How to make a map sort by value C++

Question

I was trying to make a map sort by value using a custom comparator but I couldn\'t figure out why I kept getting the error of \"no matching call to compareByVal\"

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Answer 1

Simply put, you cannot. Not sure which compiler you're using, but clang and gcc both give useful messages. with context.

clang: static_assert(__is_invocable<_Compare&, const _Key&, const _Key&>{},

gcc: if (__i == end() || key_comp()(__k, (*__i).first))

You can see that clang and gcc are both calling the compare method with only they key, and not a value. This is simply how maps work.

If you want to sort by value, you would have to create your own custom map, or, more realistically, use the value as the key instead. Creating your own map to achieve this would be more difficult than you'd think, since it would have to sort after any value is modified.