下面这种定义方式会报错: Undefined symbols for architecture x86_64

template 
     
      class longint{public:    T num;    longint (T a = 0) {        num = a;    }        friend ostream& operator<< (ostream& out, const longint
      
       & Lint);    ~longint (){        ;    }private:};template 
       
        ostream& operator<< (ostream &out, const longint
        
          & Lint) {    return out<
        
       
      
     

改法一：将重载运算符的T改成S就行了

//类内声明时template 
     
      friend ostream& operator<< (ostream& out, const longint
      & Lint);//类外定义时template 
       
        ostream& operator<< (ostream &out, const longint
         & Lint) {    return out<
       
     

改法二：直接在类内定义

friend ostream& operator<< (ostream& out, const longint
     
      & Lint) {    return out<