What requirements must std::map key classes meet to be valid keys?

All that is required of the key is that it be copiable and assignable. The ordering within the map is defined by the third argument to the template (and the argument to the constructor, if used). This defaults to std::less Just write a comparison operator (preferably as a functional object).

The answer is actually in the reference you link, under the description of the "Compare" template argument.

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