Re: [Boost-users] [BGL] Bundled properties and property maps

Subject: Re: [Boost-users] [BGL] Bundled properties and property maps
From: Andrew Sutton (andrew.n.sutton_at_[hidden])
Date: 2009-01-15 19:07:33

> 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?
>

I will admit that writing perfectly const-correct code can be very, very
difficult in many situations :) However, I'll defer to more experienced
Boosters with regards that its impossible. I will also say that
const-incorrect code isn't necessarily "bad". There are times that ignoring
const-ness is quite useful.

In this particular instance, I will cautiously say that the concept
definition is implemented incorrectly. If the constraints() funciton did
this:

property_map<Key, Graph>::type pm = get(...);

instead of:

pm = get(),

It would preserve the const'ness of the pmap object, and the compiler error
might not exist.

I also wouldn't worry about compiler optimizations in this case. The
compiler is going to elide all or most of the pmap copies and inline all off
the function calls to the underlying data structures. Generally, you'll
never see the statement (pm = get(...)) in Boost.Graph algorithms.

Andrew Sutton
andrew.n.sutton_at_[hidden]