一个类型和两个常见的已知***拟合类型，但您的模板运算符< 三种类型。

Your template has three arguments, a type, and two constants of a known best fit type, but your templated operator<< takes an instantiation of the template with three types.

template<class Int_T = int, typename Best_Fit<Int_T>::type Min_Range
                                     = std::numeric_limits<Int_T>::min(), // constant!
                            typename Best_Fit<Int_T>::type Max_Range
                                     = std::numeric_limits<Int_T>::max()  // constant!
        >
class Int
//...
template<class Int_T>
std::ostream& operator<<(std::ostream& out, 
                         const Int<Int_T, 
                                   Best_Fit<Int_T>::type, // type!
                                   Best_Fit<Int_T>::type  // type!
                         >& obj)

我通常建议类模板在类定义中定义（使用 friend 在该上下文中定义一个***函数）因为这个特殊的原因，在类模板中获取类型是很容易的，容易在其外失效。还有一些其他的差异（像事实，如果运算符被定义在类中，那么它只能通过ADL访问 - 无论你也决定在外部声明）

I usually recommend that operator overloads of class templates are defined inside the class definition (use friend to define a free function in that context) for this particular reason, it is trivial to get the types right inside the class template, and easy to fail outside of it. There are a couple other differences (like the fact that if the operator is defined inside the class then it will only be accessible through ADL --unless you also decide to declare it outside)

template<class Int_T = int, typename Best_Fit<Int_T>::type Min_Range
                                     = std::numeric_limits<Int_T>::min(), // constant!
                            typename Best_Fit<Int_T>::type Max_Range
                                     = std::numeric_limits<Int_T>::max()  // constant!
        >
class Int {
   friend                  // allows you to define a free function inside the class
   std::ostream& operator<<( std::ostream& out, 
                             Int const & obj ) {  // Can use plain Int to refer to this 
                                                  // intantiation. No need to redeclare
                                                  // all template arguments
       return out << obj.get_data();
   }
};