[boost] [conversion] review

Subject: [boost] [conversion] review
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2011-08-26 03:06:48

Hi all,

Here is my review of the Conversion library.

I like the idea of a uniform configurable casting operator, whether it's called convert_to, static_cast, convert_cast, or extrinsic_cast.  

If I had to write a conversion function, I would rather write it as an overload of a general-purpose converter, rather than giving it some ugly typeA2TypeB name.  And I would rather find out what header to include and use a generic function rather than using such some specific name.

I vote to accept the library (although obviously as Review Manager I'll follow the wishes of all reviewers).

Part of the reason I took on the challenge of managing this review is that the library is something people don't know they need.  I think uniformity of interface is a good enough reason to use the library, but when would you need a truly generic conversion function as this library offers?  

Or more to the point, what does it mean to have a "shadow conversion space" for types that don't know about each other?

We would hope that libraries are generic enough so that their types can be adapted to work with each other (prime example: Polygon & Geometry). But it's the sad truth that most libraries are not designed this way, and any non-header library can't be designed this way.  So that's where Conversion steps in to make combining those libraries cleaner.

As we've already discussed, the Achille's Heel of this library is that it requires all compilation units to agree on what conversions they will use, to avoid ODR violations.  In practice this probably means that users would have to maintain their own conversion setup headers, which is annoying.  But it's hard to imagine an openly configurable global utility that doesn't suffer from some version of this problem.

John Bytheway's suggestion of tagging libraries with type keys is an interesting idea.  I think Vicente also mentioned the possibility of a scheme that would cause linker errors instead of ODR violations, which would be an improvement.  (FWIW I suggested something involving #error but of course that would only work within a CU.)

In my mind it is very clear that a templated function<T>(x) will probably return a T and I don't find it necessary that the proposed function has to use the preposition "to".  But that's because I'm very aware that the explicit template argument must be the return type, which is understandably not clear to everyone.  

While the recursive features of this library look interesting and useful, I wonder if there isn't a more general way to do this.  I keep thinking it's an awful lot like fusion::copy or Marsden's Traversals.

Well, that's enough for now.  I hope later today to post summaries of some comments I received off-list and maybe get those people to join the conversion.  (You know who you are!)

Cheers,
Gordon