970ee117   
   From: francis.glassborow@btinternet.com   
      
   On 16/10/2011 14:29, Ethan wrote:   
   > Hello,   
   >   
   >   I'm trying to have a member function of a class to be a friend of   
   > another class, however, i get this error   
   > Am i missing anything?   
      
   Yes, in this case the compiler needs to be able to see the definition of   
   the class. In the case you show there is not reason to forward declare   
   B, just replace the declaration with the definition. I attach my version   
   of your code below. Please look at it and note the chnages I have made.   
      
   >   
   > main.cpp:10: error: invalid use of incomplete type ‘struct B’   
   > main.cpp:3: error: forward declaration of ‘struct B’   
   >   
   > #include   
   >   
   > class B;   
   >   
   > class A {   
   > int a;   
   > public : A() { a = 5; }   
   >          void sample(B);   
   >          friend void B::sample();   
   > };   
   >   
   > class B {   
   > public : void sample() {   
   >              //printf("%d\n",obj.a);   
   >          }   
   > };   
   >   
   >   
      
   My version of your code:   
      
   #include    // better to use C++ i/o when writing C++   
      
       class B {   
       public:   
          void sample();  // you can define this function later   
       };   
      
       class A {   
       public:   // need to know ordering   
         friend void B::sample();   
         A(): a(5){} // use ctor/init list   
        private:   
          int a;   
       };   
      
      
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