函数模板和友元重载运算符报"无法解析的外部符"的解决方法

解决方法：

1. 直接把定义和实现都写在类中
2. 如下：

    #include <iostream>
    using namespace std;


    template <class T>
    class Compleax
    {

        template <class S> //注意这里是S,和上面的T的名字不一样
        friend ostream & operator<< (ostream &out, Compleax <S>&c);
        public:
        Compleax(T a, T b);
        void PrintCom()
        {
            cout<<"a:"<<a<<" b:"<<b<<endl;
        }
        private:
        T a;
        T b;
    };

    template <class T>
    Compleax<T> :: Compleax(T a, T b)
    {
        this->a = a;
        this->b = b;
    }

    template <class T>
    ostream &operator << (ostream &out, Compleax<T> &c)
    {
        out<<c.a<<" "<<c.b<<endl;
        return out;
    }
    int main()
    {
        Compleax<int> c(2, 3);
        c.PrintCom();
        cout<<c<<endl;