If you need runtime polymorphism, you must not use pass-by-value.
如果你需要运行时多态，你不能使用按值传递。
Why does declaring runtime polymorphism for template instantiations result in linker error?
为什么声明为模板实例化运行时多态导致链接器错误呢？
This allows you to swap out at runtime a different class exposing the same methods, taking advantage of polymorphism.
它允许利用多态性在运行时换出一个不同的类来公开相同的方法。
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