Re: [eigen] Namespaces

[Ian Mackenzie <ian.e.mackenzie@xxxxxxxxx>]

Benoît,

Are you sure typedefs aren't supposed to work across namespaces in C++?  For 
one thing, I've been doing exactly that with Eigen 1 in my code:

typedef long double basic_scalar;
typedef Eigen::Vector<basic_scalar, 3> basic_vector;

and it compiles OK under GCC (Linux and OS X) and MSVC.  That might just have 
been compilers being lenient, but by looking through 'The C++ Programming 
Language' I found an example that did the same sort of thing (section 
8.2.8.2, page 182 in my copy):

namespace Lib2 {
	// bunch of other examples
	typedef Her_lib::String Her_string;
}

Indeed, Stroustrup explicitly mentions that "Usually, I prefer to leave a name 
unchanged when including it into a new namespace...However, sometimes a new 
name is needed or simply nice to have...There is no specific language 
mechanism for renaming.  Instead, the general mechanisms for defining new 
entities are used."

-Ian

On October 11, 2007 04:23:46 pm Benoît Jacob wrote:
> Thanks for the long e-mail, Donald. I received it as I was reverting to
> namespace Eigen, and so it almost felt good to know that you also were in
> favor of a namespace :)
>
> Anyway, this is committed. I haved added a CCMAIL in the commit message to
> this list, but apparently the mail hasn't yet been sent.
>
> So we currently have everything in a namespace Eigen. And we have a macro,
> USING_EIGEN_DATA_TYPES, that does 'using' on the Matrix class and its
> typedefs (these typedefs are not too polluting thanks to their suffixes,
> like Vector2d).
>
> So many users will be able to do USING_EIGEN_DATA_TYPES, and never have to
> write Eigen::, without polluting the global namespace with names such
> as "Object".
>
> Hope that is satisfying, and thanks Konstantin, Cyrille, Andre, Ben, Donald
> for contributing your opinions,
>
> Benoit
>
> On Thursday 11 October 2007 21:35:08 Donald Ephraim Curtis wrote:
> > Man this is a tough conversation.  I do like namespaces, they seem like
> > a proper way to do it because that's why they were put into the
> > language, (although i don't know strousup) but both ways accomplish the
> > same thing, just one is emulating namespaces.  If I had to vote I'd say
> > use the namespace Eigen because I like them.  I know we did namespaces
> > with Avogadro and I still consider that maybe AMainWindow might have
> > been better, but honestly I don't regret it.
> >
> > I have a couple things for you to consider Benoit then i'll be done.  I
> > tried to think this out well and please don't take it as an attack.
> >
> > 1) Using a namespace gives the _option_ to the developer to type out the
> > prefix 'Eigen::' everytime or allow them to do 'using Eigen::Object' or
> > to use 'using namespace Eigen'.  If you name the object EiObject you are
> > forcing them to type Ei everytime and even with autocomplete.  Having to
> > type Eigen::Object with autocomplete becomes E<tab>::O<tab> whereas
> > EiObject is EiO<tab> so maybe a little typing saved but negligable.
> >
> > 2) The only time you need to worry about the namespace is when declaring
> > a variable or calling a function in that namespace.  For the latter you
> > don't want to have global functions prefixed with eiDoSomething (in my
> > opinion).  qSort is ackward to me whereas Qt::sort.  It could be
> > cosmetic.
> >
> > 3) Namespaces are more proper.  Just cause QT doesn't use them doesn't
> > make it right.  Qt does use namespaces for their global defines (enums).
> > I find it annoying because I'd just as well do a using namespace Qt when
> > I'm using Qt functions and have it know what enum i'm talking about.  I
> > agree that just because they are proper doesn't make them the best way.
> > Consider though that namespaces are a part of a lot of formalisms (XML,
> > Java, Python) and when you look at people wanting to use Eigen for code
> > generated from UML or some higher level formalism like Model Driven
> > Architecture you're going to want that namespace.  Also i read an
> > article that when they did bindings for Ruby or something for KDE that
> > they had to use namespaces and had to work around it.
> >
> > 4) There is an argument to be made that even with namespaces that
> > ambiguity can exist.  But that's due to bad programming.  Just make sure
> > you're not doing something to make up for the problem of a bad
> > programmer.  Managing namespaces is a part of being a good programmer.
> >
> > Doesn't KDE now use namespaces?  I think it does even though it's a
> > little ugly like KDE:KMainWindow or something like that.
> >
> > Anyways, I think it's up to you.  If you like EiVector i'm totally cool
> > with it.  You do a termendous job with Eigen and also have contributed
> > to Avogadro so much I think you should do what you want.  If anyone is
> > goign to _not_ use your library for that reason then their loss.  And if
> > you decide to go Eigen:: and then change your mind i'll help you search
> > and replace with Ei prefix.
> >
> > --
> > Donald
> >
> > Also read the wiki on namespaces maybe that will help ya decide.
> > http://en.wikipedia.org/wiki/Namespace_(computer_science)
> >
> > (Thu, Oct 11, 2007 at 08:55:30PM +0200) Benoît Jacob
>
> <jacob@xxxxxxxxxxxxxxx>:
> > > Argh.
> > >
> > > Seems like aside from me, everybody is either in favor of a namespace,
> > > or doesn't care.
> > >
> > > You, Cyrille, Konstantin, Andre.
> > >
> > > I'm generally in favor of dictatorship, but every dictator has to
> > > listen to the people once in a while.
> > >
> > > So maybe, we'll revert to namespace Eigen:: instead of a prefix.
> > >
> > > But not tonight. Let's give this whole issue more time to ripen.
> > >
> > > Cheers,
> > > Benoit
> > >
> > > On Thursday 11 October 2007 20:48:44 Schleimer, Ben wrote:
> > > > > > Look at what Qt does. Qt classes are not encapsulated in any
> > > > > > namespace, instead they have the Q prefix, like "QObject".
> > > > >
> > > > > Qt was first release at a time when namespaces were not well (if at
> > > > > all) supported by most C++ compiler.
> > > >
> > > > I have to agree with Cyrille. It would be cleaner to use
> > > > Eigen::Object instead of prefixing every class with Ei. Let the
> > > > application programmer figure out which "using" statements they want
> > > > to do. It'll probably be rare to get namespace collisions if eigen is
> > > > the only matrix library used in an application. Plus, when Eigen3
> > > > comes out, it'll be easier for app programmers to which by changing
> > > > the "using EigenX" instead of the prefix.
> > > >
> > > > Cheers
> > > > Ben