在第二种情况下，RET是什么，取决于模板类型T.需要确保编译器在所有可能的实例化中都将是一种类型(并且可能不是IF某些实例化的静态成员).您可以使用typename关键字来实现.

In the second case, what RET is, depends on the template type T. The compiler needs to be assured that it is going to be a type in all possible instantiations (and not perhaps a static member of some instantiation of IF). You do so with the typename keyword.

template <typename T>
class Param
{
    typename IF< sizeof(int)<sizeof(long), T&, T* >::RET mParam;

};