C++类模板友元函数和操作符<<重载实例

#include <iostream>
using namespace std;

//template<class T> class Test;


template <typename T>
class A{
public:
    
//    friend ostream &operator <<(ostream &out,A&obj){
//        out<<obj.a<<endl;
//        return out;
//    }
   // friend ostream &operator << <T>(ostream &out,const A<T>&);

    friend void friendPrint();
    template<typename C>//这个参数的类型和 T 不同
    friend void friendPrint2(A<C> &a);
    
//    friend void frindPrint3<T>(T u);
   // friend void display(A<T> &t);
    A operator+(A &c2){
        A tmp(a+c2.a);
        return tmp;
    }
    A(T a){
        this->a = a;
    }
public:

    void printA(){
        cout<<"a :"<<a<<endl;
    }
    int getA(){
        return a;
    }
private:
    T a;
};

template <typename T>
ostream &operator <<(ostream &out, A<T>&obj){
    out<<obj.getA();
    return out;
}

//template<typename T>
void friendPrint(){
    cout<<"----"<<1111111<<endl;
}
template <typename C>
void friendPrint2(A<C> &mya){
    
    cout<<mya.a<<endl;
}

int main(int argc, const char * argv[]) {
    // insert code here...
    std::cout << "Hello, World!\n";
    
    
    A<int> a1(10);
    A<int> a2(20);
    A<int> a3 = a1+a2;
  //  a1.printA();
    cout<<a3<<endl;
    
    friendPrint();
    //friendPrint(a3);
    friendPrint2(a3);
    
    return 0;
}