当所有元素都应该是唯一的时，为什么 STL 集有 count() ?
我可以理解multiset有count()，用于计算一个值出现的次数，因为元素可以在multiset中重复.但是当所有的值都已经是唯一的时候，设置 count() 有什么意义呢?
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?
推荐答案
count 是 关联容器要求(1).
每个关联容器都需要将其作为其接口的一部分提供，即使结果始终为 0 或 1，就像 std::set 的情况一样.
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.
(1) 这是描述关联容器概念的 SGI STL 文档的链接；C++ 标准中定义的概念可能会略有不同，但不会大不相同.
(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.
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