Re: [boost] [hash] Turn on BOOST_HASH_NO_IMPLICIT_CASTS by default

Subject: Re: [boost] [hash] Turn on BOOST_HASH_NO_IMPLICIT_CASTS by default
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2012-04-18 07:44:33

Daniel James wrote:
> On 17 April 2012 15:37, Stewart, Robert
> <Robert.Stewart_at_[hidden]> wrote:
> >
> I suppose the big question is, which implicit casts should
> considered suitable. IIUC your solution has the advantage of
> only banning certain implicit casts.

Right.  By restricting the set of types that are supported to the
exact types mentioned, there can be no implicit conversions to
those types.  Nevertheless, the primary specialization can permit
implicit conversions for UDTs.

> Even if I wanted to avoid SFINAE, I could probably write a type
> trait to detect them that doesn't rely on it.

By that I presume you mean that you would use a trait to dispatch
to other functions/function templates that avoid the bool
conversion problem, right?

> I'm not sure if an implicit cast to a class' hash_function
> would always be considered safe. For example, someone might
> supply a lossy implicit conversion to std::string.

Implicit conversions cause all manner of problems.  They are
rarely a good idea, so I doubt you should worry about that.
Besides, the function template can be specialized for such a
class when the implicit conversion causes undesirable hashing.

> > You could also use std::numeric_limits<T>::is_specialized
> > to detect a possibly larger set of types to which overloads
> > in the details namespace could be applied.
>
> That's an interesting idea regardless of this issue. But I'm
> not sure how much you can infer from a numeric_limits
> specialisation. My worry is that someone might specialise it
> for a class that gives the hash function problems, something
> like a big int perhaps.

The function template can be, and very likely should be,
specialized for the big int type, so any issues can be rectified,
though not prevented with that approach.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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