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Subject: [boost] [Re:Opaque Typedef] observations on opaque typedef
From: Robert Ramey (ramey_at_[hidden])
Date: 2011-10-20 03:43:20

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.

Robert Ramey

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