This is the first sequel to my post several months ago on customization point design for library functions. That post was all about function customization points. This post is all about type-based customization points — a.k.a. traits classes — or at least what the standard library means when it says “traits class.”

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Subtitled, “The Itanium ABI ruins everything, again.”

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Highly recommended reading for people who want to know something about East Asian writing systems, or for people who already know a bit about them and enjoy reading entertaining things: “If English was written like Chinese,” by Mark Rosenfelder.

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In a previous post on borrow types, I mentioned in passing the idea of operator auto. For the past couple of years, when I’ve used that phrase, I’ve meant something like what’s proposed in Joël Falcou, Peter Gottschling, and Herb Sutter’s P0672 “Implicit Evaluation of auto Variables” (2017). That is, we have some class with a bunch of operators that return expression templates, or perhaps just proxies (like vector<bool>::iterator). Anyway, they return objects of some ugly temporary type, probably containing dangling references.

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Michael Deacon of the Telegraph writes (2013-05-10):

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I just watched Odin Holmes’s half-session from C++Now 2017 titled “Type-Based Template Metaprogramming is Not Dead.” Honestly, I found it mostly pretty hard to follow. (Note to presenters: The video recording will pick up your mouse cursor, but not your laser pointer!) But there was one “oooh” moment right around 15 minutes in. Context: compile-time performance.

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Is this code:

struct VD2 {};
struct VD2 final;

• ill-formed, requiring a diagnostic from the compiler?

• ill-formed, no diagnostic required?

• valid, and further inheritance from VD2 is diagnosed as an error?

• valid, and further inheritance from VD2 is permitted with no diagnostic?

The answer is below the break. …

Today I learned: Explicitly defaulting a class’s destructor causes the compiler not to implicitly define any move constructor at all. Thus:

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The “relocate” operation to be proposed in my upcoming P1144 (not yet published) is a high-level operation consisting of a move-construct and a destroy. This is analogous to how most libraries today define “swap” as a high-level operation consisting of a move-construct, two move-assigns, and a destroy.

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Via Mark Dominus:

There are two boxes on a table, one red and one green. One contains a treasure. The red box is labelled “exactly one of the labels is true”. The green box is labelled “the treasure is in this box.”

Can you figure out which box contains the treasure? …

Just in time for Independence Day comes this news item from my alma mater:

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Today I learned:

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Right now you can register for a full week of CppCon 2018 for the low low price of only $845. On July 7th, early-bird registration closes, and the price goes up to $995.

Some people are probably waiting to see what the program schedule looks like, before they commit to the price tag. If you’re in that category, stop waiting! …

Via David Chess:

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This is a recycling of a reply I just made to Mingxin Wang and Gašper Ažman over at std-proposals. TLDR I think “trivially swappable” is a straightforward corollary to “trivially relocatable”, but its cost-of-specification might be a bit higher, and its benefit-in-performance is essentially zero as far as any use-case I can think of.

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Today I learned that shields.io is a thing.

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I was looking at HyperRogue again this week (see my previous post). It has a really nice localization framework: every message in the game can be translated just by adding a lookup entry to a single file (like, for the Czech translation, you add entries to language-cz.cpp); and then during the build process, all the language-??.cpp files are collated together and used to produce a single language-data.cpp file with a lookup table from each English message to the same message in every other language. (Seeing all the messages at once allows us to report on how “complete” each translation is, relative to the others.)

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Currently reading Max Havelaar (1860), “the Dutch Uncle Tom’s Cabin.” From Chapter 12, in the middle of a dinner party where Max has just wound up one of his long stories:

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From Superiority (1951), by Arthur C. Clarke:

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