[基础] 模板+友元类外定义

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

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

template <typename T>
ostream& operator<< (ostream &out, const longint<T> & Lint) {
    return out<<Lint.num;
}

改法一：将重载运算符的T改成S就行了

//类内声明时
template <typename S>
friend ostream& operator<< (ostream& out, const longint<S>& Lint);

//类外定义时
template <typename S>
ostream& operator<< (ostream &out, const longint<S> & Lint) {
    return out<<Lint.num;
}

改法二：直接在类内定义

friend ostream& operator<< (ostream& out, const longint<T>& Lint) {
    return out<<Lint.num;
}