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Subject: Re: [boost] [review][mp11] Formal review of Mp11
From: Brook Milligan (brook_at_[hidden])
Date: 2017-07-19 17:47:43

Here is my formal review of Mp11.

> On Jul 15, 2017, at 4:19 AM, Bjorn Reese via Boost <boost_at_[hidden]> wrote:
> Mp11 is a C++11 metaprogramming library for compile-time manipulation of
> data structures that contain types. It’s based on template aliases and
> variadic templates and implements the approach outlined in the article
> "Simple C++ metaprogramming" [2] and its sequel [3]. These articles are
> useful background information for the review.

Background: at the time I first ran across these articles, I had been a reasonably proficient Boost.Mpl programmer, but was facing challenges and wanting to take advantage of c++11. I found these articles very helpful, copied a bunch of the code to explore, and learned quite a bit about metaprogramming in c++11. Thus, the foundation for Mp11 that Peter established early on was very helpful. Since then, I incorporated my copied and modified versions of the article code into a large project that needed some fairly tricky metaprogramming, and found Peter’s library very useful. Most recently, I have stripped out most of that code and made the Mp11 library an external dependency. That transition went perfectly smoothly and left only a handful of “custom” metafunctions built on top of Mp11. During the course of this review, Peter has already incorporated most of those in the library, because he agreed that they would be generally useful.

> 1. Should Mp11 be accepted into Boost? Please state all conditions
> for acceptance explicitly.

I feel Mp11 should definitely be accepted.

> 2. What is your evaluation of the design?

As mentioned above, the clarity of the design became evident to me early on after reading the original articles. At the time I was not a great c++11 programmer, but working with the early Mp11 code and extending it based upon the principles outlined in the articles was quite straightforward and taught me some useful things about c++11. I feel that the design has only improved since.

> 3. What is your evaluation of the implementation?

I have not spent much time with the current implementation. However, I did pour over the initial implementation to understand it and to extend it to cover a few additional use cases that I needed. I felt that the implementation made a lot of sense, especially in concert with the articles, which explained the “magic” really well.

> 4. What is your evaluation of the documentation?

The current documentation seems quite good, if a bit telegraphic at points. Keeping the links to the original articles is essential, and perhaps an abridged version of the most salient points would improve the introduction. It is hard to tell, because I read those articles long ago, learned their content, and have been using the library (or its precursor code) for quite awhile.

There are, however, a few points that I have found difficult and about which the documentation is probably too telegraphic. While most of the metafunctions are quite simple, i.e., take one or two arguments and return an obvious type that is easy to reason about, I have more difficulty with the metafunctions that are composable. For example, using algorithms together with bind and friends is something that I feel greater clarity, including examples, would help.

During the course of this review, I suggested a few metafunctions that I had found useful in my own code base that were simple extensions of Mp11. In all but the most complicated case, my implementation of the extension was exactly what Peter subsequently implemented in the library, having taken my suggestions to be useful enough for general consumption. I appreciate his adding to the library; more importantly, though, I feel that my ability to implement these metafunctions exactly like the pro, based only on the documentation available, is a strong indication that the document has served its purpose. At the same time, I implemented the final example (which involved an algorithm and bind) in a much more complicated way than Peter did; this too is an indication of my point above that the documentation on composing algorithms/bind/etc. into more complicated constructs can be improved.

> 5. What is your evaluation of the potential usefulness of the library?

I have been using this library (or its precursor) for the last year in a large code base. Thus, for me personally, it has already been very useful. I expect that it will be highly useful in general.

> 6. Did you try to use the library? With what compiler? Did you have
> any problems?

I have been using clang to work with the code base that uses Mp11.

> 7. How much effort did you put into your evaluation? A glance? A quick
> reading? In-depth study?

I have not put a great deal of extra effort into this specific review. However, I have been using the library for a year, during which time I have thoroughly studied the original articles, copied the code itself, and modified bits to support metafunctions that were not originally provided. This work has been spread out over months, but I feel it corresponds to an in-depth study at some level.

> 8. Are you knowledgeable about the problem domain?

I am familiar with metaprogramming in general and use it extensively in my own code base. I had earlier relied on Boost.MPL, but largely as a result of Peter’s articles and my experience with Mp11 have switched entirely to using c++11 style metaprogramming to take advantage of variadics, type aliases, etc. Thus, I feel quite comfortable with this domain.

Thanks to Peter for putting together such a carefully crafted library.


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