"error: use of Deleted Function"

Question

I’m having trouble compiling the example below, linux platform for esp8266, I need an example of how to return an object through a class function to another class. As an example: I need access to the object (c) defined in class C, class A.

 #include "b.h"
    #include "c.h"
    class A
    {
        public:
           int start();
        private:            
            B b;
            C c;
            int x;      


    }

    a.cpp

    int A::start()
    {
        c = b.copy();
        x = c.d;
        return x;
    }



    #include "c.h"
    class B
    {
        private:
            C c;        
        public:
            C copy();
    }

    b.cpp

    C B::copy()
    {
        c.add(1);   
        return c;
    }


    class C
    {           
        public:
            void add(int x);
            int d;
    }

    c.cpp

    void c.add(int x)
    {
        C::d = x + 10;
        return;
    }


    main()
   {   
      A a;
      int y;

      y = a.start();

      cout << y ;

   }

When trying to compile on Linux the above example shows the error : "error: use of Deleted Function".