Re: [boost] [type_erasure] Review started (July 18-27, 2012)

Subject: Re: [boost] [type_erasure] Review started (July 18-27, 2012)
From: Eric Niebler (eric_at_[hidden])
Date: 2012-07-27 15:46:13

On 7/17/2012 10:13 PM, Lorenzo Caminiti wrote:
> Please state clearly whether you think this library should be accepted
> as a Boost library.

Yep.

> Other questions you may want to consider:
> 1. What is your evaluation of the design?

Solid. One question:

Does concept_interface really need to explicitly inherit from Base? What
about mpl::inherit_linearly? I suppose this has problems with
overloading, right? Blarg. The overloading thing sure is nasty, but I
have no suggestions. :-(

> 2. What is your evaluation of the implementation?

I didn't look.

> 3. What is your evaluation of the documentation?

The reference docs are thorough. The user docs could be more gentle. I'm
afraid it will be rather imposing for most non-guru C++ programmers.

There is very little motivation given. Check out Sean Parent's BoostCon
2012 talk on value semantics[^1] to see a really good example of
building motivation using a concrete example, and then presenting the
solution. It helps get the points across. The one and only complete
example in the docs (polymorphic range formatter) has this only for
building motivation:

>  This example demonstrates using Boost.TypeErasure to implement a virtual "template" function. 

That's a pretty thin motivation. WHY is a virtual template interesting?
What problem does it solve? Is this really all that you have to say
about this example?

In short, I'd put a handful of examples at the front of the users' guide
that are relate-able and illustrative. Otherwise, you just have a
listing of features and sample usages, with no motivation, no rationale,
and no thread tying the features together.

It might even help to describe what "type erasure" is, how it's been
traditionally used, and what one of the simpler examples would look like
without Boost.TypeErasure. That way, folks can get a feel for the
drudge-work that your library saves them.

Aside: I don't much care for the name "TypeErasure". In purpose, it's
much like the proposed Boost.Interface library. And that is closer to
how the library is used: to define an interface and build a type that
implements it. The fact that it uses a mechanism called "type erasure"
to achieve that is really just an implementation detail. Some might feel
the word "interface" comes with too much OO baggage, or imply reference
semantics. I don't feel that way, but then you might consider
"StaticInterface" or "ValueInterface". Actually, I quite like the latter.

I'd also consider going through the docs and changing references to
"requirements" to "interface".

Details:

On "Using references"
> Note
> 
> _self is the default placeholder, so it is easiest to use _self&. We could use another placeholder instead. any<__typeid<_a>, _a&> has exactly the same behavior

__typeid should be typeid_

"Concept Maps"
A link here to your built-in Concept Definitions would be nice. "See
_here_ for a comprehensive list of built-in concept maps that can be
specialized like this."

"Associated Types"
> Referring to the full name of the associated type can be cumbersome when it's used many times. 

This, after a line of code that reads "any_cast<int>(*x)". After some
head-scratching, I'm inferring that *x returns an any. Of course. And
same_type is a promise that deduced<pointee<T> > will always come out to
be "int", so please just hard-code that in the interface and run
whatever checks are necessary to guarantee that that is true. This is
all clear after reflection, but it would be nice if the docs were more
explicit on this point.

> 4. What is your evaluation of the potential usefulness of the library?

Very.

> 5. Did you try to use the library?  With what compiler?  Did you have
> any problems?

No, I didn't try to use it.

> 6. How much effort did you put into your evaluation?  A glance?  A
> quick reading?  In-depth study?

2+ hrs.

> 7. Are you knowledgeable about the problem domain?

Yes.

[^1] http://www.youtube.com/watch?v=_BpMYeUFXv8&feature=plcp

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com