[boost] Re: Assignment library: review

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2004-04-10 20:13:23

"Gennadiy Rozental" <gennadiy.rozental_at_[hidden]> wrote in message
news:c59i86$39d$1_at_sea.gmane.org...
> So what about 1?

There seem to be more people who want this, but I leave it to Tom to
summarize required changes.

> > > 2. Forwarding problem
> > > This library is affected by forwarding problem. I do not know how
> > important
> > > it is, but may be we should use by-value scheme used by boost::bind?
> >
> > won't boost::ref <> "solve" this. (don't bind use &?)
>
> No and no.

(a)
My comments about boost::bind comes from
http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm

But bind's reference docs seems to use by-value.

(b)
I've attached a small example that demonstrates why we should not
use by-value:

1. it passes a reference to a temporary
2. it removes constness (and do (1))

const& is good because

1. it disallows buggy code to compile
2. references can be passes with boost:ref

You must know something I have overlooked :-)
At the latest wg21 meeting, Dietmar was worried about function<> using by
by-value. So far I agree.

> > > 3. Library doesn't allow to create const collections.
> > > Well, I am not sure whether or not it's doable but currently it's not
> > > supported
> >
> > It has already been suggested to support
> >
> > const vector<int> v = init( v )(1)(2).
>
> How you plan to implement this? init( v ) wouldn't be able to modify v
since
> it is declared const.

Yes, it's simple. Just stuff the values into a temporary and provide a
conversion operator.

 I actually plan to support two versions

1. without container argument (slightly slower, convinient):

    const vector<int> c = make_list(1)(1)(2)(3);

2. with container argument or properly specialized (slightly faster, less
convinient)

   const vector<int> c = list( c )(1)(1)(2)(3); // or
   const vector<int> c = make_vector(1)(1)(2)(3);

For stl, it could make sence to add make_vector()  etc. This kind of
anonymous list can also be used
with container algorithms:

copy( list(1)(2)(3), // bind to a const&
          ostream_iterator<int>( cout ) );

Of course, this will have to be asssesed in a post-review version provided
the library is accepted.

> > (a) is used with operator+=()
>
> You don't need it
>
>     template< typename C >
>     inline insert_assigner<C>
>     operator+=( C& c, const typename C::value_type& v )
>     {
>         return insert_assigner<C>( c ), v;
>     }
>
> The same idea with second guy.

true.

> > (b) is used with operator<<(). If this goes away, it will sill eb used
in
> eg
> >
> > assign_all( array ) = 1,2,3;
>
> Why?

no idea anymore :-)

> > > 2. Should we use Boost::pp to implement insert_assigner,
> > fixed_size_assigner
> > > e.t.c. and have configurable limit?
> >
> > Some have wanted this. I have no problems with it besides I can't see
what
> > its good for.
> > Having a function/constructor with more than X arguments (x small) is
> hardly
> > good practice.
>
> Why 6 then? Why not 4?

I agree four is even better.

> > > 4. ~fixed_size_assigner
> > > This method may throw an exception. Does it always safe to rely on
> > > uncaught_exception call?
> >
> > No, it is not always safe, but the unsafe stuff happens if you use it in
a
> > destructor; not very
> > likely. Sutter has an article about it.
>
> Where?

http://www.gotw.ca/gotw/047.htm

> > > 5. No function template ordering handling
> > > Why not use usual trick with extra long parameter?
> >
> > Sure. What's the trick and what's the issue?
>
>     template< typename C, typename V >
>     inline void make_insertion( C& c, const V& v, long )
>     {
>         c.insert( c.end(), v );
>     }
>
> ...
>
>         void insert_( const value_type& v )
>         {
>             make_insertion( c_, v, 0 ); // 0 is of type int
>         }
>
>  Now you could use generic version even for compilers that does not
support
> partial ordering and you could remove all repetition from stl.hpp

neat. I'll use it when cleaning up the code. Thanks.

br

Thorsten

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