ait allows an instance
of the smart pointer struct to behave like a reference so you can write code
that works with either references or smart pointers. The `Drop` trait allows
you to customize the code that is run when an instance of the smart pointer
goes out of scope. In this chapter, we’ll discuss both traits and demonstrate
why they’re important to smart pointers.
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<p>スマートポインタは普通、構造体を使用して実装されています。スマートポインタを通常の構造