Re: [boost] [Re:Opaque Typedef] observations on opaque typedef

Subject: Re: [boost] [Re:Opaque Typedef] observations on opaque typedef
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2011-10-20 12:51:19

Le 20/10/11 09:43, Robert Ramey a écrit :
> Vicent -
>
> I saw your post on this subject some months ago and I've reviewed the
> thread.  I didn't look at the code though.  I also looked at the C++0x
> document and I have something to say about this.
>
> a) as you know I'm the author of BOOST_STRONG_TYPEDEF which lets one assign
> a unique type to some primitive integer type.  I did this to permit
> overloads on integers used for different purposes.  The new types created
> had all the properties of integer types but they were all different types.
> So far so good.
>
> b) I eventually found the BOOST_STRONG_TYPEDEF did too much - what I really
> wanted to not to inherit all the interoperations but only some of them.  So
> I've reduced or eliminated usage of BOOST_STRONG_TYPE from the serialization
> library implementation.  Never the less, I do see it useful in other
> contexts so your efforts to create and opaque typedef seem useful to me.
>
> c) BUT now I really want something else.  I want the ability to assign a new
> type to an existing type.  The current typedef has
> the feature that if two different names are used for the same type, a new
> type is NOT created.  That is typedef creates
> a synonym rather than a new type.  I thought that C++0x template alias would
> do this.  But a careful reading of the standard
> document that I have leads me to believe that if two names are aliased to
> the same underlying type - a new type is not created
> - just like the current typedef.  So what we have in the new C++ is
> basically a templated typedef.
>
> d) what I really need is the equivalent to
>
> #define OPAQUE_TYPEDEF(name, t) \
> struct name : public t {};
>
> so I can say
>
> struct dollar_amount {
>      ...
>     operator *(const&  rhs) const {..} // don't include this operator - makes
> no sense
> };
>
> OPAQUE_TYPEDEF(debit, dollar_amount);
> OPAQUE_TYPEDEF(price, dollar_amount)
>
> and then say
> void f(debit, t){...}
> void f(price t){...}
>
> Notice in my example, I don't want debit and price types to be true types
> and not just
> synonyms for dollar_amount.
>
> I realize that this is whole different kettle of fixh than BOOST_STRONG_TYPE
> or OPAQUE_TYPE.  So this might be considered off topic.  or maybe not.
>
> Comments welcome.  Also anyone can feel free to correct my understanding of
> the template alias feature of C++0x to create new types.
Hi,

there is a feature in C++11 that could respond to your needs.

|struct debit :|dollar_amount
|  {
   using|||dollar_amount|::|||dollar_amount|;
};
|

This paper 
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2141.html 
explain its relation to Opaque types.

Best,
Vicente