最后加了一点点。

OOP/Final.md

@@ -113,4 +113,4 @@
     ```c++
     className operator++(int);
     ```
-26. `<<`和`>>`不能重载为成员函数。`=`，`[]`，`()`不能重载为友元函数。
+26. `<<`和`>>`不能重载为成员函数，必须重载为友元函数。`=`，`[]`，`()`不能重载为友元函数，必须重载为类的成员函数。

OOP/test.cpp

@@ -1,20 +1,21 @@
 #include <iostream>
-using std::cout;
 using std::cin;
+using std::cout;
 using std::endl;
 
-template <int x>
+template <int x> struct Factorial {
-struct Factorial {
+    static const unsigned long long value = x * Factorial<x - 1>::value;
-    static const unsigned long long value = x * Factorial<x-1>::value;
 };
 
-template <>
+template <> struct Factorial<0> {
-struct Factorial<0> {
     static const unsigned long long value = 1;
 };
 
 int main() {
     Factorial<20> a;
     cout << a.value << endl;
+    int x = 1;
+    double b = 3;
+    cout << x / b * 3 << endl;
     return 0;
 }