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

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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<KeyType>, which defaults to the < operator,
but there’s no requirement to use the defaults. Just write a comparison
operator (preferably as a functional object):

struct CmpMyType
{
    bool operator()( MyType const& lhs, MyType const& rhs ) const
    {
        //  ...
    }
};

Note that it must define a strict ordering, i.e. if CmpMyType()( a, b
) returns true, then CmpMyType()( b, a ) must return false, and if
both return false, the elements are considered equal (members of the
same equivalence class).

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