Constructs an empty container using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it.
Constructs an empty container using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it.使用 `a` 以及默认的哈希函数和键相等谓词构造一个空容器，并将 `il` 中的元素插入其中。
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Constructs an empty container using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it.使用 `a` 以及默认的哈希函数和键相等谓词构造一个空容器，并将 `il` 中的元素插入其中。