Re: [boost] [metal] Feature Complete - Request for Feedback

Am 21.02.2017 um 14:03 schrieb Bruno Dutra via Boost:

On Tue, Feb 21, 2017 at 10:53 AM, Hans Dembinski wrote:

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I suppose my big question is this: can't you just merge your ideas/improvements into boost::mpl? With ifdefs you could enable C++14 features like templated aliases when they are available. Even more so, it should be possible to merge your speed improvements into boost::mpl.

It does sound like a good idea from the user's point of view since Metal looks so similar to MPL, but because they are so drastically distinct internally, merging them into a single library doesn't really make any sense. Fortunately, if you are coming from some legacy metaprogram, it is easy to incrementally migrate to Metal using the helper metal::from_mpl, but there is really no reason to ever use MPL anymore on a new project.

Could you elaborate a little more on this metal::from_mpl? How is it used? Can I just replace all occurrences of "boost::mpl" in any code by "metal::from_mpl" and it works?

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I also looked at your benchmarks and they are a bit confusing. I am mostly interested in how your lib compares to the mpl, but not all benchmarks have mpl data, and those that have (see at or count_if) only use up to 50 elements. Is that a hard limitation of the current mpl implementation?

Unfortunately that is indeed a hard limit of of Boost.MPL. Just like mpl::quote, everything else in MPL had to be implemented by enumerating every possible combination of arguments, which implies that some hard limit must have been chosen. That limit is 50 to sequence sizes and 15 to mpl::quote, mpl::apply and their friends.

Just a short note: You can increase this upper limit by changing the values of some Boost.MPL macros and pre-generating headers for Boost.MPL-containers with these new values. For this, some python-scripts exist in "libs/mpl/preprocessed". However, they stopped working after the migration to Git from Subversion. And they were pretty complicated to use (and understand). Therefore, a few years ago, I created some helper python-script "boost_mpl_preprocess.py" which fixes pre-generating the headers and greatly simplifies the process. It is also located in the same directory in Boost-source since Boost 1.59. Just call it like this to see the options and its usage: python boost_mpl_preprocess.py --help At our company we had to increase the number of elements to 100 and it works fine except for the obvious reasons Bruno already talked about. It is so damn slow and wastes so much RAM that we can only compile single-threaded which slows down compile-performance even more. That is why I am so interested in a drop-in replacement for Boost.MPL and asked about metal::from_mpl above.

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Regards, Bruno

Thanks, Deniz