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Using std::map with a custom class key
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When using std::map should I overload operator== for the key type?
是否需要重载 ==
结论： operator < is enough. Equality can be checked by testing a < b and b < a both returning false.
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std::map does not care about literal unicity of the keys. It cares about keys equivalence.
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The keys a and b are equivalent by definition when neither a < b nor b < a is true.
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Note also that std::map does not directly use operator <. std::map does not know anything about operator <. Instead std::map use
