On Mon, May 20, 2013 at 02:49:36PM +0100, Garth N. Wells wrote: > On 20 May 2013 14:33, Anders Logg <[email protected]> wrote: > > On Mon, May 20, 2013 at 01:09:20PM +0100, Garth N. Wells wrote: > >> On 20 May 2013 12:44, David Ham <[email protected]> wrote: > >> > Hi all, > >> > > >> > I'm writing Dolfin-compatible wrappers for PyOP2 as previously > >> > advertised at > >> > FEniCS '13, which is causing me to bump into one of the "interesting" > >> > quirks > >> > of the Python Dolfin API. Lots of things which would appear to naturally > >> > be > >> > properties are actually methods and have to be called to be accessed. For > >> > one among many, many examples, consider the value_size method of a > >> > Function. > >> > This is accessed with: > >> > > >> > f.value_size() > >> > > >> > while > >> > > >> > f.value_size > >> > > >> > would seem more natural. Given the existence of the @property decorator > >> > in > >> > standard Python which translates the former into the latter, this is > >> > particularly mysterious. Is there a reason why this is done in Dolfin? > >> > > >> > >> A few of us discussed this in January. I agree that the latter is cleaner. > >> > >> First point, the Python interface is largely generated automatically, > >> so that's our starting position. We would save on C++ code and get the > >> syntax ' f.value_size' in many cases by not accessing member data via > >> functions. I like skipping the function, and have been doing so lately > >> with new code. The issue we discussed in January was backwards > >> compatibility - we could make a lot of C++ changes to get the syntax > >> 'f.size', but users would have to update their code (this point > >> bothers me less than it does others :)). > >> > >> In some cases we need a method, e.g. to get the size of a vector from > >> a linear algebra backend. Storing the size in a wrapper is error > >> prone. > >> > >> In summary, the reason for the interface in some parts is > >> history/convention (with no technical reason), and in other cases it's > >> a method for technical reasons. We could move more towards direct > >> access to member data. > > > > I don't agree with these conlusions. > > > > The reasons for the use of methods are: > > > > 1. Tradition: following some C++ guidelines we read 10 years back that > > member data should never be accessed directly. > > > > It's done at times in the STL, e.g std::pair.
Yes, this is a good example of where it makes sense. > > 2. Safety: Being able to access member variables means they can be > > changed from the outside (for a non-const object). This might lead to > > all kinds of unwanted behavior. A member variable can rarely be > > changed safely without changing a whole bunch of other data. > > > > If the data must be hidden, then it can be hidden behind a function. > Otherwise, if the object is const, then member data cannot be changed > and only const functions can be called on the data. > Something that would make things cleaner around the code would be to > let more objects have immutable data (e.g., not allow Vector > resizing), which allows some member data to be made const. How does one disallow changing a member variable in C++ except making it private (as we do now) or making it const (which means not even the class itself can change the value)? > At present we have annoying code duplication in numerous classes to > provide const and non-const access functions. Agree. > > 3. Consistency: Sometimes, there is no member data to be accessed but > > it gets computed or extracted by the accessor function. This means if > > we moved to more access of member data, there would be mix of both and > > it would be confusing and inconsistent. > > > > This is a major plus to accessing data directly. It makes explicit > that accessing a data member involves no computation. This is why we try to name those functions compute_foo rather than foo. > > On top of theses strong reasons (2 + 3), we would also break the > > interface. > > Which is a drawback. Just to be clear, I agree with both of you that foo.bar is prettier than foo.bar() and therefore an interesting proposal, if it can be done in a safe and consistent way. -- Anders _______________________________________________ fenics mailing list [email protected] http://fenicsproject.org/mailman/listinfo/fenics
