Why does STL set have count() when all elements ar

问题:

I can understand that multiset has count(), for counting the number of occurrences of a value, because elements can be repeated in multiset. But what's the point of having count() in set, when all the values are already unique?

回答1:

count is part of the associative container requirements⁽¹⁾.

Every associative container is required to provide it as part of its interface, even if the result is always zero or one as is the case with std::set.

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(1) This is a link to the SGI STL documentation describing the Associative Container concept; the concept as defined in the C++ standard may differ slightly, but not substantially.