totally_ordered< T, Final // public
> specific conversions
> , boost::integer_arithmetic< T, Final // public
> specific conversions
> , boost::bitwise< T, Final // public specific conversions
> > > > > > > >

That's quite a collection of mixins!

> Please, let me know if you see a error on the design.

At first blush, it seems reasonable, but you'll need to test a great variety of use cases to make sure they do only what they should and in many contexts.

Here are a few questions that spring to mind:

   Can you declare such types local to a function?

   Can you declare such types nested within a UDT?

   Can you declare such types with others as the underlying type?

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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