If an element `x` exists with key equivalent to `k`, invokes `f` with a reference to `x`. Such reference is const iff `*this` is const.
If an element `x` exists with key equivalent to `k`, invokes `f` with a reference to `x`. Such reference is const iff `*this` is const.若存在键与 k 等价的元素 x，则以 x 的引用调用函数 f。 当且仅当当前容器 *this 为常量（const）时，该引用为常量引用。
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If an element `x` exists with key equivalent to `k`, invokes `f` with a reference to `x`. Such reference is const iff `*this` is const.若存在键与 k 等价的元素 x，则以 x 的引用调用函数 f。当且仅当当前容器 *this 为常量（const）时，该引用为常量引用。