Person类仅仅用一个指针来指向某个不确定的实现，这样的类常常被称为句炳类(Handle class)或信封类(Envelope class)。（对于它们所指向的类来说，前一种情况下对应的叫法是主体类(Body class)；后一种情况下则叫信件类(Letter class)。
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envelope class
信封类
以上为机器翻译结果，长、整句建议使用 人工翻译 。
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The proposed Forever stamp is good for a first-class letter no matter when you stick it on the envelope.
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