嗯，首先，模板不是运行时多态——它们是编译时多态.

Uhm, first of all, templates are not runtime polymorphism -- they're compile-time polymorphism.

如果你想使用运行时多态性，你必须确保 ModCont> 派生自 ModCont<A<T>> -- C++ 处理多态的方式并没有使它成为默认值.或者，您可以让所有 ModCont<T> 派生自一些通用的 ModContBase，尽管目前还不清楚它是如何工作的.

If you want to use runtime polymorphism with this, you have to make sure that ModCont<B<T> > derives from ModCont<A<T> > -- the way C++ handles polymorphism does not make this the default. Alternatively, you could have all ModCont<T> derive from some general ModContBase, although it's unclear how that would work.