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Subject: [std-proposals] Separately qualifying names in structured bindings
From: Kyle Knoepfel (kyleknoepfel_at_[hidden])
Date: 2020-01-03 10:27:37


Hi all,

I know the following has been considered in the past, but I'm wondering if
the considerations have changed in the last few years. To summarize,
structured bindings allow all names to be introduced with the same
qualification:

std::map<unsigned int, std::string> employees = workers();
for (auto const [id, last_name] : employees); // 1. id and last_name are
both copies
for (auto const& [id, last_name] : employees); // 2. id and last_name are
both references

The lesser of two evils would be to pick option 2 to avoid the string copy,
even though using id as a reference is suboptimal.

I would love to be able to do something like:

for (auto const [id, &last_name] : employees); // 3. id is a copy;
last_name is a reference

This syntax parallels that of lambda expressions, and would therefore not
be "that new" of a construct to get used to. However, I know that this has
been considered before in:

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0144r1.pdf

in section 3.5, and the preference is to disallow it for various reasons.
Other proposals have been made that would enable more flexible use of
structured bindings. See Ville's proposal:

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2018/p0480r1.html

It seems that the direction the committee is taking is to leave things be
(keep it simple, for now) until pattern matching evolves a bit more. Does
anyone know if that's still the case, or should an attempt be made again to
make structured bindings more flexible a la what is/has been proposed
above? Ville, do you have any comments?

Thanks,
Kyle Knoepfel



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