将 class 成员 function 声明为模板 class 的朋友

Question

#include< iostream>  
using namespace std;  

template< class t>  
class X  
{  
  private:  
      t x;    

  public:  
        template< class u>  
         friend u y::getx(X< u> ); 

      void setx(t s)  
      {x=s;}  
           };      

class y   
{  
 public:  
     template< class t>  
     t getx(X< t> d)  
     {return d.x;}     
};  

int main()  
{  
    X< int> x1;  
    x1.setx(7);  
    y y1;  
    cout<< y1.getx(x1);    
    return 0;  
}       

The above program, when compiled, showed an error that y is neither a function nor a member function, so it cannot be declared a friend. What is the way to include getx as a friend in X ?

Answer 1

You have to arrange the classes so that the function declared as a friend is actually visible before class X. You also have to make X visible before y...

template< class t>  
class X;

class y   
{  
 public:  
     template< class t>  
     t getx(X< t> d)  
     {return d.x;}     
};

template< class t>  
class X  
{  
  private:  
      t x;    

  public:  
        template< class u>  
         friend u y::getx(X< u> ); 

      void setx(t s)  
      {x=s;}  
};      

Answer 2

You should "forward declare" class y before template class XIe, just put:

class y; // forward declaration

template class X...