c/c + + template and STL small example series < 2 > template class and friend function

For example, if a class is a template class D, there is a requirement to overload the operator < function of D, then a friend is needed.

Three steps are necessary to achieve such a friend

1. Declare the friend function above the implementation code of template class D

template<typename> class D;//Because D is used in the parameter of friend function, you should declare it here first
template<typename T>
ostream& operator<< (ostream&, const D<T> &);

2. Declare it as my friend in the implementation code of template class D

//Note that the operator < is followed by < T >                                                  
friend ostream& operator<< <T>(ostream& ,const D<T>&);

3. Implement friend function

template<typename T>
//Note that the operator < is not followed by < T > 
ostream& operator << (ostream& out,const D<T>& d){
  out << d.x;
  return out;
}

Example code:

#include <iostream>
using namespace std;

template<typename T>
class Test{
public:
  Test(T t) : data(t){}
  virtual void show() = 0;
private:
  T data;
};

template<typename> class D;
template<typename T>
ostream& operator<< (ostream&, const D<T> &);

template<typename T>
class D : public Test<T>{
  //Note that there are <T>                                                  
  friend ostream& operator<< <T>(ostream& ,const D<T>&);
public:

  //Note not Test(t1)                                           
  D(T t1, T t2) : Test<T>(t1), x(t2){}
  void show(){
    cout << x << ", " << x << endl;
  }
private:
  T x;
};

template<typename T>
ostream& operator << (ostream& out,const D<T>& d){
  out << d.x;
  return out;
}

int main(void){
  Test<int> *p = new D<int>(10, 21);
  p->show();
  D<int> d(10,20);
  cout << d << endl;
  return 0;
}

Template class inherits non template class, non template class inherits template class

The following examples have no practical significance, just look at grammar.

#include <iostream>
using namespace std;

class Foo{
public:
  Foo(int a, int b, int c) : x(a), y(b), z(c){}
  void show(){
    cout << x << "," << y << "," << z << endl;
  }
private:
  int x, y, z;
};

template <typename T>
class Goo : public Foo{
public:
  Goo(T t, int a, int b, int c):Foo(a,b,c), data(t){}
  void show(){
    cout << data << endl;
    cout << "Goo show" << endl;
  }
private:
  T data;
};

class Hoo : public Goo<int>{
public:
  Hoo(int a1,int a2,int a3,int a4,int a5):
    Goo(a1,a2,a3,a4),ho(a5){}
  void show(){
    cout << "Hoo show" << endl;
  }
private:
  int ho;
};
int main(void){
  Hoo hoo(1,2,3,4,5);
  hoo.show();
  Goo<string> goo("abc",1,2,3);
  goo.show();
  return 0;
}