如何在秒差距实现下标操作符？
对于下标操作符(操作符)的使用有一点非常重要，就是仅能提取确实已存在的元素，例如。
It is crucially important to understand that we may use the subscript operator, (the operator), to fetch only elements that actually exist. For example.
与下标和解引用操作符一样，赋值操作也返回左值。
Like the subscript and dereference operators, assignment returns an lvalue.
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