[Insight-users] itk::SmartPointer - problem making code const-correct

[Stuart Golodetz]

Hi,

Hope this is the right place to post this. I was just wondering if 
there's a reason why itk::SmartPointer was designed so as not to allow e.g.

itk::Image<int,3>::Pointer image;
const itk::Image<int,3>::ConstPointer& cimage = image;

?

The equivalent with boost::shared_ptr is allowed, e.g.

boost::shared_ptr<int> p(new int);
const boost::shared_ptr<const int>& cp = p;

This doesn't seem like a major problem, until you start writing 
functions taking const itk::Image<...>::ConstPointer& parameters - at 
which point it won't let you pass a normal Pointer in without explicitly 
constructing a ConstPointer from it. Now the types are often quite long, 
and it's annoying to have to add extra typedefs in the calling code just 
for that purpose. Duplicating the functions with const 
itk::Image<...>::Pointer& parameters doesn't work either, because you 
get a combinatorial explosion when you have multiple such parameters. 
For instance, with 3 parameters, you have to create functions with 
combinations:

const Pointer&, const Pointer&, const Pointer&
const Pointer&, const Pointer&, const constPointer&
const Pointer&, const ConstPointer& const Pointer&
// more here
const ConstPointer&, const ConstPointer& const ConstPointer&

This seems like an unproductive way to spend one's time, to say the 
least. The only other "reasonable" alternative I've managed to come up 
with that doesn't either (a) clutter up the call site or (b) cause the 
combinatorial explosion just outlined, is to just use the non-const 
Pointers everywhere and abandon the idea of making the code 
const-correct. But that seems defeatist to me  :)  Please could you tell 
me if there's something I'm missing? (And if so, what?)

Cheers,
Stuart

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