avoid copies, when comparing maps #537
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There were two points, where copies were made: 1. When binding a map for use in `partial`. This could be avoided, by using `std::ref`, but then either `partial` would have to handle `std::ref` in special way, by unwrapping the reference at the call point or the wrapped function would have to specify the accepted argument type explicitly to force cast. Instead, partial was replaced with a `compare_at_key` helper struct capturing the `maps` by reference. 2. When returning the value from `find` wrapped into optional. This is avoided by using `at_key` instead of `find`.
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The alternative approaches can be checked at: |
ricejasonf
reviewed
Dec 13, 2025
| // Check if non-copyable objects can be compared | ||
| auto map_unique_1 = hana::make_map(hana::make_pair(hana::integral_constant<int, 0>{}, std::make_unique<int>(0))); | ||
| auto map_unique_2 = hana::make_map(hana::make_pair(hana::integral_constant<int, 0>{}, std::make_unique<int>(0))); | ||
| BOOST_HANA_RUNTIME_CHECK(map_unique_1 != map_unique_2); |
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Maybe add map_unique_1 == map_unique_1. I am not seeing that hana::equal tries to short circuit identical types (or even address).
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There were two points, where copies were made:
partial.This could be avoided, by using
std::ref, but then eitherpartialwould have to handlestd::refin special way, by unwrapping the reference at the call point or the wrapped function would have to specify the accepted argument type explicitly to force cast.Instead, partial was replaced with a
compare_at_keyhelper struct capturing themapsby reference.findwrapped into optional.This is avoided by using
at_keyinstead offind.