Subject: Re: [Boost-users] [BGL] Bundled properties and property maps
From: Geoff Hilton (geoff.hilton_at_[hidden])
Date: 2009-01-15 15:44:39
Andrew Sutton wrote:
> const property_map<...>::const_type map; // or something similar.
>
> For all property maps, map = get(...) is a valid expression,
> regardless of whether your map is a ::type or ::const_type. By
> declaring it const, and then instantiating a template with
> the const
> pmap, you're going to run into problems - probably the
> problem you
> reported earlier.
>
>
>
>
> Okay, so if I understand correctly I shouldn't use const prop maps
> with const_type at all?
>
> It seems to me like a line such as the above should still
> theoretically compile. At least from the perspective of a concept
> check for Readability; this is by definition what const is meant to
> restrict objects to so in theory in should be allowable no? If I'm
> right it would be a compilation error caused by the underlying
> implementation. Either that or a debatable foible that should be
> documented? *shrug*.
>
>
> I don't think you should be using const property maps at all - with type
> or const_type. For example:
Okay.
>
> /* 1 */ property_map<...>::const_type p; // Good
> /* 2 */ const property_map<...>::const_type p; // Bad
>
> The const_type in 1 forces the p to operate on its underlying reference
> in a const way. Returing const references, no put() operation, etc. The
> leading const in 2 means that you can't write:
>
> p = q; // Assuming q is type property_map<...>::type
>
> It's a compiler error since p is not modifiable.
I agree, both situations are perfectly reasonable.
>
> If you look at the concept definition in boost/property_map.hpp for
> ReadablePropertyMapConcept, you'll find, in the constraints() member,
> this line:
>
> val = get(pmap, k)
>
> So apparently, the concept definition actually requires that property
> maps are never const - which is admittedly a little weird.
It sounds like we're now on the same page! The above essentially means
that it becomes literally impossible to write perfectly const-correct
code (from a user standpoint). While const-incorrect code is bad enough,
it also means that I may be missing out on potential compiler
optimizations, that bugs me quite a bit more.
What may be done to remedy this?
>
> Andrew Sutton
> andrew.n.sutton_at_[hidden]
Thanks,
Geoff