C++函数、类、类的成员函数作为 类模板的友元 实例

//program 18.5.1.cpp 函数、类、类的成员函数作为 类模板的友元
#include<iostream>
using namespace std;

template <class T>
class Tmpl;

template <class T>
void Func1(Tmpl<T> &x)
{
   
   cout << "this is a global function :Func1()" << endl<<"\t";
   x.print();

}
/*模板类B*/
template <class T>
class B
{
public:
   void Func(Tmpl<T>& x);
};
/*类模板Tmpl*/
template <class T>
class Tmpl 
{
   T a;
public:
   friend void Func1();//全局友元函数
   friend class A;//友元类A

   template<class T>
   friend void B<T>::Func(Tmpl<T>&);//类B的成员函数作为 类模板Tmpl的友元;
   Tmpl(T a_=0):a(a_)//构造函数
   {
      cout << "the Tmpl<T>::constructor was called!" << endl;
   }
   void print()
   {
      cout << "a=" << a << endl;
   }
};

class A
{
   Tmpl<double> obj_in_A;
public:
   void printA()
   {
      cout << "this is a function of class A" << endl;
      
   }
   A(double a_):obj_in_A(a_)
   {
      //obj_in_A.a = a_;
      cout << "从友元类A(普通类A)中访问Tmpl<T>的私有成员a:" <<endl;
      cout << "a=" << obj_in_A.a << endl;
   }
};

template<class T>//不可少
void B<T>:: Func(Tmpl<T>& x)
{
   cout << "this is a function of class B:" << endl;
   cout << "从友元函数(类模板B<T>的函数Func(Tmpl<T>& x))中访问Tmpl<T>的私有成员a:" << endl;
   cout << "x.a=" << x.a<< endl;
}

int main()
{
   Tmpl<int> i(7); 
   i.print();
   Func1(i);//全局友元函数

   Tmpl<double> f(7.889);
   f.print();

   A obj1(3.14);//类A中有Tmpl<double>类的成员对象,会先调用Tmpl<double>的构造函数.
   B<double> obj2;
   obj2.Func(f);

}