类在不必要的时候不要包含虚方法。

Classes do not contain virtual methods when they are not needed.

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同样，我们的实现覆盖了其中一些do_foo虚方法。

Likewise, our implementation overrides some of these virtual do_foo methods.

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把虚方法调用转换成直接方法调用，叫做单形monomorphic调用变换。

Converting a virtual method call to a direct method call is called monomorphic call transformation.

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