front在TestSolution中没有被覆盖，它已被重载.

front is not overridden in TestSolution, it is overloaded.

您可以将重载函数视为完全不同的函数，例如名称不同的函数.

You can regard an overloaded function as a completely different function, like one with a different name.

所以t.front(a)会调用Tyre中的那个，而a会隐式转换为long.

So t.front(a) will call the one in Tyre, with an a implicitly converted to long.