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Comment by jcelerier | Hacker News Reader

    for (auto [s, n]: my_map)

copies all the data needlessly, better to use

    for (const auto& [s, n]: my_map)

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That's why some newer languages have 'const are the default'.

I'm pretty sure it's the reference that makes the data not be copied.

Without the const the key would still be const.

Copying the data to a const makes little sense in this case. The extra & choice that has emerged makes things more complicated than needed. The sad faith of this old language.

what a language

just consider const reference to be the default for non-primitive types

My rule of thumb is to use const references when the sizeof the type is larger than the sizeof a pointer or reference.

Might be worth using values/copies even a bit bigger, so long as it's "simple" data. This[1] short post argues for passing `std::string_view` (~2 pointers) by value, for

- Omitting pointer indirections (loads),

- Not forcing the pointee to have an address (i.e. gotta be in memory, not just registers), and

- Eliminating aliasing questions, potentially leading to better codegen if the function isn't inlined.

1: https://quuxplusone.github.io/blog/2021/11/09/pass-string-vi...

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