charleyb123 . wrote:
http://pdimov.com/cpp2/simple_cxx11_metaprogramming.html
Very nice. Very clean and impressive post.
Peter respondeth:
Thank you. <snip, read-it-fast>
I happened to be looking at the screen when you posted, and opened the link. Still, I think it's greatly an attribute of the (very) high quality of your post. This was my "clean" reference: Your post is very well written, very well organized, and you express very clear assertions with supporting rationale and example(s). (Works for me, anyway. ;-)) Bravo -- I'm sure it took you a LONG time to write. There's a lot there, and you clearly needed to experiment with the compiler quite-a-bit to feel-your-way-through. Greatly to your credit, I think this an especially timely topic. I've been thinking about similar things for a while, but not with the clarity and understanding that you expressed. Your "opening" grabbed my attention: (a) <snip, historic reference to Boost-MPL and C++03 approaches> (b) <snip, C++11 changed the playing field because of variadic templates and associated parameter packs> IMHO, we're "re-learning" the TMP thing with C++11, C++14, and C++17. I've been watching Boost-Hana closely, and it's kind of blowing my mind. I particularly liked these two assertions:
(a) mp_size to compute type-list length is a truly generic primitive which is, "...so nice that [you'd] argue that all our metaprogramming primitives ought to have this property."
(b) "Lack-of-higher-order-metaprogramming", suggesting that in C++11 and beyond, we largely should not need metafunctions such as compose, bind, or a lambda library.
The only problem with (b) is that you can't define a template alias inside a function, which I myself found more than a bit annoying. Lambda functions a-la Hana have a distinct advantage here - they can be defined inside functions.
Still. :-)
Good point, I'll have to think-on-that. It doesn't take away from your "main-thrust", though, which is that an extreme elegance is now possible *without* metafunctions. That's Really Nice, because they seem kind of "klunky" to me, and I like the idea where we can increasingly get-away from them. The other thing that I wanted to mention (which I almost put in my first response, but then took-it-out) is that I really agree with your approach to use a prefix ("mp_" in your case) rather than relying on an enclosing namespace. I agree that it's nice to be able to exercise more control over exactly what the compiler is finding-and-expanding without worrying about weird namespace "discoveries". ;-)) Again, thank you very much for the article: It shouts loudly that we have new and more elegant options for template metaprogramming, and I really like the direction you suggest. I really think yours is going to be one of those "classic-posts" that we'll still be talking about for the next few years. (So, if I ever run into you, drinks are "on-me" -- you deserve it.) --charley