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Subject: Re: [boost] [TTI] Review
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-07-18 20:41:11

On 7/18/2011 1:07 AM, Gregory Crosswhite wrote:
> Are you knowledgeable about the problem domain?
> -----------------------------------------------
> I have done some metaprogramming and made some use of SFINAE, but my
> knowledge of the program domain is not extensive.
> How much effort did you put into your evaluation? A glance? A quick
> reading? In-depth study?
> --------------------------------------------------------------------------------------------
> I have glanced over the documentation a few times over the last few days
> in order to give myself a chance to ponder it subconsciously, but I have
> probably only spent about an hour or two in total actively examining and
> thinking about the library, plus about an hour writing this review.
> Do you think the library should be accepted as a Boost library?
> ---------------------------------------------------------------
> I would say yes from what I have seen and understand about the library,
> but based on my relative lack of knowledge in the program domain and the
> relatively small amount of time I have spent looking at the library my
> vote should be weighted accordingly.


> What is your evaluation of the design?
> --------------------------------------
> 1) I agree with others that it makes more sense to inject the
> metafunctions at the current scope rather at the absolute scope boost::tti.

You are absolutely correct.

> 2) I strongly think that "CREATE_METAFUNCTION_FOR" (or something
> similar) should be inserted into all of the macro names because
> otherwise they strike me as being misleading. If I were not familiar
> with this library and I saw the macro BOOST_TTI_HAS_TYPE(X) in someone
> else's code, I would be confused because the name of the macro makes it
> sound like it is asking if X has some type even though the result
> doesn't seem to be used. By contrast, if I saw the macro
> clear to me that the purpose of this macro is to create a metafunction.

I can do that but I wonder if programmers really want to type that much
<g>. I do not think they do even though I fully agree with you that your
macro names are much clearer than the shorter ones.

> I say this based in part on my own experience. When I was learning how
> this library worked I started by skimming through the documentation to
> get a feel for what was going on, and the macro names confused me
> because I couldn't tell what they were actually doing. I did eventually
> figure this out when I read through the documentation more closely, but
> if they had names following the convention I am recommending then I
> would have immediately understood what they were doing.
> Although my proposed change would require additional typing (though
> unlikely not much, since I suspect most people are like me and just copy
> and paste from a template), I think that the benefit of improving the
> clarity would be well worth it.

It is a decent suggestion but I do explain after all that the macro
metafunctions create metafunctions for introspecting an inner element
with a given name.

> 3) I wonder if it would be better to instead of giving the metafunctions
> names such as has_type_X, it would make more sense to put them in their
> own namespace so they they would be referred to as for example
> has_type::X. This would save use from having to use macro pasting to
> compute the metafunction names, and would also have the advantage of
> addressing Lorenzo's concern that a double-underscore might
> inadvertently be put in a type name. However, I do not have enough
> experience here to get a full sense of the design implications of
> choosing this convention over the current convention.

Then we are back to injecting a namespace when a macro metafunction is
invoked. I do not want to do this for all the reasons others cited.

> Another possibility is to hide the actual name of the metafunction by
> using a macro to access it. If my recommendation to insert
> "CREATE_METAFUNCTION_FOR" in all of the macro names is followed, it
> would free up the current macro names to be used to refer to the
> metafunction. That is, rather than code looking like
> boost::tti::has_type_X<T>::...
> it would look like
> which would have the advantage of hiding the exact name and location of
> the metafunction as an implementation detail.
> To be clear, point #3 is just me pondering other possibilities to invite
> discussion, and should not be considered a necessary change that needs
> to be made.

I actually have a set of macros for generating the metafunction names.
In the topic "General Functionality" look at the "Macro Metafunction
Name Generation" section.

> What is your evaluation of the implementation?
> ----------------------------------------------
> I glanced at the code and it looked reasonable on the surface but I am
> not an expert in metaprogramming so I did not look at it closely.
> What is your evaluation of the documentation?
> ---------------------------------------------
> As others have pointed out, the documentation desperately needs more
> examples. In particular, unless I missed something I do not believe that
> there are any examples of actually using the library to accomplish
> something; all of the examples only show which macros should be used to
> create the metafunctions and what the resulting metafunction is called.
> In no example did I see an example where the metafunction was actually
> applied to accomplish something, such as disabling a function using SFINAE.
> I suspect that the author assumed that such in-depth examples would not
> be necessary since anyone reading the documentation would probably be
> very experienced with metaprogramming and so would already know what to
> do once they had the introspection metafunctions available to them, but
> not everyone thinking about using this library will be so experienced at
> metaprogramming (myself being one member of this ignorant group) and so
> some examples of how the library could be used to accomplish goals would
> be incredibly useful to us so that we could get a better sense of
> exactly how this library works and how we can leverage it to solve
> practical problems.

I agree with you and intend to add some examples, and a section of the
documentation which presents these examples inlined.

> What is your evaluation of the potential usefulness of the library?
> -------------------------------------------------------------------
> It seems like this library could be incredibly useful to those who need
> introspection functionality, but since I do not generally do the type of
> metaprogramming that this library facilitates this should be considered
> as just my speculation.
> Did you try to use the library? With what compiler? Did you have any
> problem?
> -----------------------------------------------------------------------------
> I played with it a little bit and compiled a random selection of the
> examples successfully.
> I used gcc version 4.2.1 on OSX.
> Cheers,
> Greg

Thanks very much for your review and suggestions.

Eddie Diener

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